BGD - Latest Articles

Assessing biogeochemical effects and best management practice for a wheat–maize cropping system using the DNDC model

2013-05-22T00:00:00+02:00

Assessing biogeochemical effects and best management practice for a wheat–maize cropping system using the DNDC model

Biogeosciences Discussions, 10, 8561-8609, 2013

Author(s): F. Cui, X. H. Zheng, C. Y. Liu, K. Wang, Z. X. Zhou, and J. Deng

Contemporary agriculture is shifting from a single-goal to a multi-goal strategy, which in turn requires choosing best management practice (BMP) based on assessment of the biogeochemical effects of management alternatives. The bottleneck is the capacity of predicting the simultaneous effects of different management practice scenarios on multiple goals and choosing BMP among scenarios. The denitrification-decomposition (DNDC) model may provide an opportunity to solve this problem. We validated the DNDC model (version 95) using the observations of soil moisture and temperature, crop yields, aboveground biomass and fluxes of net ecosystem exchange of carbon dioxide, methane, nitrous oxide (N₂O), nitric oxide (NO) and ammonia (NH₃) from a wheat-maize cropping site in northern China. The model performed well for these variables. Then we used this model to simulate the effects of management practices on the goal variables of crop yields, NO emission, nitrate leaching, NH₃ volatilization and net emission of greenhouse gases in the ecosystem (NEGE). Results showed that no-till and straw-incorporated practices had beneficial effects on crop yields and NEGE. Use of nitrification inhibitors decreased nitrate leaching and N₂O and NO emissions, but they significantly increased NH₃ volatilization. Irrigation based on crop demand significantly increased crop yield and decreased nitrate leaching and NH₃ volatilization. Crop yields were hardly decreased if nitrogen dose was reduced by 15% or irrigation water amount was reduced by 25%. Two methods were used to identify BMP and resulted in the same BMP, which adopted the current crop cultivar, field operation schedules and full straw incorporation and applied nitrogen and irrigation water at 15% and 25% lower rates, respectively, than the current use. Our study indicates that the DNDC model can be used as a tool to assess biogeochemical effects of management alternatives and identify BMP.

The iron budget in ocean surface waters in the 20th and 21st centuries: projections by the Community Earth System Model version 1

2013-05-22T00:00:00+02:00

The iron budget in ocean surface waters in the 20th and 21st centuries: projections by the Community Earth System Model version 1

Biogeosciences Discussions, 10, 8505-8559, 2013

Author(s): K. Misumi, K. Lindsay, J. K. Moore, S. C. Doney, F. O. Bryan, D. Tsumune, and Y. Yoshida

We investigated the simulated iron budget in ocean surface waters in the 1990s and 2090s using the Community Earth System Model version 1 and the Representative Concentration Pathway 8.5 future CO₂ emission scenario. We assumed that exogenous iron inputs did not change during the whole simulation period; thus, iron budget changes were attributed solely to changes in ocean circulation and mixing in response to projected global warming. The model simulated the major features of ocean circulation and dissolved iron distribution for the present climate reasonably well. Detailed iron budget analysis revealed that roughly 70% of the iron supplied to surface waters in high-nutrient, low-chlorophyll (HNLC) regions is contributed by ocean circulation and mixing processes, but the dominant supply mechanism differed in each HNLC region: vertical mixing in the Southern Ocean, upwelling in the eastern equatorial Pacific, and deposition of iron-bearing dust in the subarctic North Pacific. In the 2090s, our model projected an increased iron supply to HNLC surface waters, even though enhanced stratification was predicted to reduce iron entrainment from deeper waters. This unexpected result could be attributed largely to changes in the meridional overturning and gyre-scale circulations that intensified the advective supply of iron to surface waters, especially in the eastern equatorial Pacific. The simulated primary and export productions in the 2090s decreased globally by 6% and 13%, respectively, whereas in the HNLC regions, they increased by 11% and 6%, respectively. Roughly half of the elevated production could be attributed to the intensified iron supply. The projected ocean circulation and mixing changes are consistent with recent observations of responses to the warming climate and with other Coupled Model Intercomparison Project model projections. We conclude that future ocean circulation and mixing changes will likely elevate the iron supply to HNLC surface waters and will potentially buffer future reductions in ocean productivity. External inputs of iron to the oceans are likely to be modified with climate change. Future work must incorporate robust estimates of these processes affecting the marine iron cycle.

Comparison between eddy covariance and automatic chamber techniques for measuring net ecosystem exchange of carbon dioxide in cotton and wheat fields

2013-05-22T00:00:00+02:00

Comparison between eddy covariance and automatic chamber techniques for measuring net ecosystem exchange of carbon dioxide in cotton and wheat fields

Biogeosciences Discussions, 10, 8467-8503, 2013

Author(s): K. Wang, C. Liu, X. Zheng, M. Pihlatie, B. Li, S. Haapanala, T. Vesala, H. Liu, Y. Wang, G. Liu, and F. Hu

Static and transparent automatic chamber (AC) technique is a~necessary choice for measuring net ecosystem exchange (NEE) of carbon dioxide (CO₂) in circumstances where eddy covariance (EC) technique is not applicable. However, a comparison of the two techniques for measurements on croplands has seldom been undertaken. We carried out NEE observations in a cotton field (for one year) and a winter wheat field (for one cropping season) using both AC and EC techniques, to (a) compare the NEE fluxes measured using each technique, and (b) test the NEE measurement performance of an automatic chamber system (AMEG), which was designed for simultaneous flux measurements of multiple gases. The half-hourly NEE fluxes measured with the two techniques were in approximate agreement, with the AC fluxes being 0.78 (cotton) and 1.06 (wheat) times those of the EC. When integrated to daily timescale, the fluxes of the two techniques were in better agreement, showing an average ratio of 0.94 and 1.00 for the cotton and wheat, respectively. During the periods with comparable field conditions and normal performance of both instruments, the cumulative NEE fluxes revealed small differences between the two techniques (–9.0 ~ 6.7%, with a mean of 0.1%). The measurements resulted in annual cumulative NEE of –40 g C m^–2 yr^–1 (EC) and –42 g C m^–2 yr^–1 (AC) in the cotton field and seasonal cumulative NEE of –251 g C m^–2 (EC) and –205 g C m^–2 (AC) in the wheat field. Our results indicate that, for cropland populated by short plants, the AMEG system and the data processing procedures applied in this study are able to provide NEE estimates comparable to those from EC measurements, although either technique may lead to an overestimation of the loss rate (or underestimation of the gain rate) of the soil organic carbon stock of an ecosystem, in particular with calcareous soils exposed to increasing atmospheric acid deposition.

Biology and air–sea gas exchange controls on the distribution of carbon isotope ratios (δ¹³C) in the ocean

2013-05-21T00:00:00+02:00

Biology and air–sea gas exchange controls on the distribution of carbon isotope ratios (δ¹³C) in the ocean

Biogeosciences Discussions, 10, 8415-8466, 2013

Author(s): A. Schmittner, N. Gruber, A. C. Mix, R. M. Key, A. Tagliabue, and T. K. Westberry

Analysis of observations and sensitivity experiments with a new three-dimensional global model of stable carbon isotope cycling elucidate the processes that control the distribution of δ¹³C in the contemporary and preindustrial ocean. Biological fractionation dominates the distribution of δ¹³C_DIC of dissolved inorganic carbon (DIC) due to the sinking of isotopically light δ¹³C organic matter from the surface into the interior ocean. This process leads to low δ¹³C_DIC values at dephs and in high latitude surface waters and high values in the upper ocean at low latitudes with maxima in the subtropics. Air–sea gas exchange provides an important secondary influence due to two effects. First, it acts to reduce the spatial gradients created by biology. Second, the associated temperature dependent fractionation tends to increase (decrease) δ¹³C_DIC values of colder (warmer) water, which generates gradients that oppose those arising from biology. Our model results suggest that both effects are similarly important in influencing surface and interior δ¹³C_DIC distributions. However, air-sea gas exchange is slow, so biological effect dominate spatial δ¹³C_DIC gradients both in the interior and at the surface, in constrast to conclusions from some previous studies. Analysis of a new synthesis of δ¹³C_DIC measurements from years 1990 to 2005 is used to quantify preformed (δ¹³C_pre) and remineralized (δ¹³C_rem) contributions as well as the effects of biology (Δδ¹³C_bio) and air–sea gas exchange (δ¹³C^*). The model reproduces major features of the observed large-scale distribution of δ¹³C_DIC, δ¹³C_pre, δ¹³C_rem, δ¹³C^*, and Δδ¹³C_bio. Residual misfits are documented and analyzed. Simulated surface and subsurface δ¹³C_DIC are influenced by details of the ecosystem model formulation. For example, inclusion of a simple parameterization of iron limitation of phytoplankton growth rates and temperature-dependent zooplankton grazing rates improves the agreement with δ¹³C_DIC observations and satellite estimates of phytoplankton growth rates and biomass, suggesting that δ¹³C can also be a useful test of ecosystem models.

The full greenhouse gases budget of Africa: synthesis, uncertainties and vulnerabilities

2013-05-17T00:00:00+02:00

The full greenhouse gases budget of Africa: synthesis, uncertainties and vulnerabilities

Biogeosciences Discussions, 10, 8343-8413, 2013

Author(s): R. Valentini, A. Arneth, A. Bombelli, S. Castaldi, R. Cazzolla Gatti, F. Chevallier, P. Ciais, E. Grieco, J. Hartmann, M. Henry, R. A. Houghton, M. Jung, W. L. Kutsch, Y. Malhi, E. Mayorga, L. Merbold, G. Murray-Tortarolo, D. Papale, P. Peylin, B. Poulter, P. A. Raymond, M. Santini, S. Sitch, G. Vaglio Laurin, G. R. van der Werf, C. A. Williams, and R. J. Scholes

This paper, developed under the framework of the RECCAP initiative, aims at providing improved estimates of the carbon and GHG (CO₂, CH₄ and N₂O) balance of continental Africa. The various components and processes of the African carbon and GHG budget were considered, and new and available data derived by different methodologies (based on inventories, ecosystem fluxes, models, and atmospheric inversions) were integrated. The related uncertainties were quantified and current gaps and weakness in knowledge and in the monitoring systems were also considered in order to provide indications on the future requirements. The vast majority of the results seem to agree that Africa is probably a small sink of carbon on an annual scale, with an average value of −0.61 ± 0.58 Pg C yr⁻¹. Nevertheless the emissions of CH₄ and N₂O may turn Africa into a source in terms of CO₂ equivalents. At sub-regional level there is a significant spatial variability in both sources and sinks, mainly due to the biome's differences and the different anthropic impacts, with southern Africa as the main source and central Africa, with its evergreen tropical forests, as the main sink. Emissions from land use change in Africa are significant (around 0.32 ± 0.05 Pg C yr⁻¹) and even higher than the fossil fuel ones; this is a unique feature among all the continents. In addition there can be significant carbon losses from land even without changes in the land use (forest), as results from the impact of selective logging. Fires also play a significant role, with 1.03 ± 0.22 Pg C yr⁻¹ of carbon emissions, mainly (90%) originated by savanna and woodland burning. But whether fire carbon emissions are compensated by CO₂ uptake during the growing season, or are a non-reversible loss of CO₂, remains unclear. Most of these figures are subjected to a significant interannual variability, on the order of ± 0.5 Pg C yr⁻¹ in standard deviation, accounting for around 25% of the year-to-year variation in the global carbon budget.

These results, even if still highly uncertain, show the important role that Africa plays in the carbon cycle at global level, both in terms of absolute values and variability.

Physiological compensation for environmental acidification is limited in the deep-sea urchin Strongylocentrotus fragilis

2013-05-17T00:00:00+02:00

Physiological compensation for environmental acidification is limited in the deep-sea urchin Strongylocentrotus fragilis

Biogeosciences Discussions, 10, 8313-8341, 2013

Author(s): J. R. Taylor, C. Lovera, P. J. Whaling, K. R. Buck, E. F. Pane, and J. P. Barry

Anthropogenic CO₂ is now reaching depths over 1000 m in the Eastern Pacific, overlapping the Oxygen Minimum Zone (OMZ). Deep-sea animals – particularly, calcifiers – are suspected to be especially sensitive to environmental acidification associated with global climate change. We have investigated the effects of hypercapnia and hypoxia on the deep-sea urchin Strongylocentrotus fragilis, during two long-term exposure experiments (1 month and 4 month) at three levels of reduced pH at in situ O₂ levels of approx. 10% saturation, and also to control pH at 100% O₂ saturation. During the first experiment, internal acid-base balance was investigated during a one-month exposure; results show S. fragilis has limited ability to compensate for the respiratory acidosis brought on by reduced pH, due in part to low non-bicarbonate extracellular fluid buffering capacity. During the second experiment, longer-term effects of hypercapnia and variable O₂ on locomotion, feeding, growth, and gonadosomatic index (GSI) were investigated; results show significant mortality and correlation of all measured parameters with environmental acidification at pH 6.6. Transient adverse effects on locomotion and feeding were seen at pH 7.2, without compromise of growth or GSI. Based on the expected changes in ocean pH and oxygen, results suggest extinction of S. fragilis in the eastern North Pacific is unlikely. Rather, we expect a shoaling and contraction of its bathymetric range.

Ocean acidification from 1997 to 2011 in the subarctic western North Pacific Ocean

2013-05-16T00:00:00+02:00

Ocean acidification from 1997 to 2011 in the subarctic western North Pacific Ocean

Biogeosciences Discussions, 10, 8283-8311, 2013

Author(s): M. Wakita, S. Watanabe, M. Honda, A. Nagano, K. Kimoto, K. Matsumoto, H. Kawakami, T. Fujiki, M. Kitamura, K. Sasaki, K. Sasaoka, Y. Nakano, and A. Murata

Rising atmospheric CO₂ contents have led to greater CO₂ uptake by the oceans, lowering both pH due to increasing hydrogen ions and CaCO₃ saturation states due to declining carbonate ion (CO₃²⁻). Here, we used previously compiled data sets and new data collected in 2010 and 2011 to investigate ocean acidification of the North Pacific western subarctic gyre. In winter, the western subarctic gyre is a source of CO₂ to the atmosphere because of convective mixing of deep waters rich in dissolved inorganic carbon (DIC). We calculated pH in winter mixed layer from DIC and total alkalinity (TA), and found that it decreased at the rate of −0.001 ± 0.0004 yr⁻¹ from 1997 to 2011. This decrease rate is slower than that expected under condition of seawater/atmosphere equilibration, and it is also slower than the rate in the subtropical regions (−0.002 yr⁻¹). The slow rate is caused by a reduction of CO₂ emission in winter due to an increase in TA. Below the mixed layer, the calcite saturation horizon (~185 m depth) shoaled at the rate of 2.9 ± 0.9 m yr⁻¹ as the result of the declining CO₃²⁻ concentration (−0.03 ± 0.01 μmol k⁻¹yr⁻¹). Between 200 m and 300 m depth, pH decline during the study period (−0.0051 ± 0.0010 yr⁻¹) was larger than ever reported in the open North Pacific. This enhanced acidification rate below the calcite saturation horizon reflected not only the uptake of anthropogenic CO₂ but also the increase in the decomposition of organic matter evaluated from the increase in AOU, which suggests that the dissolution of CaCO₃ particles increased.

Asynchronism in leaf and wood production in tropical forests: a study combining satellite and ground-based measurements

2013-05-16T00:00:00+02:00

Asynchronism in leaf and wood production in tropical forests: a study combining satellite and ground-based measurements

Biogeosciences Discussions, 10, 8247-8281, 2013

Author(s): F. Wagner, V. Rossi, C. Stahl, D. Bonal, and B. Hérault

The fixation of carbon in tropical forests mainly occurs through the production of wood and leaves, both being the principal components of net primary production. Currently field and satellite observations are independently used to describe the forest carbon cycle, but the link between satellite-derived forest phenology and field-derived forest productivity remains opaque. We used a unique combination of a MODIS EVI dataset, a climate-explicit wood production model and direct litterfall observations at an intra-annual time scale in order to question the synchronism of leaf and wood production in tropical forests. Even though leaf and wood biomass fluxes had the same range (respectively 2.4 ± 1.4 Mg C ha⁻¹yr⁻¹ and 2.2 ± 0.4 Mg C ha⁻¹yr⁻¹), they occured separately in time. EVI increased with the magnitude of leaf renewal at the beginning of the dry season when solar irradiance was at its maximum. At this time, wood production stopped. At the onset of the rainy season when new leaves were fully mature and water available again, wood production quickly increased to reach its maximum in less than a month, reflecting a change in carbon allocation from short lived pools (leaves) to long lived pools (wood). The time lag between peaks of EVI and wood production (109 days) revealed a substantial decoupling between the irradiance-driven leaf renewal and the water-driven wood production. Our work is a first attempt to link EVI data, wood production and leaf phenology at a seasonal time scale in a tropical evergreen rainforest and pave the way to develop more sophisticated global carbon cycle models in tropical forests.

Bottom-up and top-down controls on picoplankton in the East China Sea

2013-05-16T00:00:00+02:00

Bottom-up and top-down controls on picoplankton in the East China Sea

Biogeosciences Discussions, 10, 8203-8245, 2013

Author(s): C. Guo, H. Liu, L. Zheng, S. Song, B. Chen, and B. Huang

Dynamics of picoplankton population distribution in the East China Sea (ECS), a marginal sea in the western North Pacific Ocean, were studied during two "CHOICE-C" cruises in August 2009 (summer) and January 2010 (winter). Dilution experiments were conducted during the two cruises to investigate the growth and grazing among picophytoplantkon populations. Picoplankton accounted for an average of ~29% (2% to 88%) of community carbon biomass in the ECS on average, with lower percentages in plume region than in shelf and kuroshio regions. Averaged growth rates (μ) for Prochlorococcus (Pro), Synechococcus (Syn) and picoeukaryotes (peuk) were 0.36, 0.89, 0.90 d⁻¹, respectively, in summer, and 0.46, 0.58, 0.56 d⁻¹, respectively, in winter. Seawater salinity and nutrient availability exerted significant controls on picoplankton growth rate. Averaged grazing mortality (m) were 0.46, 0.63, 0.68 d⁻¹ in summer, and 0.22, 0.32, 0.22 d⁻¹ in winter for Pro, Syn and peuk respectively. The three populations demonstrated very different distribution patterns regionally and seasonally affected by both bottom-up and top-down controls. In summer, Pro, Syn and peuk were dominant in Kuroshio, transitional and plume regions respectively. Protist grazing consumed 84%, 78%, 73% and 45%, 47%, 57% of production for Pro, Syn and peuk in summer and winter respectively, suggesting more significant top-down controls in summer. In winter, all three populations tended to distribute in offshore regions, although the area of coverage was different (peuk > Syn > Pro). Bottom-up factors can explain as much as 91.5%, 82% and 81.2% of Pro, Syn and peuk abundance variance in winter, while only 59.1% and 43.7% for Pro and peuk in summer. Regionally, Yangtze River discharge plays a significant role in affecting the intensity of top-down control, indicated by significant and negative association between salinity and grazing mortality of all three populations and higher grazing mortality to growth rate ratio (m / μ) in plume region than Kuroshio region in summer. The gradient of bottom-up factors caused by Yangtze River input and Kuroshio warm current intrusion also exerted important influence on picoplankton abundance evidenced by the significant correlations. Vertically, picoplankton exhibited highest abundance at subsurface layer around 20 m thick in summer, while at surface in winter. Both growth rate and grazing mortality were higher at surface than at the deep chlorophyll maximum (DCM) layer. Our study first systematically described the bottom-up and top-down regulations of different picoplankton populations between contrasting seasons, different depths, and among different regions in the ECS, which provide insights for better understanding the population dynamics of picoplankton and trophic transfer in microbial food web in highly dynamic shelf ecosystems and in general.

Microbial activity and carbonate isotope signatures as a tool for identification of spatial differences in methane advection: a case study at the Pacific Costa Rican margin

2013-05-15T00:00:00+02:00

Microbial activity and carbonate isotope signatures as a tool for identification of spatial differences in methane advection: a case study at the Pacific Costa Rican margin

Biogeosciences Discussions, 10, 8159-8201, 2013

Author(s): S. Krause, P. Steeb, C. Hensen, V. Liebetrau, A. W. Dale, M. Nuzzo, and T. Treude

Subduction of the oceanic Cocos plate offshore Costa Rica causes strong advection of methane-charged fluids. Presented here are the first direct measurements of microbial anaerobic oxidation of methane (AOM) and sulfate reduction (SR) rates in sediments from the two mounds, applying radiotracer techniques in combination with numerical modeling. In addition, analysis of carbonate δ¹⁸O, δ¹³C, and ⁸⁷Sr / ⁸⁶Sr signatures constrain the origin of the carbonate-precipitating fluid. Average rates of microbial activities showed differences with a factor of 4.8 to 6.3 between Mound 11 [AOM 140.71 (±40.84 SD); SR 117.25 (±82.06 SD) mmol m⁻² d⁻¹, respectively] and Mound 12 [AOM 22.37 (±0.85 SD); SR 23.99 (±5.79 SD) mmol m⁻² d⁻¹, respectively]. Modeling results yielded flow velocities of 50 cm a⁻¹ at Mound 11 and 8–15 cm a⁻¹ at Mound 12. Analysis of oxygen and carbon isotope variations of authigenic carbonates from the two locations revealed higher values for Mound 11 (δ¹⁸O: 4.7 to 5.9‰, δ¹³C: −21.0 to −29.6‰), compared to Mound 12 (δ¹⁸O: 4.1 to 4.5‰, δ¹³C: −45.7 to −48.9‰). Analysis of carbonates ⁸⁷Sr / ⁸⁶Sr indicated temporal changes of deep-source fluid admixture at Mound 12. The present study is in accordance with previous work supporting considerable differences of methane flux between the two Mounds. It also strengthens the hypothesis of a predominantly deep fluid source for Mound 11 versus a rather shallow source of biogenic methane for Mound 12. The results demonstrate that methane-driven microbial activity is a valid ground truthing tool for geophysical measurements of fluid advection and constraining of recent methane fluxes in the study area. The study further shows that the combination of microbial rate measurements, numerical modeling, and authigenic carbonate analysis provide a suitable approach to constrain temporal and spatial variations of methane charged fluid flow at the Pacific Costa Rican margin.

Distribution of phytoplankton functional types in high-nitrate low-chlorophyll waters in a new diagnostic ecological indicator model

2013-05-15T00:00:00+02:00

Distribution of phytoplankton functional types in high-nitrate low-chlorophyll waters in a new diagnostic ecological indicator model

Biogeosciences Discussions, 10, 8103-8157, 2013

Author(s): A. P. Palacz, M. A. St. John, R. J. W. Brewin, T. Hirata, and W. W. Gregg

Modeling and monitoring plankton functional types (PFTs) is challenged by insufficient amount of field measurements to ground-truth both plankton models and bio-optical algorithms. In this study, we combine remote sensing data and a dynamic plankton model to simulate an ecologically-sound spatial and temporal distribution of phyto-PFTs. We apply an innovative ecological indicator approach to modeling PFTs, and focus on resolving the question of diatom-coccolithophore co-existence in the subpolar high-nitrate and low-chlorophyll regions. We choose an artificial neural network as our modeling framework because it has the potential to interpret complex nonlinear interactions governing complex adaptive systems, of which marine ecosystems are a prime example. Using ecological indicators that fulfill the criteria of measurability, sensitivity and specificity, we demonstrate that our diagnostic model correctly interprets some basic ecological rules similar to ones emerging from dynamic models. Our time series highlight a dynamic phyto-PFT community composition in all high latitude areas, and indicate seasonal co-existence of diatoms and coccolithophores. This observation, though consistent with in situ and remote sensing measurements, was so far not captured by state-of-the-art dynamic models which struggle to resolve this "paradox of the plankton". We conclude that an ecological indicator approach is useful for ecological modeling of phytoplankton and potentially higher trophic levels. Finally, we speculate that it could serve as a powerful tool in advancing ecosystem-based management of marine resources.

Vertical activity distribution of dissimilatory nitrate reduction in coastal marine sediments

2013-05-14T00:00:00+02:00

Vertical activity distribution of dissimilatory nitrate reduction in coastal marine sediments

Biogeosciences Discussions, 10, 8065-8101, 2013

Author(s): A. Behrendt, D. de Beer, and P. Stief

The relative importance of two dissimilatory nitrate reduction pathways, denitrification (DEN) and dissimilatory nitrate reduction to ammonium (DNRA), was investigated in intact sediment cores from five different coastal marine field sites. The vertical distribution of DEN activity was examined using the acetylene inhibition technique combined with N₂O microsensor measurements, whereas NH₄⁺ production via DNRA was measured with a recently developed gel probe-stable isotope technique. At all field sites, dissimilatory nitrate reduction was clearly dominated by DEN (> 59% of the total NO₃⁻ reduced) rather than by DNRA, irrespective of the sedimentary inventories of electron donors such as organic carbon, sulfide, and iron. Ammonium production via DNRA (8.9% of the total NO₃⁻ reduced) was exclusively found at one site with very high concentrations of total sulfide and NH₄⁺ in the layer of NO₃⁻ reduction and below. Sediment from two field sites, one with and one without DNRA activity in the core incubations, was also used for slurry incubations. Now, in both sediments high DNRA activity was detected accounting for 37–77% of the total NO₃⁻ reduced. These contradictory results can be explained by enhanced NO₃⁻ availability for DNRA bacteria in the sediment slurries compared to the core-incubated sediments.

It can be argued that the gel probe technique gives more realistic estimates of DNRA activity in diffusion-dominated sediments, while slurry incubations are more suitable for advection-dominated sediments.

Fertilization success of an arctic sea urchin species, Strongylocentrotus droebachiensis (O. F. Müller, 1776) under CO₂-induced ocean acidification

2013-05-14T00:00:00+02:00

Fertilization success of an arctic sea urchin species, Strongylocentrotus droebachiensis (O. F. Müller, 1776) under CO₂-induced ocean acidification

Biogeosciences Discussions, 10, 8027-8064, 2013

Author(s): D. Bögner, U. Bickmeyer, and A. Köhler

Sea urchins as broadcasting spawners, release their gametes into open water for fertilization, thus being particularly vulnerable to ocean acidification. In this study, we assessed the effects of different pH scenarios on fertilization success of Strongylocentrotus droebachiensis, collected at Spitsbergen, Arctic. We achieved acidification by bubbling CO₂ into filtered seawater using partial pressures (pCO₂) of 180, 380, 980, 1400 and 3000 μatm}. Untreated filtered seawater was used as control. We recorded fertilization rates and diagnosed morphological aberrations after post-fertilization periods of 1 h and 3 h under different exposure conditions in experiments with and without pre-incubation of the eggs prior to fertilization. In parallel, we conducted measurements of intracellular pH changes using BCECF/AM in unfertilized eggs exposed to a range of acidified seawater. We observed increasing rates of polyspermy in relation to higher seawater pCO₂, which might be due to failures in the formation of the fertilization envelope. In addition, our experiments showed anomalies in fertilized eggs: incomplete lifting-off of the fertilization envelope and blebs of the hyaline layer. Other drastic malformations consisted of constriction, extrusion, vacuolization or degeneration (observed as a gradient from the cortex to the central region of the cell) of the egg cytoplasm, and irregular cell divisions until 2- to 4-cell stages. The intracellular pH (pH_i) decreased significantly from 1400 μatm on. All results indicate a decreasing fertilization success at CO₂ concentrations from 1400 μatm upwards. Exposure time to low pH might be a threatening factor for the cellular buffer capacity, viability, and development after fertilization.

The calcareous nannofossil Prinsiosphaera achieved rock-forming abundances in the latest Triassic of western Tethys: consequences for the δ¹³C of bulk carbonate

2013-05-14T00:00:00+02:00

The calcareous nannofossil Prinsiosphaera achieved rock-forming abundances in the latest Triassic of western Tethys: consequences for the δ¹³C of bulk carbonate

Biogeosciences Discussions, 10, 7989-8025, 2013

Author(s): N. Preto, C. Agnini, M. Rigo, M. Sprovieri, and H. Westphal

The onset of pelagic biomineralization marked a milestone in the history of the long term inorganic carbon cycle: as soon as calcareous nannofossils became major limestone producers, the pH and supersaturation state of the global ocean were stabilized (the so-called Mid Mesozoic Revolution). But although it is known that calcareous nannofossils were abundant already by the end of the Triassic, no estimates exist on their contribution to hemipelagic carbonate sedimentation. With this work, we estimate the volume proportion of Prinsiosphaera, the dominant Late Triassic calcareous nannofossil, in hemipelagic and pelagic carbonates of western Tethys. The investigated Upper Triassic lime mudstones are composed essentially of microspar and tests of calcareous nannofossils, plus minor bioclasts. Prinsiosphaera became a significant component of lime mudstones since the late Norian, and was contributing up to ca. 60% of the carbonate by the late Rhaetian in periplatform environments with hemipelagic sedimentation. The increasing proportion of Prinsiosphaera in upper Rhaetian hemipelagic lime mudstones is paralleled by a increase of the δ¹³C of bulk carbonate. We interpreted this isotopic trend as related to the diagenesis of microspar, which incorporated respired organic carbon with a low δ¹³C when it formed during shallow burial. As the proportion of nannofossil tests increased, the contribution of microspar with low δ¹³C diminished, determining the isotopic trend. We suggest that a similar diagenetic effect may be observed in many Mesozoic limestones with a significant, but not yet dominant, proportion of calcareous plankton.

Catchment-scale carbon exports across a subarctic landscape gradient

2013-05-14T00:00:00+02:00

Catchment-scale carbon exports across a subarctic landscape gradient

Biogeosciences Discussions, 10, 7953-7988, 2013

Author(s): R. Giesler, S. W. Lyon, C.-M. Mörth, J. Karlsson, E. J. Jantze, G. Destouni, and C. Humborg

Climatic change is currently enhancing permafrost thawing and hydrological cycling in subarctic and arctic catchments with major consequences for the carbon export to aquatic ecosystems. We studied stream water carbon export in several tundra dominated catchments in northern Sweden. There were clear seasonal differences in both dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC) concentrations. The highest DOC concentrations occurred during the spring freshet while the highest DIC concentrations were always observed during winter baseflow conditions for the six catchments considered in this study. In these subarctic catchments, DIC accounted for at least about half of the annual mass of C exported. Further, there was a direct relationship between both hydrologic flow pathway length and the maximum flow to minimum flow ratio (which serves as a proxy for fractioning between surface and subsurface flow pathways) and annual carbon fluxes for these six catchments. Further, these relationships were more prevalent for annual DIC exports than annual DOC exports in this region. These results highlight that there can be large regional differences in high latitude ecosystems and emphasize the importance of proper representation of subsurface hydrogeological conditions. This is particularly relevant in subarctic environments were thawing permafrost and changes to subsurface ice due to global warming can influence stream water fluxes of C. The large proportion of stream water DIC flux also has implications on regional C budgets and needs to be considered in order to understand climate induced feedback mechanisms across the landscape.

Benthic fluxes of dissolved organic nitrogen in the Lower St. Lawrence Estuary and implications for selective organic matter degradation

2013-05-13T00:00:00+02:00

Benthic fluxes of dissolved organic nitrogen in the Lower St. Lawrence Estuary and implications for selective organic matter degradation

Biogeosciences Discussions, 10, 7917-7952, 2013

Author(s): M. Alkhatib, P. A. del Giorgio, Y. Gelinas, and M. F. Lehmann

The distribution of dissolved organic nitrogen (DON) and carbon (DOC) in sediment pore waters was determined at nine locations along the St. Lawrence Estuary and in the Gulf of St. Lawrence. The study area is characterized by gradients in the sedimentary particulate organic matter (POM) reactivity, bottom water oxygen concentrations, as well as benthic respiration rates. Based on pore water profiles we estimated the benthic diffusive fluxes of DON and DOC. Our results show that DON fluxed out of the sediments at significant rates (110 to 430 μmol m⁻² d⁻¹). DON fluxes were positively correlated with sedimentary POM reactivity and sediment oxygen exposure time (OET), suggesting direct links between POM quality, aerobic remineralization and the release of DON to the water column. DON fluxes were on the order of 30% to 64% of the total benthic inorganic fixed N loss due to denitrification, and often exceeded the diffusive nitrate fluxes into the sediments. Hence they represented a large fraction of the total benthic N exchange. This result is particularly important in light of the fact that DON fluxes are usually not accounted for in estuarine and coastal zone nutrient budgets. The ratio of the DON to nitrate flux increased from 0.6 in the Lower Estuary to 1.5 in the Gulf. In contrast to DON, DOC fluxes did not show any significant spatial variation along the Laurentian Channel (LC) between the Estuary and the Gulf (2100 ± 100μmol m⁻² d⁻¹), suggesting that production and consumption of labile DOC components proceed at similar rates, irrespective of the overall benthic characteristics and the reactivity of POM. As a consequence, the molar C/N ratio of dissolved organic matter (DOM) in pore water and the overlying bottom water varied significantly along the transect, with lowest C/N in the Lower Estuary (5–6) and highest C/N (> 10) in the Gulf. We observed large differences between the C/N of pore water DOM with respect to POM, and the degree of the C– versus –N element partitioning seems to be linked to POM reactivity and/or redox conditions in the sediment pore waters. Our results thus highlight the variable effects selective OM degradation and preservation can have on bulk sedimentary C/N ratios, decoupling the primary source C/N signatures from those in sedimentary archives. Our study further underscores that the role of estuarine sediments as efficient sinks of bioavailable nitrogen is strongly influenced by the release of DON during early diagenetic reactions, and that DON fluxes from continental margin sediments represent an important internal source of N to the ocean.

Ocean acidification state in western Antarctic surface waters: drivers and interannual variability

2013-05-08T00:00:00+02:00

Ocean acidification state in western Antarctic surface waters: drivers and interannual variability

Biogeosciences Discussions, 10, 7879-7916, 2013

Author(s): M. Mattsdotter Björk, A. Fransson, and M. Chierici

Each December during four years from 2006 to 2010, the surface water carbonate system was measured and investigated in the Amundsen Sea and Ross Sea, western Antarctica as part of the Oden Southern Ocean expeditions (OSO). The I/B Oden started in Punta Arenas in Chile and sailed southwest, passing through different regimes such as, the marginal/seasonal ice zone, fronts, coastal shelves, and polynyas. Discrete surface water was sampled underway for analysis of total alkalinity (A_T), total dissolved inorganic carbon (C_T) and pH. Two of these parameters were used together with sea-surface temperature (SST), and salinity to obtain a full description of the surface water carbonate system, including pH in situ and calcium carbonate saturation state of aragonite (Ω_Ar) and calcite (Ω_Ca). Multivariate analysis was used to investigate interannual variability and the major controls (sea-ice concentration, SST, salinity and chlorophyll a) on the variability in the carbonate system and Ω. This analysis showed that SST and chlorophyll a were the major drivers of the Ω variability in both the Amundsen and Ross seas. In 2007, the sea-ice edge was located further south and the area of the open polynya was relatively small compared to 2010. We found the lowest pH in situ (7.932) and Ω = 1 values in the sea-ice zone and in the coastal Amundsen Sea, nearby marine out flowing glaciers. In 2010, the sea-ice coverage was the largest and the areas of the open polynyas were the largest for the whole period. This year we found the lowest salinity and A_T, coinciding with highest chl a. This implies that the highest Ω_Ar in 2010 was likely an effect of biological CO₂ drawdown, which out-competed the dilution of carbonate ion concentration due to large melt water volumes. We predict and discuss future Ω values, using our data and reported rates of oceanic uptake of anthropogenic CO₂, suggesting that the Amundsen Sea will become undersaturated with regard to aragonite about 20 yr sooner than predicted by models.

Time-series measurements of biochemical and physical properties in the southwestern East/Japan Sea during the spring transition in 2010

2013-05-08T00:00:00+02:00

Time-series measurements of biochemical and physical properties in the southwestern East/Japan Sea during the spring transition in 2010

Biogeosciences Discussions, 10, 7831-7878, 2013

Author(s): Y.-T. Son, K.-I. Chang, S.-T. Yoon, Y.-B. Kim, T. Rho, C. K. Kang, and K.-R. Kim

An ocean buoy, UBIM, deployed during the spring transition from February and May, 2010 reveals for the first time highly-resolved temporal variation of biochemical properties of the upper layer of the Ulleung Basin in the southwestern East/Japan Sea. Meteorological data shows the typical spring transition occurred during the mooring period, weakening of wind speed, increase in shortwave radiation, and change in total heat flux from net cooling to net heating. Power spectrum of chlorophyll fluorescence (CF) peaks at semidiurnal tidal, near-inertial, diurnal, and subtidal frequencies. The diurnal variation of CF is characterized by high CF during the daytime and low CF at night. Dissolved oxygen and CF are correlated with high (low) dissolved oxygen accompanied by high (low) CF, indicating the dissolved oxygen is mainly determined by biological activities. The time series measurement captured the onset of subsurface spring bloom at 30 m, and collocated temperature and current data gives an insight into a mechanism that triggers the onset of the spring bloom not documented so far. The entire mooring period can be divided into pre-bloom period from the beginning of the mooring to early April, and bloom period afterwards. Mean CF values during the pre-bloom and bloom periods are 0.9 μg L⁻¹ and 1.9 μg L⁻¹, respectively. Mean mixed layer depth (MLD) shoaled from 22 m during the pre-bloom period to 15 m during the bloom period. Despite of the increase in shortwave radiation, average PAR values at 20 m show lower value during the bloom period as compared to that during the pre-bloom period. Low-frequency modulation of MLD ranging from 10 m to 53 m during the entire mooring period is mainly determined by shoaling and deepening of isothermal (isopycnal) depths. Temperature structure in the upper 110 m is characterized by alternating uplifting and lowering of isotherms, which is caused by the placement of the mooring site on the cold (cyclonic) or warm side of the frontal jet, the East Korean Warm Current. The frontal variability is thought to be due to the low-frequency path variatio of the East Korean Warm Current. The occurrence of the spring bloom at 30 m is concomitant with the appearance of colder East Sea Intermediate Water (ESIW) at buoy UBIM that results in the subsurface cooling and the shoaling of isotherms to the shallower depth levels than those occurred during the pre-bloom period. It is suggested that the springtime spreading of the ESIW is one of the important factors that triggers the onset of subsurface spring bloom below the mixed layer. The time lag between the peaks of CF and the occurrence of the shallowest isothermal depths is about several days, which appears to be the timescale for the growth of phytoplankton.

Modeling ocean circulation and biogeochemical variability in the Gulf of Mexico

2013-05-08T00:00:00+02:00

Modeling ocean circulation and biogeochemical variability in the Gulf of Mexico

Biogeosciences Discussions, 10, 7785-7830, 2013

Author(s): Z. Xue, R. He, K. Fennel, W.-J. Cai, S. Lohrenz, and C. Hopkinson

A three-dimensional coupled physical-biogeochemical model is applied to simulate and examine temporal and spatial variability of circulation and biogeochemical cycling in the Gulf of Mexico (GoM). The model is driven by realistic atmospheric forcing, open boundary conditions from a data assimilative global ocean circulation model, and observed freshwater and terrestrial nutrient input from major rivers. A 7 yr model hindcast (2004–2010) was performed, and validated against satellite observed sea surface height, surface chlorophyll, and in-situ observations including coastal sea-level, ocean temperature, salinity, and nutrient concentration. The model hindcast revealed clear seasonality in nutrient, phytoplankton and zooplankton distributions in the GoM. An Empirical Orthogonal Function analysis indicated a phase-locked pattern among nutrient, phytoplankton and zooplankton concentrations. The GoM shelf nutrient budget was also quantified, revealing that on an annual basis ~80% of nutrient input was denitrified on the shelf and ~17% was exported to the deep ocean.

Food availability and pCO₂ impacts on planulation, juvenile survival, and calcification of the azooxanthellate scleractinian coral, Balanophyllia elegans

2013-05-07T00:00:00+02:00

Food availability and pCO₂ impacts on planulation, juvenile survival, and calcification of the azooxanthellate scleractinian coral, Balanophyllia elegans

Biogeosciences Discussions, 10, 7761-7783, 2013

Author(s): E. D. Crook, H. Cooper, D. C. Potts, T. Lambert, and A. Paytan

Ocean acidification, the assimilation of atmospheric CO₂ by the oceans that decreases the pH and CaCO₃ saturation state (Ω) of seawater, is projected to have severe consequences for calcifying organisms. Strong evidence suggests that tropical reef-building corals containing algal symbionts (zooxanthellae) will experience dramatic declines in calcification over the next century. The responses of azooxanthellate corals to ocean acidification are less well understood, and because they cannot obtain extra photosynthetic energy from symbionts, they provide a system for studying the direct effects of acidification on the energy available for calcification. The orange cup coral Balanophyllia elegans is a solitary, azooxanthellate scleractinian species common on the California coast where it thrives in the low pH waters of an upwelling regime. During an 8 month study, we addressed the effects of three pCO₂ treatments (410, 770, and 1230 μatm) and two feeding frequencies (High Food and Low Food) on adult Balanophyllia elegans planulation (larval release) rates, and on the survival, growth, and calcification of their juvenile offspring. Planulation rates were affected by food level but not pCO₂, while juvenile survival was highest under 410 μatm and High Food conditions. Our results suggest that feeding rate has a greater impact on calcification of B. elegans than pCO₂. Net calcification was positive even at 1230 μatm (~ 3 times current atmospheric pCO₂), although the increase from 410 to 1230 μatm reduced overall calcification by ~ 25–45%, and reduced skeletal density by ~ 35–45%. Higher pCO₂ also altered aragonite crystal morphology significantly. We discuss how feeding frequency affects azooxanthellate coral calcification, and how B. elegans may respond to ocean acidification in coastal upwelling waters.

Testing the applicability of neural networks as a gap-filling method using CH₄ flux data from high latitude wetlands

2013-05-03T00:00:00+02:00

Testing the applicability of neural networks as a gap-filling method using CH₄ flux data from high latitude wetlands

Biogeosciences Discussions, 10, 7727-7759, 2013

Author(s): S. Dengel, D. Zona, T. Sachs, M. Aurela, M. Jammet, F. J. W. Parmentier, W. Oechel, and T. Vesala

Since the advancement in CH₄ gas analyser technology and its applicability to eddy covariance flux measurements, monitoring of CH₄ emissions is becoming more widespread. In order to accurately determine the greenhouse gas balance, high quality gap-free data is required. Currently there is still no consensus on CH₄ gap-filling methods, and methods applied are still study-dependent and often carried out on low resolution daily data.

In the current study, we applied artificial neural networks to six distinctively different CH₄ time series from high latitudes in order to recover missing data points, explained the method and tested its functionality. We discuss the applicability of neural networks in CH₄ flux studies, the advantages and disadvantages of this method, and what information we were able to extract from such models.

In keeping with the principle of parsimony, we included only five standard meteorological variables traditionally measured at CH₄ flux measurement sites. These included drivers such as air and soil temperature, barometric air pressure, solar radiation, and in addition wind direction (indicator of source location). Four fuzzy sets were included representing the time of day. High Pearson correlation coefficients (r) of 0.76–0.93 achieved in the final analysis are indicative for the high performance of neural networks and their applicability as a gap-filling method for CH₄ flux data time series. This novel approach that we showed to be appropriate for CH₄ fluxes is a step towards standardising CH₄ gap-filling protocols.

Summer and winter living coccolithophores in the Yellow Sea and the East China Sea

2013-05-03T00:00:00+02:00

Summer and winter living coccolithophores in the Yellow Sea and the East China Sea

Biogeosciences Discussions, 10, 7677-7726, 2013

Author(s): X. Y. Gu, Y. Y. Feng, S. F. Jin, W. S. Jiang, H. Y. Jin, J. F. Chen, and J. Sun

To date, very little information on living coccolithophores species composition and distribution, especially the vertical profile has been reported around the world. This paper tries to fill this gap by descripting on living coccolithophores (LCs) distribution in the Yellow Sea and the East China Sea in summer and winter time in detail, and its relationship among enviromental factors by canonical correspondence analysis (CCA). We carried out the investigations on LC distribution in the Yellow Sea and the East China Sea in July and December 2011. 210 samples from different depths were collected from 44 stations in summer and 217 samples were collected from 45 stations in winter. Totally 20 taxa belonging to coccolithophyceae were identified using a polarized microscope at the 1000 × magnification. The dominant species of the two seasons were Gephyrocapsa oceanica, Emiliania huxleyi, Helicosphaera carteri, and Algirosphaera robusta. In summer the abundance of cells and coccoliths ranged 0 ~ 176.40 cells mL⁻¹, and 0 ~ 2144.98 coccoliths mL⁻¹, with the average values of 8.45 cells mL⁻¹, and 265.42 coccoliths mL⁻¹, respectively. And in winter the abundance of cells and coccoliths ranged 0 ~ 71.66 cells mL⁻¹, and 0 ~ 4698.99 coccoliths mL⁻¹, with the average values of 13.91 cells mL⁻¹ and 872.56 coccoliths mL⁻¹ respectively. In summer the LCs in surface layer were mainly observed on the coastal belt and southern part of the survey area. The highest abundance was found at the bloom station. In winter the LCs in surface layer had high value in the continental shelf area of section P. The comparison among section A, section F, section P and section E indicated lower species diversity and less abundance in the Yellow Sea than those of the East China Sea in both seasons. Temperature and the nitrate concentration may be the major environmental factors controlling the distribution and species composition of LCs in the studying area based on CCA.

Dynamics of seawater carbonate chemistry, production, and calcification of a coral reef flat, Central Great Barrier Reef

2013-05-03T00:00:00+02:00

Dynamics of seawater carbonate chemistry, production, and calcification of a coral reef flat, Central Great Barrier Reef

Biogeosciences Discussions, 10, 7641-7676, 2013

Author(s): R. Albright, C. Langdon, and K. R. N. Anthony

Ocean acidification is projected to shift coral reefs from a state of net accretion to one of net dissolution this century. Presently, our ability to predict global-scale changes to coral reef calcification is limited by insufficient data relating seawater carbonate chemistry parameters to in situ rates of reef calcification. Here, we investigate natural trends in carbonate chemistry of the Davies Reef flat in the central Great Barrier Reef on diel and seasonal timescales and relate these trends to benthic carbon fluxes by quantifying net ecosystem calcification (nec) and net community production (ncp). Results show that seawater carbonate chemistry of the Davies Reef flat is highly variable over both diel and seasonal timescales. pH (total scale) ranged from 7.92 to 8.17, pCO₂ ranged from 272 to 542 μatm, and aragonite saturation state (Ω_arag) ranged from 2.9 to 4.1. Diel cycles in carbonate chemistry were primarily driven by ncp, and warming explained 35% and 47% of the seasonal shifts in pCO₂ and pH, respectively. Daytime ncp averaged 36 ± 19 mmol C m⁻² h⁻¹ in summer and 33 ± 13 mmol C m⁻² h⁻¹ in winter; nighttime ncp averaged −22 ± 20 and −7 ± 6 mmol C m⁻² h⁻¹ in summer and winter, respectively. Daytime nec averaged 11 ± 4 mmol CaCO₃ m⁻² h⁻¹ in summer and 8 ± 3 mmol CaCO₃ m⁻² h⁻¹ in winter, whereas nighttime nec averaged 2 ± 4 mmol and −1 ± 3 mmol CaCO₃ m⁻² h⁻¹ in summer and winter, respectively. Net ecosystem calcification was positively correlated with Ω_arag for both seasons. Linear correlations of nec and Ω_arag indicate that the Davies Reef flat may transition from a state of net calcification to net dissolution at Ω_arag values of 3.4 in summer and 3.2 in winter. Diel trends in Ω_arag indicate that the reef flat is currently below this calcification threshold 29.6% of the time in summer and 14.1% of the time in winter.

Respiration of Mediterranean cold-water corals is not affected by ocean acidification as projected for the end of the century

2013-05-02T00:00:00+02:00

Respiration of Mediterranean cold-water corals is not affected by ocean acidification as projected for the end of the century

Biogeosciences Discussions, 10, 7617-7640, 2013

Author(s): C. Maier, F. Bils, M. G. Weinbauer, P. Watremez, M. A. Peck, and J.-P. Gattuso

The rise of CO₂ has been identified as a major threat to life in the ocean. About one-third of the anthropogenic CO₂ produced in the last 200 yr has been taken up by the ocean, leading to ocean acidification. Surface seawater pH is projected to decrease by about 0.4 unit between the pre-industrial revolution and 2100. The branching cold-water corals Madrepora oculata and Lophelia pertusa are important, habitat-forming species in the deep Mediterranean Sea. Although previous research has investigated the abundance and distribution of these species, little is known regarding their ecophysiology and potential responses to global environmental change. A previous study indicated that the rate of calcification of these two species remained constant up to 1000 μatm CO₂ a value that is at the upper end of changes projected to occur by 2100. We examined whether the ability to maintain calcification rates in the face of rising pCO₂ affected the energetic requirements of these corals. Over the course of three months, rates of respiration were measured at a pCO₂ ranging between 350 and 1100 μatm to distinguish between short-term response and longer-term acclimation. Respiration rates ranged from 0.074 to 0.266 μmol O₂ (g skeletal dry weight)⁻¹ h⁻¹ and 0.095 to 0.725 μmol O₂ (g skeletal dry weight)⁻¹ h⁻¹ for L. and M. oculata, respectively, and were independent of pCO₂. Respiration increased with time likely due to regular feeding which may have provided an increased energy supply to sustain coral metabolism. Future studies are needed to confirm whether the insensitivity of respiration to increasing pCO₂ is a general feature of deep-sea corals in other regions.

Deformities in larvae and juvenile European lobster (Homarus gammarus) exposed to lower pH at two different temperatures

2013-05-02T00:00:00+02:00

Deformities in larvae and juvenile European lobster (Homarus gammarus) exposed to lower pH at two different temperatures

Biogeosciences Discussions, 10, 7579-7615, 2013

Author(s): A.-L. Agnalt, E. S. Grefsrud, E. Farestveit, M. Larsen, and F. Keulder

Trends of increasing temperatures and ocean acidification are expected to influence benthic marine resources, especially calcifying organisms. The European lobster (Homarus gammarus) is among those species at risk. A project was initiated in 2011 aiming to investigate long-term synergistic effects of temperature and projected increases in ocean acidification on the life cycle of lobster. Larvae were exposed to pCO₂ levels of ambient water (water intake at 90 m depth, tentatively of 380 μatm pCO₂), 727 and 1217 μatm pCO₂, at temperatures 10 and 18 °C. Long-term exposure lasted until 5 months of age. Thereafter the surviving juveniles were transferred to ambient water at 14 °C. At 18 °C the development from Stage 1 to 4 lasted from 14 to 16 days, as predicted under normal pH values. Growth was very slow at 10 °C and resulted in only two larvae reaching Stage 4 in the ambient treatment. There were no significant differences in carapace length at the various larval stages between the different treatments, but there were differences in total length and dry weight at Stage 1 at 10 °C, Stage 2 at both temperatures, producing larvae slightly larger in size and lighter by dry weight in the exposed treatments. Stage 3 larvae raised in 18 °C and 1217 μatm pCO₂ were also larger in size and heavier by dry weight compared with 727 μatm. Unfortunate circumstances precluded a full comparison across stages and treatment. Deformities were however observed in both larvae and juveniles. At 10 °C, about 20% of the larvae exposed to elevated pCO₂were deformed, compared with 0% in larvae raised in pH above 8.0. At 18 °C and in high pCO₂ treatment, 31.5% of the larvae were deformed. Occurrence of deformities after 5 months of exposure was 33 and 44% in juveniles raised in ambient and low pCO₂, respectively, and 20% in juveniles exposed to high pCO₂. Some of the deformities will possibly affect the ability to find food, sexual partner (walking legs, claw and antenna), respiration (carapace), and ability to swim (tail-fan damages).

On the consistency in variations of chlorophyll a concentration in the South China Sea as revealed by three remote sensing datasets

2013-05-02T00:00:00+02:00

On the consistency in variations of chlorophyll a concentration in the South China Sea as revealed by three remote sensing datasets

Biogeosciences Discussions, 10, 7549-7578, 2013

Author(s): S. L. Shang, Q. Dong, C. M. Hu, G. Lin, Y. H. Li, and S. P. Shang

Chlorophyll a (Chl) concentrations derived from satellite measurements have been used in oceanographic research, for example to interpret eco-responses to environmental changes on global and regional scales. However, it is unclear how existing Chl products compare with each other in terms of accuracy and consistency in revealing temporal and spatial patterns, especially in the optically complex marginal seas. In this study, we examined three MODIS Chl data products that have been made available to the community by the US NASA using community-accepted algorithms and default parameterization. These included the products derived from the OC3M, GSM and GIOP algorithms. We compared their temporal variations and spatial distributions in the South China Sea. We found that the three products appeared to capture general features such as unique winter peak at the Southeast Asian Time-series Study station (SEATS, 18° N, 116° E), strong upwelling induced bloom off the Vietnam and the Pearl River plume associated bloom in summer, their absolute magnitude, however, may be questionable. Further error statistics using field measured Chl as the truth demonstrated that the three MODIS Chl products may contain high degree of uncertainties in the study region. Root mean square error (RMSE) of the products from OC3M and GSM (on a log scale) was about 0.4 and average percentage error (ε) was ~150% (Chl between 0.03–7.67 mg m^–3, n = 63). In contrast, the GIOP with default parameterization led to higher errors (ϵ = 349%). This study thus advocates more careful interpretation of Chl spatio-temporal variations when using standard Chl products, and also points to the need of local tuning of algorithm parameterization for the study region.

Comparing soil biogeochemical processes in novel and natural boreal forest ecosystems

2013-04-30T00:00:00+02:00

Comparing soil biogeochemical processes in novel and natural boreal forest ecosystems

Biogeosciences Discussions, 10, 7521-7548, 2013

Author(s): S. A. Quideau, M. J. B. Swallow, C. E. Prescott, S. J. Grayston, and S.-W. Oh

Emulating the variability that exists in the natural landscape prior to disturbance should be a goal of soil reconstruction and land reclamation efforts following resource extraction. Long-term ecosystem sustainability within reclaimed landscapes can only be achieved with the re-establishment of biogeochemical processes between reconstructed soils and plants. In this study, we assessed key soil biogeochemical attributes (nutrient availability, organic matter composition, and microbial communities) in reconstructed, novel, anthropogenic ecosystems covering different reclamation treatments following open-cast mining for oil extraction. We compared the attributes to those present in a range of natural soils representative of mature boreal forest ecosystems in the same area of northern Alberta. Soil nutrient availability was determined in situ with resin probes, organic matter composition was described with ¹³C nuclear magnetic resonance spectroscopy and soil microbial community structure was characterized using phospholipid fatty acid analysis. Significant differences among natural ecosystems were apparent in nutrient availability and seemed more related to the dominant tree cover than to soil type. When analyzed together, all natural forests differed significantly from the novel ecosystems, in particular with respect to soil organic matter composition. However, there was some overlap between the reconstructed soils and some of the natural ecosystems in nutrient availability and microbial communities, but not in organic matter characteristics. Hence, our results illustrate the importance of considering the range of natural landscape variability, and including several soil biogeochemical attributes when comparing novel, anthropogenic ecosystems to the mature ecosystems that constitute ecological targets.

Understanding soil erosion impacts in temperate agroecosystems: bridging the gap between geomorphology and soil ecology

2013-04-30T00:00:00+02:00

Understanding soil erosion impacts in temperate agroecosystems: bridging the gap between geomorphology and soil ecology

Biogeosciences Discussions, 10, 7491-7520, 2013

Author(s): C. Baxter, J. S. Rowan, B. M. McKenzie, and R. Neilson

Soil is a key asset of natural capital, providing a myriad of goods and ecosystem services that sustain life through regulating, supporting and provisioning roles, delivered by chemical, physical and biological processes. One of the greatest threats to soil is accelerated erosion, which raises a natural process to unsustainable levels, and has downstream consequences (e.g. economic, environmental and social). Global intensification of agroecosystems is a major cause of soil erosion which, in light of predicted population growth and increased demand for food security, will continue or increase. Elevated erosion and transport is common in agroecosystems and presents a multi-disciplinary problem with direct physical impacts (e.g. soil loss), other less tangible impacts (e.g. loss of ecosystem productivity), and indirect downstream effects that necessitate an integrated approach to effectively address the problem. Climate is also likely to increase susceptibility of soil to erosion. Beyond physical response, the consequences of erosion on soil biota have hitherto been ignored, yet biota play a fundamental role in ecosystem service provision. To our knowledge few studies have addressed the gap between erosion and consequent impacts on soil biota. Transport and redistribution of soil biota by erosion is poorly understood, as is the concomitant impact on biodiversity and ability of soil to deliver the necessary range of ecosystem services to maintain function. To investigate impacts of erosion on soil biota a two-fold research approach is suggested. Physical processes involved in redistribution should be characterised and rates of transport and redistribution quantified. Similarly, cumulative and long-term impacts of biota erosion should be considered. Understanding these fundamental aspects will provide a basis upon which mitigation strategies can be considered.

Dissolved Fe across the Weddell Sea and Drake Passage: impact of DFe on nutrients uptake in the Weddell Sea

2013-04-29T00:00:00+02:00

Dissolved Fe across the Weddell Sea and Drake Passage: impact of DFe on nutrients uptake in the Weddell Sea

Biogeosciences Discussions, 10, 7433-7489, 2013

Author(s): M. B. Klunder, P. Laan, H. J. W. De Baar, I. Neven, R. Middag, and J. Van Ooijen

This manuscript reports the first full depth distributions of dissolved iron (DFe) over a high resolution Weddell Sea and Drake Passage transect. Very low dissolved DFe concentrations (0.01–0.1 nM range) were observed in the surface waters in the Weddell Sea, and within the Polar regime in the Drake Passage. Locally, enrichment in surface DFe was observed, likely due to recent ice melt (Weddell Sea) or dust deposition (Drake Passage). In the Weddell Sea, the low DFe concentrations can be partly explained by high POC export and/or primary production (indicated by chlorophyll fluorescence). As expected, in high DFe regions a strong silicate drawdown compared to nitrate drawdown was observed. However, this difference in drawdown between these nutrients appears not related to biological activity on the Peninsula shelf. In the Western Weddell Sea transect, with relatively small diatoms, no relationship between N:P and N:Si removal ratios and DFe was observed. For comparison, nutrient depletion is also presented for a transect along the Greenwich Meridian (Klunder et al., 2011), where diatoms are significantly larger, the N:P and N:Si removal ratio increased with increasing DFe. These findings confirm the important role of DFe in Southern Ocean (biologically mediated) nutrient cycles.

Over the shelf around the Antarctic Peninsula, higher DFe concentrations (> 1.5 nM) were observed. These elevated concentrations of Fe were transported into Drake Passage along isopycnal surfaces. At the South American continent, high (> 2 nM) DFe concentrations were caused by fluvial/glacial input of DFe.

On the Weddell Sea side of the Peninsula region, formation of deep water (by downslope convection) caused relatively high Fe (0.6–0.8 nM) concentrations in the bottom waters relative to the water masses at mid depth (0.2–0.4 nM). During transit of Weddell Sea Bottom Water to Drake Passage, through the Scotia Sea, extra DFe is taken up from seafloor sources, resulting in highest bottom water concentrations in the southernmost part of the Drake Passage of > 1 nM. The Weddell Sea Deep Water concentrations (~ 0.32 nM) were consistent with the lowest DFe concentrations observed in Atlantic AABW.

Effects of ocean acidification on the larval growth of olive flounder (Paralichthys olivaceus)

2013-04-29T00:00:00+02:00

Effects of ocean acidification on the larval growth of olive flounder (Paralichthys olivaceus)

Biogeosciences Discussions, 10, 7413-7431, 2013

Author(s): K.-S. Kim, J. H. Shim, and S. Kim

Little is known about how marine fishes respond to the reduced pH condition caused by the increased CO₂ in the atmosphere. We investigated the effects of CO₂ concentration on the growth of olive flounder (Paralichthys olivaceus) larvae. Newly hatched larvae were reared in three different concentrations of CO₂ (574, 988 and 1297 μatm CO₂) in temperature-controlled water tanks until metamorphosis (4 weeks). Body lengths, weights, and the concentration of some chemical elements in larval tissue were measured at the completion of each experiment, and experiment was repeated three times in May, June, and July 2011. Results indicated that body length and weight of flounder larvae were significantly increased with increasing CO₂ concentration (P < 0.05). Daily growth rates of flounder larvae were higher (0.391 mm) from the high CO₂ concentration (1297 μatm) than those (0.361 mm and 0.360 mm) from the lower ones (988 and 574 μatm).The measurement on some chemical elements (Ca, Fe, Cu, Zn and Sr) in fish tissue also revealed the increasing tendency of element concentration with increasing CO₂ in seawater, although statistical significance cannot be tested due to the single measurement. It suggests that there are enrichment processes of these cations in larval tissue in the low pH condition.

Data-based assessment of environmental controls on global marine nitrogen fixation

2013-04-29T00:00:00+02:00

Data-based assessment of environmental controls on global marine nitrogen fixation

Biogeosciences Discussions, 10, 7367-7412, 2013

Author(s): Y.-W. Luo, I. D. Lima, D. M. Karl, and S. C. Doney

There are a number of hypotheses for the environmental controls on marine nitrogen fixation (NF). Most of these hypotheses have not been assessed against direct measurements on the global scale. In this study, we use ~ 500 depth-integrated field measurements of NF covering the Pacific and Atlantic Oceans to test whether the spatial variance of these measurements can be explained by the commonly hypothesized environmental controls, including measurement-based surface solar radiation, mixed layer depth, sea surface temperature, surface nitrate and phosphate concentrations, surface excess phosphate (P^*), atmospheric dust deposition and surface wind speed, as well as minimum dissolved oxygen in upper 500 m to identify possible subsurface denitrification zones. By conducting simple linear regression and stepwise multiple linear regression (MLR) analyses, solar radiation and/or sea surface temperature as well as subsurface dissolved oxygen are identified as the predictors explaining the most spatial variance in the observed NF data, while dust deposition and wind speed do not appear to influence the spatial patterns of NF on global scale. Our study suggests that marine NF is coupled to regional loss of fixed nitrogen induced by subsurface low oxygen concentration, with its magnitude constrained by solar radiation or temperature. By applying the MLR-derived equation, we estimate the global-integrated NF at 71 (error range 49–104) Tg N yr⁻¹ in the open ocean, acknowledging that it could be substantially higher as the ¹⁵N₂-assimilation method used by most of the field samples underestimates NF. Our conclusion suggests that marine NF will increase in the future if subsurface nitrogen-losses increase as a consequence of developing deoxygenation with the global warming, a projection that will be modulated by other factors such as warming, elevated carbon dioxide, and changes in macro- and micro-nutrient distributions. More field NF samples in the Pacific and Indian Oceans, particularly in the oxygen minimum zones, are needed to reduce uncertainties in our conclusion.

Simulating boreal forest carbon dynamics after stand-replacing fire disturbance: insights from a global process-based vegetation model

2013-04-24T00:00:00+02:00

Simulating boreal forest carbon dynamics after stand-replacing fire disturbance: insights from a global process-based vegetation model

Biogeosciences Discussions, 10, 7299-7366, 2013

Author(s): C. Yue, P. Ciais, S. Luyssaert, P. Cadule, J. Harden, J. Randerson, V. Bellassen, T. Wang, S. L. Piao, B. Poulter, and N. Viovy

Stand-replacing fires are the dominant fire type in North American boreal forest and leave a historical legacy of a mosaic landscape of different aged forest cohorts. To accurately quantify the role of fire in historical and current regional forest carbon balance using models, one needs to explicitly simulate the new forest cohort that is established after fire. The present study adapted the global process-based vegetation model ORCHIDEE to simulate boreal forest fire CO₂ emissions and follow-up recovery after a stand-replacing fire, with representation of postfire new cohort establishment, forest stand structure and the following self-thinning process. Simulation results are evaluated against three clusters of postfire forest chronosequence observations in Canada and Alaska. Evaluation variables for simulated postfire carbon dynamics include: fire carbon emissions, CO₂ fluxes (gross primary production, total ecosystem respiration and net ecosystem exchange), leaf area index (LAI), and biometric measurements (aboveground biomass carbon, forest floor carbon, woody debris carbon, stand individual density, stand basal area, and mean diameter at breast height). The model simulation results, when forced by local climate and the atmospheric CO₂ history on each chronosequence site, generally match the observed CO₂ fluxes and carbon stock data well, with model-measurement mean square root of deviation comparable with measurement accuracy (for CO₂ flux ~100 g C m⁻² yr⁻¹, for biomass carbon ~1000 g C m⁻² and for soil carbon ~2000 g C m⁻²). We find that current postfire forest carbon sink on evaluation sites observed by chronosequence methods is mainly driven by historical atmospheric CO₂ increase when forests recover from fire disturbance. Historical climate generally exerts a negative effect, probably due to increasing water stress caused by significant temperature increase without sufficient increase in precipitation. Our simulation results demonstrate that a global vegetation model such as ORCHIDEE is able to capture the essential ecosystem processes in fire-disturbed boreal forests and produces satisfactory results in terms of both carbon fluxes and carbon stocks evolution after fire, making it suitable for regional simulations in boreal regions where fire regimes play a key role on ecosystem carbon balance.

Effects of anomalous high temperatures on carbon dioxide, methane, dissolved organic carbon and trace element concentrations in thaw lakes in Western Siberia in 2012

2013-04-24T00:00:00+02:00

Effects of anomalous high temperatures on carbon dioxide, methane, dissolved organic carbon and trace element concentrations in thaw lakes in Western Siberia in 2012

Biogeosciences Discussions, 10, 7257-7297, 2013

Author(s): O. S. Pokrovsky, L. S. Shirokova, S. N. Kirpotin, S. P. Kulizhsky, and S. N. Vorobiev

During the anomalous hot summer in 2012, surface air temperatures in Western Siberia were 5 to 10 °C higher than those observed during the previous period of > 30 yr. This unusual climate phenomenon provided an opportunity to examine the effects of short-term natural heating of water in thermokarst ponds and lakes in discontinuous permafrost zones and compare these observations to previous field results obtained when the temperature was normal during the summer of 2010 in the same region. Thermokarst bodies of water shrank significantly, water levels dropped approximately 50 cm in large lakes and small (< 10–100 m²) ponds, and shallow soil depressions disappeared. Based on samples from ~ 40 bodies of water collected previously and in 2012, first-order features of changes in chemical composition in response to increased water temperatures (from 14.1 ± 2.2 to 23.8 ± 2.3 °C in 2010 and 2012, respectively) were established. In these thermokarst bodies of water that covered a full range of surface areas, the average conductivity and pH were almost unchanged, whereas dissolved organic carbon (DOC), Cl⁻ and SO₄²⁻ concentrations were higher by a factor of ~ 2 during summer 2012 compared to periods with normal temperatures. Similarly, most divalent metals and insoluble trivalent and tetravalent elements were more concentrated by a factor of 1.7–2.4 in the summer of 2012 than normal periods. The average concentrations of dissolved CO₂ and CH₄ during the hot summer of 2012 increased by factors of 1.4 and 4.9, respectively. For most of the trace elements bound to colloids, the degree of colloidal binding decreased by a factor of 1.44 ± 0.33 (for an average of 40 elements) during the hot summer of 2012 compared to normal periods. Increases in CO₂ and CH₄ concentrations with the decreasing size of the body of water were well-pronounced during the hot summer of 2012. The concentrations of CO₂ and CH₄ significantly increased by factors of 5 and 150, respectively, in small (≤ 10² m²) compared to large (≥ 10⁴ m²) thermokarst (thaw) lakes. Taken together, these trends suggest that, for a conservative scenario of lake size distribution, lake water warming at high latitudes will produce (1) a significant increase in methane emission capacity from thaw lake surfaces; (2) decrease of molecular sizes of TE complexes and increase of potential bioavailability of metal micronutrients in water columns; and (3) relatively conservative responses by CO₂, DOC and trace element concentrations.

The effect of vertically-resolved soil biogeochemistry and alternate soil C and N models on C dynamics of CLM4

2013-04-23T00:00:00+02:00

The effect of vertically-resolved soil biogeochemistry and alternate soil C and N models on C dynamics of CLM4

Biogeosciences Discussions, 10, 7201-7256, 2013

Author(s): C. D. Koven, W. J. Riley, Z. M. Subin, J. Y. Tang, M. S. Torn, W. D. Collins, G. B. Bonan, D. M. Lawrence, and S. C. Swenson

Soils are a crucial component of the Earth System; they comprise a large portion of terrestrial carbon stocks, mediate the supply and demand of nutrients, and influence the overall response of terrestrial ecosystems to perturbations. In this paper, we develop a new soil biogeochemistry model for the Community Land Model, version 4 (CLM4). The new model includes a vertical dimension to carbon (C) and nitrogen (N) pools and transformations, a more realistic treatment of mineral N pools, flexible treatment of the dynamics of decomposing carbon, and a radiocarbon (¹⁴C) tracer. We describe the model structure, comparison against site-level and global observations, and overall effect of the revised soil model on CLM carbon dynamics. Site-level comparisons to radiocarbon and bulk soil C observations support the idea that soil C turnover is reduced at depth beyond what is expected from environmental controls by temperature, moisture, and oxygen that are considered in the model. The revised soil model predicts substantially more and older soil C, particularly at high latitudes, where it resolves a permafrost soil C pool, in better agreement with observations. In addition the 20th century C dynamics of the model are more realistic than the baseline model, with more terrestrial C uptake over the 20th century due to reduced N downregulation and longer turnover times of decomposing C.

Multiresolution quantification of deciduousness in West Central African forests

2013-04-23T00:00:00+02:00

Multiresolution quantification of deciduousness in West Central African forests

Biogeosciences Discussions, 10, 7171-7200, 2013

Author(s): G. Viennois, N. Barbier, I. Fabre, and P. Couteron

The characterization of leaf phenology in tropical forests is of major importance and improves our understanding of earth-atmosphere-climate interactions. The availability of satellite optical data with a high temporal resolution has permitted the identification of unexpected phenological cycles, particularly over the Amazon region. A primary issue in these studies is the relationship between the optical reflectance of pixels of 1 km or more in size and ground information of limited spatial extent. In this paper, we demonstrate that optical data with high to very-high spatial resolution can help bridge this scale gap by providing snapshots of the canopy that allow discernment of the leaf-phenological stage of trees and the proportions of leaved crowns within the canopy. We also propose applications for broad-scale forest characterization and mapping in West Central Africa over an area of 141 000 km².

Eleven years of the Moderate Resolution Imaging Spectroradiometer (MODIS) Enhanced Vegetation Index (EVI) data were averaged over the wet and dry seasons to provide a dataset of optimal radiometric quality at a spatial resolution of 250 m. Sample areas covered at a very-high (GeoEye) and high (SPOT-5) spatial resolution were used to identify forest types and to quantify the proportion of leaved trees in the canopy. The dry season EVI was positively correlated with the proportion of leaved trees in the canopy. This relationship allowed the conversion of EVI into canopy deciduousness at the regional level. On this basis, ecologically important forest types could be mapped, including young secondary, open Marantaceae, Gilbertiodendron dewevrei and swamp forests. We show that in west central African forests, a large share of the variability in canopy reflectance, as captured by the EVI, is due to variation in the proportion of leaved trees in the upper canopy, thereby opening new perspectives for biodiversity and carbon-cycle applications.

A~nitrogen budget for the Strait of Georgia, British Columbia

2013-04-23T00:00:00+02:00

A~nitrogen budget for the Strait of Georgia, British Columbia

Biogeosciences Discussions, 10, 7135-7169, 2013

Author(s): J. N. Sutton, S. C. Johannessen, and R. W. Macdonald

Balanced budgets for dissolved inorganic N (DIN) and particulate N (PN) were constructed for the Strait of Georgia (SoG), a semi-enclosed coastal sea off the west coast of British Columbia, Canada. The dominant control on the N budget is the advection of DIN into and out of the SoG via Haro Strait. The annual influx of DIN by advection from the Pacific Ocean is 29 990 (±19 500)Mmol yr⁻¹. The DIN flux advected out of the SoG is 24 300 (±15 500)Mmol yr⁻¹. Most of the DIN that enters the SoG (∼23 400 Mmol yr⁻¹) is converted to particulate N (PN) in situ by primary production. However, most of the PN produced by primary production is remineralized (∼22 000 Mmol yr⁻¹) back into DIN within the top 50 m. The PN budget for the SoG was further constrained by nitrogen isotope composition (δ¹⁵N) that indicated regional differences in the source of PN. The southern Strait receives a much higher proportion of terrigenous PN, relative to marine PN, than does the northern Strait. The difference is due to the influence of the Fraser River, which discharges 1950 Mmol yr⁻¹ of PN and 1660 Mmol yr⁻¹ of DIN into the southern Strait. The overall anthropogenic contribution of PN and DIN to the SoG is minimal relative to natural sources (>30 000 Mmol yr⁻¹). It is unlikely that the Strait will be affected by eutrophication in the near future, although anthropogenic N sources, such as wastewater outfalls, may have significant local effects.

Effects of topography, soil type and forest age on the frequency and size distribution of canopy gap disturbances in a tropical forest

2013-04-23T00:00:00+02:00

Effects of topography, soil type and forest age on the frequency and size distribution of canopy gap disturbances in a tropical forest

Biogeosciences Discussions, 10, 7103-7133, 2013

Author(s): E. Lobo and J. W. Dalling

Treefall gaps are the major source of disturbance in most tropical forests. The frequency and size of these gaps have important implications for forest ecosystem processes as they can influence the functional trait distribution of tree communities, stand-level above-ground biomass and productivity. However, we still know little about the relative importance of environmental drivers of gap disturbance regimes because existing studies vary greatly in criteria used for defining gaps, in the spatial extent of the study area, and the spatial resolution of canopy height measurements. Here we use LiDAR (light detecting and ranging) to explore how forest age, topography and soil type affect canopy disturbance patterns across a 1500 ha tropical forest landscape in central Panama. We characterize disturbance based on the frequency distribution of gap sizes (the "gap size distribution"), and the area of the forest affected by gaps (the "gap area fraction"). We found that slope and forest age had significant effects on the gap size distribution, with a higher frequency of large gaps associated with old-growth forests and more gentle slopes. Slope and forest age had similar effects on the gap area fraction, however gap area fraction was also affected by soil type and by aspect. We conclude that variation in disturbance patterns across the landscape can be linked to factors that act at the fine scale (such as aspect or slope), and factors that show heterogeneity at coarser scales (such as forest age or soil type). Awareness of the role of different environmental factors influencing gap formation can help scale-up the impacts of canopy disturbance on forest communities measured at the plot scale to landscape and regional scales.

Distributions and assemblages of larval fish in the East China Sea in the northeasterly and southwesterly monsoon seasons 2008

2013-04-22T00:00:00+02:00

Distributions and assemblages of larval fish in the East China Sea in the northeasterly and southwesterly monsoon seasons 2008

Biogeosciences Discussions, 10, 7075-7102, 2013

Author(s): W. Y. Chen, M. A. Lee, K. W. Lan, and G. C. Gong

A total of 8459 larval fish were collected in the southern East China Sea during the winter northeasterly monsoon season and the summer southwesterly monsoon seasons in 2008. They were composed of 184 species belonging to 105 families and 162 genera. The abundance in terms of CPUE (number of individuals/1000 m³) of the larvae was about six times higher in the southwesterly monsoon season than that in the northeasterly monsoon season. The primary environmental factors affecting the larval abundance were found to be water temperature in the northeasterly monsoon season but food availability in the southwesterly monsoon season. Three larval fish assemblages were recognized; the inshore assemblage, the offshore assemblage, and the summer coastal assemblage. The distribution and species composition of the larvae in the assemblages reflected the hydrographic conditions and water currents resulted from the seasonal monsoons.

Seasonal variations of air-sea CO₂ fluxes in the largest tropical marginal sea (South China Sea) based on multiple-year underway measurements

2013-04-19T00:00:00+02:00

Seasonal variations of air-sea CO₂ fluxes in the largest tropical marginal sea (South China Sea) based on multiple-year underway measurements

Biogeosciences Discussions, 10, 7031-7074, 2013

Author(s): W.-D. Zhai, M.-H. Dai, B.-S. Chen, X.-H. Guo, Q. Li, S.-L. Shang, C.-Y. Zhang, W.-J. Cai, and D.-X. Wang

Based upon fourteen field surveys conducted between 2003 and 2008, we showed that the seasonal pattern of sea surface partial pressure of CO₂ (pCO₂) and air–sea CO₂ fluxes differed among four different physical-biogeochemical domains in the South China Sea (SCS) proper. The four domains were located between 4 and 23° N and 109 and 121° E, covering ~ 38% of the surface area of the entire SCS. In the area off the Pearl River Estuary, relatively low pCO₂ values of 320 to 390 μatm were observed in all four seasons and both the biological productivity and CO₂ uptake were enhanced in summer in the Pearl River plume waters. In the northern SCS slope/basin area, a typical seasonal cycle of relatively high pCO₂ in the warmer seasons and relatively low pCO₂ in the cold seasons was revealed. In the central/southern SCS area, moderately high sea surface pCO₂ values of 360 to 425 μatm were observed throughout the year. In the area west of the Luzon Strait, a major exchange pathway between the SCS and the Pacific Ocean, pCO₂ was particularly dynamic in winter, when northeast monsoon induced upwelling events and strong outgassing of CO₂. These episodic events might have dominated the annual air–sea CO₂ flux in this particular area. The estimate of annual sea–air CO₂ fluxes showed that, most areas of the SCS proper served as weak sources to the atmospheric CO₂, with sea–air CO₂ flux values of 0.46 ± 0.43 mol m⁻² yr⁻¹ in the northern SCS slope/basin, 1.37 ± 0.55 mol m⁻² yr⁻¹ in the central/southern SCS, and 1.21 ± 1.47 mol m⁻² yr⁻¹ in the area west of the Luzon Strait. However, the annual sea–air CO₂ exchange was nearly in equilibrium (−0.44 ± 0.65 mol m⁻² yr⁻¹) in the area off the Pearl River Estuary. Overall the four domains released (18 ± 10) × 10¹² g C yr⁻¹ into the atmosphere. The CO₂ release rate of the South China Sea essentially exceeded the average CO₂ emission level of most tropical oceans.

Symbiosis increases coral tolerance to ocean acidification

2013-04-19T00:00:00+02:00

Symbiosis increases coral tolerance to ocean acidification

Biogeosciences Discussions, 10, 7013-7030, 2013

Author(s): S. Ohki, T. Irie, M. Inoue, K. Shinmen, H. Kawahata, T. Nakamura, A. Kato, Y. Nojiri, A. Suzuki, K. Sakai, and R. van Woesik

Increasing the acidity of ocean waters will directly threaten calcifying marine organisms such as reef-building scleractinian corals, and the myriad of species that rely on corals for protection and sustenance. Ocean pH has already decreased by around 0.1 pH units since the beginning of the industrial revolution, and is expected to decrease by another 0.2–0.4 pH units by 2100. This study mimicked the pre-industrial, present, and near-future levels of pCO² using a precise control system (±5% pCO²), to assess the impact of ocean acidification on the calcification of recently-settled primary polyps of Acropora digitifera, both with and without symbionts, and adult fragments with symbionts. The increase in pCO² of 100 μatm between the pre-industrial period and the present had more effect on the calcification rate of adult A. digitifera than the anticipated future increases of several hundreds of micro-atmospheres of pCO². The primary polyps with symbionts showed higher calcification rates than primary polyps without symbionts, suggesting that (i) primary polyps housing symbionts are more tolerant to near-future ocean acidification than organisms without symbionts, and (ii) corals acquiring symbionts from the environment (i.e. broadcasting species) will be more vulnerable to ocean acidification than corals that maternally acquire symbionts.

Integrating O₃ influences on terrestrial processes: photosynthetic and stomatal response data available for regional and global modeling

2013-04-18T00:00:00+02:00

Integrating O₃ influences on terrestrial processes: photosynthetic and stomatal response data available for regional and global modeling

Biogeosciences Discussions, 10, 6973-7012, 2013

Author(s): D. Lombardozzi, J. P. Sparks, and G. Bonan

Plants have a strong influence on climate by controlling the transfer of carbon dioxide and water between the biosphere and atmosphere during the processes of photosynthesis and transpiration. Chronic exposure to surface ozone (O₃) differentially affects photosynthesis and transpiration because it damages stomatal conductance, the common link that controls both processes, in addition to the leaf biochemistry that only affects photosynthesis. Because of the integral role of O₃ in altering plant interactions with the atmosphere, there is a strong motivation to incorporate the influence of O₃ into regional and global models. However, there are currently no analyses documenting both photosynthesis and stomatal conductance responses to O₃ exposure through time using a standardized O₃ parameter that can be easily incorporated into models. Therefore, models often rely on photosynthesis data derived from the responses of one or a few plant species that exhibit strong negative correlations with O₃ exposure to drive both rates of photosynthesis and transpiration, neglecting potential divergence between the two fluxes. Using data from the peer-reviewed literature, we have compiled photosynthetic and stomatal responses to chronic O₃ exposure for all plant types with data available in the peer-reviewed literature as a standardized function of cumulative uptake of O₃ (CUO), which integrates O₃ flux into leaves through time. These data suggest that stomatal conductance decreases ~ 11% after chronic O₃ exposure, while photosynthesis independently decreases ~ 21%. Despite the overall decrease in both variables, high variance masked any correlations between the decline in photosynthesis or stomatal conductance with increases in CUO. Though correlations with CUO are not easily generalized, existing correlations demonstrate that photosynthesis tends to be weakly but negatively correlated with CUO while stomatal conductance is more often positively correlated with CUO. Results suggest that large-scale models using data with strong negative correlations that only affect photosynthesis need to reconsider the generality of their response. Data from this analysis are now available to the scientific community and can be incorporated into global models to improve estimates of photosynthesis, global land carbon sinks, hydrology, and indirect radiative forcing that are influenced by chronic O₃ exposure.

Impact of the Kuroshio intrusion on the nutrient inventory in the upper northern South China Sea: insights from an isopycnal mixing model

2013-04-18T00:00:00+02:00

Impact of the Kuroshio intrusion on the nutrient inventory in the upper northern South China Sea: insights from an isopycnal mixing model

Biogeosciences Discussions, 10, 6939-6972, 2013

Author(s): C. Du, Z. Liu, M. Dai, S.-J. Kao, Z. Cao, Y. Zhang, T. Huang, L. Wang, and Y. Li

Based on four cruises covering a seasonal cycle in 2009–2011, we examined the impact of the Kuroshio intrusion, featured by extremely oligotrophic waters, on the nutrient inventory in the central northern South China Sea (NSCS). The nutrient inventory in the upper 100 m of the water column in the study area ranged from ∼200 to ∼290 mmol m⁻² for N + N (nitrate plus nitrite), from ~ 13 to ∼24 mmol m⁻² for soluble reactive phosphate and from ∼210 to ∼430 mmol m⁻² for silicic acid. The nutrient inventory showed a clear seasonal pattern with the highest value appearing in summer, while the N + N inventory in spring and winter had a reduction of ∼13% and ∼30%, respectively, relative to that in summer. To quantify the extent of the Kuroshio intrusion, an isopycnal mixing model was adopted to derive the proportional contribution of water masses from the SCS proper and the Kuroshio along individual isopycnal surfaces. The derived mixing ratio along the isopycnal plane was then employed to predict the genuine gradients of nutrients under the assumption of no biogeochemical alteration. These predicted nutrient concentrations, denoted as N_m, are solely determined by water mass mixing. Results showed that the nutrient inventory in the upper 100 m of the NSCS was overall negatively correlated to the Kuroshio water fraction, suggesting that the Kuroshio intrusion significantly influenced the nutrient distribution in the SCS and its seasonal variation. The difference between the observed nutrient concentrations and their corresponding N_m allowed us to further quantify the nutrient removal/addition associated with the biogeochemical processes on top of the water mass mixing. We revealed that the nutrients in the upper 100 m of the water column had a net consumption in both winter and spring but a net addition in fall.

Inter-annual variation of chlorophyll in the northern South China Sea observed at the SEATS Station and its asymmetric responses to climate oscillation

2013-04-18T00:00:00+02:00

Inter-annual variation of chlorophyll in the northern South China Sea observed at the SEATS Station and its asymmetric responses to climate oscillation

Biogeosciences Discussions, 10, 6899-6938, 2013

Author(s): K.-K. Liu, L.-W. Wang, M. Dai, C.-M. Tseng, Y. Yang, C.-H. Sui, L. Oey, K.-Y. Tseng, and S.-M. Huang

It is widely recognized that the variation of average surface chlorophyll a concentration (Chl) in the South China Sea (SCS) is closely related to wind forcing, especially during the intense winter monsoon. In this study we demonstrate that, after removal of the seasonal cycles, the variation of Chl showed strong asymmetric responses to wind speed under El Niño or La Niña conditions. The analysis was based on a time-series of Chl in the study area (115–117° E, 17–19° N) around the SEATS (South-East Asian Time-series Study) station located in the central northern SCS from September 1997 to the end of 2011, which was constructed by merging the SeaWiFS data (1997–2006) and MODIS data (2003–2011). The merged daily data were validated by shipboard observations at the SEATS station. The non-seasonal variations of monthly mean Chl, wind speed, sea surface height (SSH) and sea surface temperature (SST) were examined against the multivariate ENSO index (MEI). The analysis reveals strongly asymmetric correlations of Chl and SST with positive MEI (El Niño) or negative MEI (La Niña). Under El Niño conditions, both showed significant correlations with MEI or wind speed; under La Niña conditions, both showed weak or insignificant correlations. The contrast was more pronounced for Chl than for SST. The subdued responses of Chl to wind forcing under La Niña conditions were probably attributed to a deepened thermocline, for which wind driven nutrient pumping is less efficient. A deeper thermocline, which was observed during the 1999–2000 La Niña event and inferred by positive SSH anomalies during other La Niña events, was probably caused by reduced SCS throughflow under La Niña conditions. Intrusion of the nutrient-depleted Kuroshio water in the surface layer as observed during the 1999–2000 La Niña could be partially responsible for the suppressed Chl response.

Technical Note: Simultaneous measurement of sedimentary N₂ and N₂O production and new ¹⁵N isotope pairing technique

2013-04-17T00:00:00+02:00

Technical Note: Simultaneous measurement of sedimentary N₂ and N₂O production and new ¹⁵N isotope pairing technique

Biogeosciences Discussions, 10, 6861-6898, 2013

Author(s): T.-C. Hsu and S.-J. Kao

Dinitrogen (N₂) and/or nitrous oxide (N₂O) are produced through denitrification, anaerobic ammonium oxidation (anammox) or nitrification in sediments, of which entangled processes obfuscate the absolute rate estimation of gaseous nitrogen production from individual pathway. Recently, the classical isotope pairing technique (IPT), the most common ¹⁵N-nitrate enrichment method to quantify denitrification, has been modified by different researchers to (1) discriminate relative contribution of N₂ production by denitrification from anammox or to (2) provide more accurate denitrification rate by considering both N₂O and N₂ productions. Both modified methods, however, have deficiencies such as overlooking N₂O production in case 1 and neglecting anammox in case 2. In this paper, a new method was developed to refine previous methods. We installed cryogenic traps to pre-concentrate N₂ and N₂O separately, thus, allowing simultaneous measurement for two gases generated by one sample. The precision is better than 2% for N₂ (m/z 28, m/z 29 and m/z 30), and 1.5% for N₂O (m/z 44, m/z 45 and m/z 46). Based on the six m/z peaks of the two gases, we further revised IPT formulae to truthfully resolve the production rates of N₂ and N₂O contributed from 3 specific nitrogen removal processes, i.e. N₂ and N₂O from denitrification, N₂ from anammox and N₂O from nitrification. To validate the applicability of our new method, incubation experiments were conducted using sediment cores taken from the Danshuei estuary in Taiwan. We successfully determined the rates of aforementioned nitrogen removal processes. Moreover, N₂O yield was as high as 66%, which no doubt would significantly bias previous IPT approaches when N₂O was not considered. Our new method not only complements the previous IPT but also provides more comprehensive information to advance our understanding of nitrogen dynamics through the water-sediment interface.

Calcification response to climate change in the Pliocene?

2013-04-17T00:00:00+02:00

Calcification response to climate change in the Pliocene?

Biogeosciences Discussions, 10, 6839-6860, 2013

Author(s): C. V. Davis, M. P. S. Badger, P. R. Bown, and D. N. Schmidt

As a result of anthropogenic pCO₂ increases future oceans are growing warmer and lower in pH and oxygen, conditions that are likely to impact planktic communities. Past intervals of elevated and changing pCO₂ and temperatures can offer a glimpse into the response of marine calcifying plankton to changes in surface oceans under conditions similar to those projected for the future. Here we present new records of planktic foraminiferal and coccolith calcification from Deep Sea Drilling Project Site 607 (mid North Atlantic) and Ocean Drilling Program Site 999 (Caribbean Sea) from the Pliocene, the last time that pCO₂ was similar to today, and extending through a global cooling event into the Intensification of Northern Hemisphere Glaciation (3.3 to 2.6 million years ago). Test weights of both surface-dwelling foraminifera Globigerina bulloides and thermocline-dwelling foraminifera Globorotalia puncticulata vary, with a potential link to regional temperature variation in the North Atlantic, whereas in the tropics Globigerinoides ruber test weight remains stable. In contrast, reticulofenestrid coccoliths show a narrowing size range and a decline in the largest lith diameters over this interval. Our results suggest no major changes in plankton calcification during the high pCO₂ Pliocene or during the transition into an icehouse world.

Modelling changes in nitrogen cycling to sustain increases in forest productivity under elevated atmospheric CO₂ and contrasting site conditions

2013-04-17T00:00:00+02:00

Modelling changes in nitrogen cycling to sustain increases in forest productivity under elevated atmospheric CO₂ and contrasting site conditions

Biogeosciences Discussions, 10, 6783-6837, 2013

Author(s): R. F. Grant

If increases in net primary productivity (NPP) caused by rising concentrations of atmospheric CO₂ (C_a) are to be sustained, key N processes such as soil mineralization, biological fixation, root uptake and plant translocation must be hastened. Simulating the response of these processes to elevated C_a is therefore vital for models used to project the effects of rising C_a on NPP. In this modelling study, hypotheses are proposed for changes in soil mineralization, biological fixation, root uptake and plant translocation with changes in C_a. Algorithms developed from these hypotheses were tested in the ecosystem model ecosys against changes in N and C cycling measured over several years under ambient vs. elevatedC_a in Free Air CO₂ Enrichment (FACE) experiments at the Duke Forest in North Carolina, the Oak Ridge National Laboratory forest in Tennessee, and the USDA research forest in Wisconsin, USA. Simulating more rapid soil N mineralization was found to be vital for modelling sustained increases in NPP measured under elevated vs. ambient C_a at all three FACE sites. This simulation was accomplished by priming decomposition of N-rich humus from increases in microbial biomass generated by increased litterfall modelled under elevated C_a. Simulating more rapid nonsymbiotic N₂ fixation, root N uptake and plant N translocation under elevated C_a was found to make much smaller contributions to modelled increases in NPP, although such contributions might be greater over longer periods and under more N-limited conditions than those simulated here. Greater increases in NPP with C_a were also modelled with increased temperature and water stress, and with coniferous vs. deciduous plant functional types. These increases were also associated with changes in N cycling.

Repercussions of differential settling on sediment assemblages and multi-proxy palaeo-reconstructions

2013-04-16T00:00:00+02:00

Repercussions of differential settling on sediment assemblages and multi-proxy palaeo-reconstructions

Biogeosciences Discussions, 10, 6763-6781, 2013

Author(s): A. G. M. Caromel, D. N. Schmidt, and J. C. Phillips

Microfossils preserved in marine sediments are at the centre of numerous proxies for palaeoenvironmental reconstructions. Their precision is based on the assumption that they accurately represent the overlying watercolumn properties and faunas. In this paper, we assess the possibility of a pre-depositional bias in sediment assemblages caused by horizontal drift, due to differential settling velocities of sedimenting particles based on their shape, size and density. We calculate the lateral transport undergone by planktic foraminifera and a range of other proxy carriers in several regions with high current velocities. Lateral transport of different planktic foraminiferal species is minimal due to high settling velocities; no significant shape- or size-dependent sorting occurs before reaching the sediment, making planktic foraminiferal ideal proxy carriers. Diatoms, radiolaria and faecal pellets can be transported up to 500 km in some areas. This transport bias suggests that sediment assemblages could contain different proportions of local and imported particles, decreasing the precision of proxies based on these groups and the accuracy of the temperature reconstruction. For example in the Agulhas current, transport can lead to differences of up to 2 °C in temperature reconstructions between different proxies. For future palaeoenvironmental reconstructions, further sediment-trapping studies and multi-proxy analyses should attempt to quantify the margin of error associated with particle transport.

Spatial variations in the Kuroshio nutrient transport from the East China Sea to south of Japan

2013-04-16T00:00:00+02:00

Spatial variations in the Kuroshio nutrient transport from the East China Sea to south of Japan

Biogeosciences Discussions, 10, 6737-6762, 2013

Author(s): X. Y. Guo, X.-H. Zhu, Y. Long, and D. J. Huang

Based on absolute geostrophic velocity calculated from repeated hydrographic data of 39 cruises from 2000 to 2009 and nitrate concentrations measured at the same sections from 1964 to 2011, we obtained temporally averaged nitrate flux (the product of velocity and nitrate concentration) and nitrate transport (integration of flux over a section) through 4 sections along the Kuroshio path from the East China Sea (sections PN and TK) to south of Japan (sections ASUKA and 137E). In addition, we examined section OK east of the Ryukyu Islands in order to understand the contribution of Ryukyu Current to the Kuroshio nutrient transport south of Japan. The mean nitrate flux shows a subsurface maximum core with a value of 10, 10, 11, 11, and 6 mol m^–2 s^–1 at sections PN, TK, ASUKA, 137E, and OK, respectively. The depth of subsurface maximum core changes among five sections and is approximately 400, 500, 500, 400, and 800 m at sections PN, TK, ASUKA, 137E, and OK respectively. The mean downstream nitrate transport is 199.3, 176.3, 909.2, 1385.5, and 341.2 kmol m^–1 at sections PN, TK, ASUKA, 137E, and OK respectively. The nutrient transports at these sections suggest the presence of Kuroshio nutrient stream from its upstream region to downstream. The deep current structure of Ryukyu Current (section OK) makes it contribute more nitrate transport than the Kuroshio in the East China Sea (section TK) to the Kuroshio south of Japan. In addition, the positive difference between the downstream nitrate transport through section ASUKA and the sum of nitrate transports through sections TK and OK, as well as the positive difference of downstream nitrate transport between sections 137E and ASUKA, suggest that the Kuroshio recirculation significantly intensifies the downstream (eastward) nitrate transport by the Kuroshio.

Prominent bacterial heterotrophy and sources of ¹³C-depleted fatty acids to the interior Canada Basin

2013-04-11T00:00:00+02:00

Prominent bacterial heterotrophy and sources of ¹³C-depleted fatty acids to the interior Canada Basin

Biogeosciences Discussions, 10, 6695-6736, 2013

Author(s): S. R. Shah, D. R. Griffith, V. Galy, A. P. McNichol, and T. I. Eglinton

In recent decades, the Canada Basin of the Arctic Ocean has experienced rapidly decreasing summer sea ice coverage and freshening of surface waters. It is unclear how these changes translate to depth, particularly as our baseline understanding of organic carbon cycling in the deep basin is limited. In this study, we describe full-depth profiles of the abundance, distribution and carbon isotopic composition of fatty acids from suspended particulate matter at a seasonally ice-free station and a semi-permanently ice-covered station. Fatty acids, along with suspended particulate organic carbon (POC), are more concentrated under ice cover than in ice-free waters. But this influence, apparent at 50 m depth, does not propagate downward below 150 m depth, likely due to the weak biological pump in the central Canada Basin. Branched fatty acids have δ¹³C values that are similar to suspended POC at all depths and are ¹³C-enriched compared to even-numbered saturated fatty acids at depths above 3000 m. These are likely to be produced in situ by heterotrophic bacteria incorporating organic carbon that is isotopically similar to total suspended POC. A source of saturated even-numbered fatty acids is also suggested below surface waters which could represent contributions from laterally advected organic carbon or from chemoautotrophic bacteria. At 3000 m depth and below, a greater relative abundance of long-chain (C_20–24), branched and unsaturated fatty acids is consistent with a stronger influence of re-suspended sedimentary organic carbon on benthic particulate matter. At these deep depths, two individual fatty acids (C₁₂ and iso-C₁₇) are significantly depleted in ¹³C, allowing for the possibility that methane oxidizing bacteria contribute fatty acids, either directly to suspended particulate matter or to shallow sediments that are subsequently mobilized and incorporated into suspended particulate matter within the deep basin.

Solute specific scaling of inorganic nitrogen and phosphorus uptake in streams

2013-04-10T00:00:00+02:00

Solute specific scaling of inorganic nitrogen and phosphorus uptake in streams

Biogeosciences Discussions, 10, 6671-6693, 2013

Author(s): R. O. Hall Jr., M. A. Baker, E. J. Rosi-Marshall, and J. L. Tank

Stream ecosystem processes such as nutrient cycling may vary with stream position in the watershed. Using a scaling approach, we examined the relationship between stream size and nutrient uptake length, which represents the mean distance that a dissolved solute travels prior to removal from the water column. Ammonium uptake length increased proportionally with stream size measured as specific discharge (discharge/stream width) with a scaling exponent = 1.01. In contrast, the scaling exponent for nitrate (NO₃⁻) was 1.19 and for soluble reactive phosphorus (SRP) was 1.35, suggesting that uptake lengths for these nutrients increased more rapidly than increases in specific discharge. Additionally, the ratio of nitrogen (N) uptake length to SRP uptake length declined with stream size; there was lower demand for SRP relative to N as stream size increased. Ammonium and NO₃⁻ uptake velocity positively related with stream metabolism, while SRP did not. Finally, we related the scaling of uptake length and specific discharge to that of stream length using Hack's law and downstream hydraulic geometry. Ammonium uptake length increased less than proportionally with distance from the headwaters, suggesting a strong role for larger streams and rivers in regulating nutrient transport.

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