BGD - Latest Articles

High production of nitrous oxide (N₂O), methane (CH₄) and dimethylsulphoniopropionate (DMSP) in a massive marine phytoplankton culture

2010-09-02T00:00:00+02:00

High production of nitrous oxide (N₂O), methane (CH₄) and dimethylsulphoniopropionate (DMSP) in a massive marine phytoplankton culture

Biogeosciences Discussions, 7, 6705-6723, 2010

Author(s): L. Florez-Leiva, E. Tarifeño, M. Cornejo, R. Kiene, and L. Farías

The production of large amounts of algal biomass for different purposes such as aquaculture or biofuels, may cause impacts on the marine environment. One such impact is the production of radiatively active trace gases and aerosols with climate cooling (dimethyl sulfide DMS and its precursor DMSP) and warming (N₂O and CH₄) effects. Total and dissolved DMSP, N₂O and CH₄, together with other environmental variables were monitored daily for 46 days within a massive microalgae monoculture of Nannochloris (Chlorophyceae) in an open pond system. The growth of this green microalgae was stimulated by the addition of N- and P-rich salts, resulting in exponential growth (growth phase) during the first 17 days observed by cell abundance (1 × 10⁶ to 4.4 × 10⁶ cell mL⁻¹) and Chl-a levels (from 1.4 to 96 mg Chl-a m⁻³) followed by a decrease in both Chl-a and cell abundance (senescence phase). Total DMSP (from 6.3 to 142 μmol m⁻³), dissolved DMSP i.e. 5.8 to 137 μmol m⁻³ and N₂O (from 8 to 600 μmol m⁻³) abruptly peaked during the senescence phase, whereas CH₄ steadily increased between 2 and 10 μmol m⁻³ during the growth phase. Different ratios between tracers and Chl-a during both phases reveal different biochemical processes involved in the cycling of these gases and tracers. Our results show that despite the consumption of large quantities of CO₂ by the massive algal culture, a minor amount of DMS and huge amounts of greenhouse gases were produced, in particular N₂O, which has a greater radiative effect per molecule than CO₂. These findings have important implications for biogeochemical studies and for environmental management of aquaculture activities.

Synoptic relationships quantified between surface Chlorophyll-a and diagnostic pigments specific to phytoplankton functional types

2010-09-01T00:00:00+02:00

Synoptic relationships quantified between surface Chlorophyll-a and diagnostic pigments specific to phytoplankton functional types

Biogeosciences Discussions, 7, 6675-6704, 2010

Author(s): T. Hirata, N. J. Hardman-Mountford, R. J. W. Brewin, J. Aiken, R. Barlow, K. Suzuki, T. Isada, E. Howell, T. Hashioka, M. Noguchi-Aita, and Y. Yamanaka

Error-quantified, synoptic-scale relationships between chlorophyll-a (Chla) and phytoplankton pigment groups at the sea surface are presented. A total of nine pigment groups were considered to represent nine phytoplankton functional types (PFTs) including microplankton, nanoplankton, picoplankton, diatoms, dinoflagellates, green algae, picoeukaryotes, prokaryotes and Prochlorococcus sp. The observed relationships between Chla and pigment groups were well-defined at the global scale to show that Chla can be used as an index of not only phytoplankton abundance but also community structure; large (micro) phytoplankton monotonically increase as Chla increases, whereas the small (pico) phytoplankton community generally decreases. Within these relationships, we also found non-monotonic variations with Chla for certain pico-plankton (pico-eukaryotes, Prokaryotes and Prochlorococcus sp.) and for Green Algae and nano-sized phytoplankton. The relationships were quantified with a least-square fitting approach in order to estimate the PFTs from Chla alone. The estimated uncertainty of the relationships quantified depends on both phytoplankton types and Chla concentration. Maximum uncertainty over all groups (34.7% Chla) was found from diatom at approximately Chla = 1.07 mg m⁻³. However, the mean uncertainty of the relationships over all groups was 5.8 [% Chla] over the entire Chla range observed (0.02 < Chla < 6.84 mg m⁻³). The relationships were applied to SeaWiFS satellite Chla data from 1998 to 2009 to show the global climatological fields of the surface distribution of PFTs. Results show that microplankton are present in the mid and high latitudes, constituting ~9.0 [% Chla] of the phytoplankton community at the global surface, in which diatoms explain ~6.0 [% Chla]. Nanoplankton are ubiquious throught much of the global surface oceans except subtropical gyres, acting as a background population, constituting ~44.2 [% Chla]. Picoplankton are mostly limited in subtropical gyres, constituting ~46.8 [% Chla] globally, in which prokaryotes are the major species explaining 32.3 [% Chla] (prochlorococcus sp. explaining 21.5 [% Chla]), while pico-eukaryotes are notably abundant in the Southern Pacific explaining ~14.5 [% Chla]. These results may be used to constrain or validate global marine ecosystem models.

Desert dust and anthropogenic aerosol interactions in the Community Climate System Model coupled-carbon-climate model

2010-09-01T00:00:00+02:00

Desert dust and anthropogenic aerosol interactions in the Community Climate System Model coupled-carbon-climate model

Biogeosciences Discussions, 7, 6617-6673, 2010

Author(s): N. Mahowald, K. Lindsay, D. Rothenberg, S. C. Doney, J. K. Moore, P. Thornton, J. T. Randerson, and C. D. Jones

Coupled-carbon-climate simulations are an essential tool for predicting the impact of human activity onto the climate and biogeochemistry. Here we incorporate prognostic desert dust and anthropogenic aerosols into the CCSM3.1 coupled carbon-climate model and explore the resulting interactions with climate and biogeochemical dynamics through a series of transient anthropogenic simulations (20th and 21st centuries) and sensitivity studies. The inclusion of prognostic aerosols into this model has a small net global cooling effect on climate but does not significantly impact the globally averaged carbon cycle; we argue that this is likely to be because the CCSM3.1 model has a small climate feedback onto the carbon cycle. We propose a mechanism for including desert dust and anthropogenic aerosols into a simple carbon-climate feedback analysis to explain the results of our and previous studies. Inclusion of aerosols has statistically significant impacts on regional climate and biogeochemistry, in particular through the effects on the ocean nitrogen cycle and primary productivity of altered iron inputs from desert dust deposition.

Experimental nitrogen, phosphorus, and potassium deposition decreases summer soil temperatures, water contents, and soil CO₂ concentrations in a northern bog

2010-08-31T00:00:00+02:00

Experimental nitrogen, phosphorus, and potassium deposition decreases summer soil temperatures, water contents, and soil CO₂ concentrations in a northern bog

Biogeosciences Discussions, 7, 6589-6616, 2010

Author(s): S. Wendel, T. Moore, J. Bubier, and C. Blodau

Ombrotrophic peatlands depend on airborne nitrogen (N), whose deposition has increased in the past and lead to disappearance of mosses and increased shrub biomass in fertilization experiments. The response of soil water content, temperature, and carbon gas concentrations to increased nutrient loading is poorly known and we thus determined these data at the long-term N fertilization site Mer Bleue bog, Ontario, during a two month period in summer. Soil temperatures decreased with NPK addition in shallow peat soil primarily during the daytime (t-test, p<0.05) owing to increased shading, whereas they increased in deeper peat soil (t-test, p<0.05), probably by enhanced thermal conductivity. _RMANOVA suggested interactions between N and PK addition in particular soil layers and strong interactions between soil temperatures and volumetric water contents (p<0.05). Averaged over all fertilized treatments, the mean soil temperatures at 5 cm depth decreased by 1.3 °C and by 4.7 °C (standard deviation 0.9 °C) at noon. Water content was most strongly affected by within-plot spatial heterogeneity but also responded to both N and PK load according to _RMANOVA (p<0.05). Overall, water content and CO₂ concentrations in the near-surface peat (t-test, p<0.05) were lower with increasing N load, suggesting more rapid soil gas exchange. The results thus suggest that changes in bog ecosystem structure with N deposition have significant ramifications for physical parameters that in turn control biogeochemical processes.

Specific rates of leucine incorporation by marine bacterioplantkon in the open Mediterranean Sea in summer using cell sorting

2010-08-30T00:00:00+02:00

Specific rates of leucine incorporation by marine bacterioplantkon in the open Mediterranean Sea in summer using cell sorting

Biogeosciences Discussions, 7, 6545-6588, 2010

Author(s): A. Talarmin, F. Van Wambeke, P. Catala, C. Courties, and P. Lebaron

Cell-specific leucine incorporation rates were determined in early summer across the open stratified Mediterranean Sea along vertical profiles from 0 to 200 m. During the period of our study, the bulk leucine incorporation rate was on average 5.0 ± 4.0 (n=31) pmol leu l⁻¹ h⁻¹. After ³H-radiolabeled leucine incorporation and SyBR Green I staining, populations were sorted using flow cytometry. Heterotrophic prokaryotes (Hprok) were divided in several clusters according to the cytometric properties of side scatter and green fluorescence of the cells: the low nucleic acid content cells (LNA) and the high nucleic acid content cells (HNA), with high size and low size (HNA-hs and HNA-ls, respectively). LNA cells represented 45 to 63% of the Hprok abundance between surface and 200 m, and significantly contributed to the bulk activity, from 17 to 55% all along the transect. The HNA/LNA ratio of cell-specific activities was on average 2.1 ± 0.7 (n=31). Among Hprok populations from surface samples (0 down to the deep chlorophyll depth, DCM), HNA-hs was mostly responsible for the leucine incorporation activity. Its cell-specific activity was up to 13.3 and 6.9-fold higher than that of HNA-ls and LNA, respectively, and it varied within a wide range of values (0.9–54.3×10⁻²¹ mol leu cell⁻¹ h⁻¹). At the opposite, ratios between the specific activities of the 3 populations tended to get closer to each other, below the DCM, implying a potentially higher homogeneity in activity of Hprok in the vicinity of nutriclines. Prochlorococcus cells were easily sorted near the DCM and displayed cell-specific activities equally high, sometimes higher than the HNA-hs group (2.5–55×10⁻²¹ mol leu cell⁻¹ h⁻¹). We then showed that all the sorted populations were key-players in leucine incorporation into proteins. The mixotrophic feature of certain photosynthetic prokaryotes and the non-negligible activity of LNA cells all over Mediterranean were reinforced.

Influence of the Amazon River on dissolved and intra-cellular metal concentrations in Trichodesmium colonies along the western boundary of the sub-tropical North Atlantic Ocean

2010-08-27T00:00:00+02:00

Influence of the Amazon River on dissolved and intra-cellular metal concentrations in Trichodesmium colonies along the western boundary of the sub-tropical North Atlantic Ocean

Biogeosciences Discussions, 7, 6523-6543, 2010

Author(s): A. Tovar-Sanchez and S. A. Sañudo-Wilhelmy

Despite the ecological importance of Trichodesmium spp. for the global oceanic nitrogen budget, there is limited information on their trace metal composition in field samples. We report dissolved (<0.22 μm) metal concentrations measured in surface waters (Ag, Cd, Co, Cu, Fe, Mo, Ni, P, Pb and V) and in the total and the intracellular pool (Ag, Al, Cd, Co, Cu, Fe, Mn, Mo, Ni, P, Pb, V) of Trichodesmium populations collected in the western subtropical North Atlantic Ocean (April–May 2003) within the influence of the Amazon River plume. Dissolved element distributions were strongly influenced by the River discharge, with concentrations of some elements varying directly (i.e. Cd, Mo and V) or inversely (Ag, Co, Cu, Fe, Ni, P and Pb) with surface salinity. Intracellular metal values to phosphorous ratios (mol:mol) for Cd, Co, Cu, Fe, Mn, Mo, Ni and V ranged from 9.0×10⁻⁶ for Cd to 4.4×10⁻² for Fe. Although total metal composition was significantly correlated with the intracellular content in the Trichodesmium colonies for some elements (e.g., Co, Cu, V), metal pools in the phytoplankton did not co-vary with the dissolved metal concentrations, suggesting that water column measurements may not be good predictors of the intracellular metal concentrations. The impact of physical parameters and bioactive elements on biological processes in Trichodesmium such as nitrogen fixation, carbon drawdown and biomass production was explored by using a principal component analysis test (PCA). The analysis indicates that the biological drawdown of dissolved inorganic carbon (DIC) by Trichodesmium seems to be influenced by the internal content of Fe, Co, Cd, Cu and Mn, while nitrogen fixation seems more influenced by the internal concentration of Mo, Ni and V and by the dissolved phosphorous concentrations.

Ecosystem metabolism in a temporary Mediterranean marsh (Doñana National Park, SW Spain)

2010-08-26T00:00:00+02:00

Ecosystem metabolism in a temporary Mediterranean marsh (Doñana National Park, SW Spain)

Biogeosciences Discussions, 7, 6495-6521, 2010

Author(s): O. Geertz-Hansen, C. Montes, C. M. Duarte, K. Sand-Jensen, N. Marbá, and P. Grillas

The metabolic balance of the open waters supporting submerged macrophytes of the Doñana marsh (SW Spain) was investigated in spring, when community production is highest. The marsh community was net autotrophic with net community production rates averaging 0.61 g C m⁻² d⁻¹, and gross production rates exceeding community respiration rates by, on average, 43%. Net community production increased greatly with increasing irradiance, with the threshold irradiance for communities to become net autotrophic being 42 to 255 μE m⁻² s⁻¹, below which communities became net heterotrophic. Examination of the contributions of the benthic and the pelagic compartments showed the pelagic compartment to be strongly heterotrophic (average P/R ratio = 0.27), indicating that the metabolism of the pelagic compartment is strongly subsidised by excess organic carbon produced in the strongly autotrophic benthic compartment (average P/R = 1.58).

Seasonal and inter annual variability of energy exchange above a boreal Scots pine forest

2010-08-25T00:00:00+02:00

Seasonal and inter annual variability of energy exchange above a boreal Scots pine forest

Biogeosciences Discussions, 7, 6441-6494, 2010

Author(s): S. Launiainen

Twelve-years of eddy-covariance measurements conducted above a boreal Scots pine forest in Hyytiälä, Southern Finland, were analyzed to assess the seasonal and inter-annual variability of surface conductance (g_s) and energy partitioning. The g_s had distinct annual course, driven by the seasonal cycle of the Scots pine. Low g_s (2–3 mm s⁻¹ in April) restricted transpiration in springtime and caused the sensible heat flux to peak in May–June while evapotranspiration takes over later in July–August when g_s is typically 5–7 mm s⁻¹. Hence, during normal years Bowen ratio decreases from 4–6 in April to 0.7–0.9 in August. Sensitivity of g_s to ambient vapor pressure deficit (D) was relatively constant but the reference value at D=1 kPa varied seasonally and between years. Only two drought episodes when volumetric soil moisture content in upper mineral soil decreased below 0.15 m³ m⁻³ occurred during the period. Below this threshold value transpiration was strongly reduced, which promoted sensible heat exchange increasing Bowen ratio to 3–4. Annual evapotranspiration varied between 218 and 361 mm and accounted between 50% and 90% of equilibrium evaporation. The forest floor contributed between 16 and 25% of the total evapotranspiration on annual scale. The fraction stayed similar over the observed range of environmental conditions including drought. The inter-annual variability of evapotranspiration could not be linked to any mean climate parameter while the summertime sensible heat flux and net radiation were well explained by global radiation. The energy balance closure varied annually between 0.66 and 0.95 and had a distinct seasonal cycle with worse closure in spring when large proportion of available energy is partitioned into sensible heat.

A seasonal study of dissolved cobalt in the Ross Sea, Antarctica: micronutrient behavior, absence of scavenging, and relationships with Zn, Cd, and P

2010-08-25T00:00:00+02:00

A seasonal study of dissolved cobalt in the Ross Sea, Antarctica: micronutrient behavior, absence of scavenging, and relationships with Zn, Cd, and P

Biogeosciences Discussions, 7, 6387-6439, 2010

Author(s): M. A. Saito, T. J. Goepfert, A. E. Noble, E. M. Bertrand, P. N. Sedwick, and G. R. DiTullio

We report the distribution of cobalt (Co) in the Ross Sea polynya during austral summer 2005–2006 and the following austral spring 2006. The vertical distribution of total dissolved Co (dCo) was similar to soluble reactive phosphate (PO₄^3-), with dCo and PO₄^3- showing a significant correlation throughout the water column (r²=0.87). A strong seasonal signal for dCo was observed, with most spring samples having concentrations ranging from ~45–85 pM, whereas summer dCo values were depleted below these levels by biological activity. Surface transect data from the summer cruise revealed concentrations at the low range of this seasonal variability (~30 pM dCo), with concentrations as low as 20 pM observed in some regions where PO₄^3- was depleted to ~0.1 μM. Both complexed Co, defined as the fraction of dCo bound by strong organic ligands, and labile Co, defined as the fraction of dCo not bound by these ligands, were typically observed in significant concentrations throughout the water column. This contrasts the depletion of labile Co observed in the euphotic zone of other ocean regions, suggesting a much higher bioavailability for Co in the Ross Sea. An ecological stoichiometry of 37.6 μmol Co:mol⁻¹ PO₄^3- calculated from dissolved concentrations was similar to values observed in the subarctic Pacific, but approximately tenfold lower than values in the Eastern Tropical Pacific and Equatorial Atlantic. The ecological stoichiometries for dissolved Co and Zn suggest a greater overall use of Zn relative to Co in the shallow waters of the Ross Sea, with a Co:PO₄^3-/Zn:PO₄^3- ratio of 1:17. Comparison of these observed stoichiometries with values estimated in culture studies suggests that Zn is a key micronutrient that likely influences phytoplankton diversity in the Ross Sea. In contrast, the observed ecological stoichiometries for Co were below values necessary for the growth of eukaryotic phytoplankton in laboratory culture experiments conducted in the absence of added zinc, implying the need for significant Zn nutrition in the Zn-Co cambialistic enzymes. The lack of an obvious kink in the dissolved Co:PO₄^3- relationship was in contrast to Zn:PO₄^3- and Cd:PO₄^3- kinks previously observed in the Ross Sea. A mechanism for kink formation is proposed where Zn and Cd uptake in excess of that needed for optimal growth occurs at the base of the euphotic zone, and no clear Co kink occurs because its abundances are too low for excess uptake. An unusual characteristic of Co geochemistry in the Ross Sea is an apparent lack of Co scavenging processes, as inferred from the absence of dCo removal below the euphotic zone. We hypothesize that this vertical distribution reflects a low rate of Co scavenging by Mn oxidizing bacteria, perhaps due to Mn scarcity, relative to the timescale of the annual deep winter mixing in the Ross Sea. Thus Co exhibits nutrient-like behavior in the Ross Sea, in contrast to its hybrid-type behavior in other ocean regions, with implications for the possibility of increased marine Co inventories and utility as a paleooceanographic proxy.

CO₂ maximum in the oxygen minimum zone (OMZ)

2010-08-25T00:00:00+02:00

CO₂ maximum in the oxygen minimum zone (OMZ)

Biogeosciences Discussions, 7, 6353-6385, 2010

Author(s): A. Paulmier, D. Ruiz-Pino, and V. Garçon

Oxygen minimum zones (OMZs), known as suboxic layers mainly localized in the Eastern Boundary Upwelling Systems, are expanding since the 20th "high CO₂" century, probably due to the global warming. OMZs are also known to contribute significantly to the oceanic production of N₂O, a greenhouse gas (GHG) more efficient than CO₂. However, the contribution of the OMZs on the oceanic sources and sinks budget of CO₂, the main GHG, still remains to be established.

We present here the dissolved inorganic carbon (DIC) structure, associated locally with the Chilean OMZ and globally with the main most intense OMZs (O₂<20 μmol/kg) in the open ocean. To achieve this, we jointly examine simultaneous DIC and O₂ data collected off Chile during 4 cruises and a monthly monitoring (2000–2002) in one of the shallowest OMZ, along with international DIC and O₂ databases for other OMZs.

High DIC concentrations (>2225 μmol/kg, up to 2350 μmol/kg) have been reported over the whole OMZ thickness, allowing to define for all studied OMZs a Carbon Maximum Zone (CMZ). The CMZs-OMZs constitute the largest carbon reserves of the ocean in subsurface waters and could induce a positive feedback for the atmosphere during upwelling activity, as potential direct local sources of CO₂. The CMZ paradoxically presents a slight "carbon deficit" in its core, meaning a DIC increase from the oxygenated ocean to the OMZ lower than the corresponding O₂ decrease (assuming classical C/O molar ratios). This "carbon deficit" would be related to thermal mechanisms affecting faster O₂ than DIC (due to the carbonate buffer effect) and occurring upstream in warm waters (e.g., in the Equatorial Divergence), where the CMZ-OMZ core originates. The "carbon deficit" in the CMZ core would be mainly compensated locally at the oxycline, by a "carbon excess" induced by a specific remineralization. Indeed, a possible co-existence of bacterial heterotrophic and autotrophic processes usually occurring at different depths could stimulate an intense aerobic-anaerobic remineralization, inducing deviation of C/O molar ratio from the canonical Redfield ratios. Further studies to confirm these results for all OMZs are required to understand the OMZ effects on both climatic feedback mechanisms and marine ecosystem perturbations.

ENSO and IOD teleconnections for African ecosystems: evidence of destructive interference between climate oscillations

2010-08-25T00:00:00+02:00

ENSO and IOD teleconnections for African ecosystems: evidence of destructive interference between climate oscillations

Biogeosciences Discussions, 7, 6323-6352, 2010

Author(s): C. A. Williams and N. P. Hanan

Rainfall and vegetation across Africa are known to resonate with the coupled ocean-atmosphere phenomena of El Niño Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD). However, the regional-scale implications of sea surface temperature variability for Africa's carbon sources and sinks have received little focused attention, particularly in the case of IOD. Furthermore, studies exploring the interactive effects of ENSO and IOD when coincident are lacking. This analysis uses remotely sensed vegetation change plus a land surface model driven with observed meteorology to investigate how rainfall, vegetation, and photosynthesis across Africa respond to these climate oscillations. In addition to the relatively well-known ENSO forcing, the IOD induces large departures of photosynthesis across much of Africa associated with anomalies in rainfall and vegetation greenness. More importantly, sizeable independent effects can be suppressed or even reversed by destructive interferences during periods of simultaneous ENSO and IOD activity. For example, effects of positive IOD on southeastern Africa tended to dominate those of El Niño during their coincidence spanning 1997–1998, with sign reversal of El Niño's typically strong suppression of photosynthesis in this region. These findings call into question past analyses examining teleconnections to ENSO or IOD in isolation, and indicate the need to consider their simultaneous states when examining influences on hydroclimatic and ecological conditions across Africa.

Long-term CH₃Br and CH₃Cl flux measurements in temperate salt marshes

2010-08-23T00:00:00+02:00

Long-term CH₃Br and CH₃Cl flux measurements in temperate salt marshes

Biogeosciences Discussions, 7, 6295-6322, 2010

Author(s): E. Blei, M. R. Heal, and K. V. Heal

Fluxes of CH₃Br and CH₃Cl and their relationship with potential drivers such as sunlight, temperature and soil moisture, were monitored at fortnightly to monthly intervals for more than two years at two contrasting temperate salt marsh sites in Scotland. Manipulation experiments were conducted to further investigate possible links between drivers and fluxes. Mean (± 1 sd) annually and diurnally-weighted net emissions from the two sites were found to be 300 ± 44 ng m⁻² h⁻¹ for CH₃Br and 662 ± 266 ng m⁻² h⁻¹ for CH₃Cl. A tentative scale-up indicates that salt marshes account for 0.5–3.2% and 0.05–0.33%, respectively, of currently-estimated total global production of these two gases, in line with previous findings from this and other research groups, but consistently lower than past global scale-up estimates from Southern Californian salt marshes. Fluxes followed both seasonal and diurnal trends with highest fluxes during summer days and lowest (negative) fluxes during winter nights. Statistical analysis generally did not demonstrate a strong link between temperature or sunlight levels and methyl halide fluxes, although it is likely that temperatures have a weak direct influence on emissions, and both certainly have indirect influence via the annual and daily cycles of the vegetation. CH₃Cl flux magnitudes from different measurement locations depended on the plant species enclosed whereas such dependency was not discernible for CH₃Br fluxes. In 14 out of 19 collars CH₃Br and CH₃Cl net fluxes were significantly correlated. The CH₃Cl/CH₃Br net-emission mass ratio was 2.2, a magnitude lower than mass ratios of global methyl halide budgets (~22) or emissions from tropical rainforests (~60). This is likely due to preference for CH₃Br production by the relatively high bromine content in the salt marsh plant material.

Implications for chloro- and pheopigment synthesis and preservation from combined compound-specific δ¹³C, δ¹⁵N, and Δ¹⁴C analysis

2010-08-20T00:00:00+02:00

Implications for chloro- and pheopigment synthesis and preservation from combined compound-specific δ¹³C, δ¹⁵N, and Δ¹⁴C analysis

Biogeosciences Discussions, 7, 6265-6294, 2010

Author(s): S. Kusch, Y. Kashiyama, N. O. Ogawa, M. Altabet, M. Butzin, J. Friedrich, N. Ohkouchi, and G. Mollenhauer

Chloropigments and their derivative pheopigments preserved in sediments can directly be linked to photosynthesis. Their carbon and nitrogen stable isotopic compositions have been shown to be a good recorder of recent and past surface ocean environmental conditions tracing the carbon and nitrogen sources and dominant assimilation processes of the phytoplanktonic community. In this study we report results from combined compound-specific radiocarbon and stable carbon and nitrogen isotopic analysis to examine the timescales of synthesis and fate of chlorophyll-a and its degradation products pheophytin-a, pyropheophytin-a, and 13²,17³-cyclopheophorbide-a-enol until burial in Black Sea surface sediments. The pigments are mainly of marine phytoplanktonic origin as implied by their stable isotopic compositions. Pigment δ¹⁵N values indicate nitrate as the major uptake substrate but ¹⁵N-depletion towards the open marine setting indicates contribution from N₂-fixation. Radiocarbon concentrations translate into minimum and maximum pigment ages of approximately 40 to 1200 years. This implies that protective mechanisms against decomposition such as association with minerals or eutrophication-induced hypoxia and light limitation are much more efficient than previously thought. However, seasonal variations of nutrient source, growth period, and habitat and their associated isotopic variability are likely at least as strong as long-term trends. Combined triple isotopic analysis of sedimentary chlorophyll and its primary derivatives is a powerful tool to delineate biogeochemical and diagenetic processes in the surface water and assess their precise timescales.

Assessing the ecological status of plankton in Anjos Bay: a flow cytometry approach

2010-08-19T00:00:00+02:00

Assessing the ecological status of plankton in Anjos Bay: a flow cytometry approach

Biogeosciences Discussions, 7, 6243-6264, 2010

Author(s): G. C. Pereira, A. R. de Figuiredo, P. M. Jabor, and N. F. F. Ebecken

This aim of this paper is to assess the use of the heterotrophic/autotrophic ratio as an early indicator of trophic status as a part of development of a real time monitoring program at Anjos Bay, Rio de Janeiro, Brazil. An in-situ flow cytometer was used to quantify the abundances of phytoplankton and cyanobacteria, which were identified by chlorophyll and phycoerythrin autofluorescence, respectively. Heterotrophic prokaryotes and viruses were quantified by DNA-binding fluorochromes; merozooplankton larvae were collected by plankton net and quantified by stereomicroscopy. The temporal and spatial distributions of these variables were evaluated on the basis of weekly observations from August 2006 to September 2007. The heterotrophic/autotrophic ratio and the viral abundance were correlated with upwelling events and assume an apparently seasonal pattern. A possible control mechanism and influential factors are discussed, and it is concluded that this ecosystem is bottom-up controlled under eutrophic conditions and top-down controlled under oligotrophic conditions.

Litter quality and pH are strong drivers of carbon turnover and distribution in alpine grassland soils

2010-08-16T00:00:00+02:00

Litter quality and pH are strong drivers of carbon turnover and distribution in alpine grassland soils

Biogeosciences Discussions, 7, 6207-6242, 2010

Author(s): K. Budge, J. Leifeld, E. Hiltbrunner, and J. Fuhrer

Alpine soils are expected to contain large amounts of labile carbon (C) which may become a further source of atmospheric CO₂ as a of global warming. However, there is little data available on these soils, and understanding of the influence of environmental factors on soil organic matter (SOM) turnover is limited. We extracted 30 cm deep cores from five grassland sites along a small elevation gradient from 2285 to 2653 m above sea level (a.s.l.) in the central Swiss Alps. Our aim was to determine the quantity, degree of stabilization and mean residence time (MRT) of SOM in relation to site factors such as temperature, soil pH, vegetation, and organic matter (OM) structure. Soil fractions obtained by size and density fractionation revealed a high proportion of labile particulate organic matter C (POM C %) mostly in the uppermost soil layers. POM C in the top 20 cm across the gradient ranged from 39.6–57.6% in comparison to 7.2–29.6% reported in previous studies for lower elevation soils (810–1960 m a.s.l.). At the highest elevation, MRTs measured by means of radiocarbon dating and turnover modelling, increased between fractions of growing stability from 90 years in free POM (fPOM) to 534 years in the mineral-associated fraction (mOM). Depending on elevation and pH, plant community data indicated considerable variation in the quantity and quality of litter input, and these patterns could be reflected in the dynamics of soil C. ¹³C NMR data confirmed the direct relationship of OM composition to MRT. While temperature is likely to be a major cause for the slow turnover rate observed, other factors such as litter quality and soil pH, as well as the combination of all factors, play an important role in causing small-scale variability of SOM turnover. Ignoring this interplay of controlling factors may impair the performance of models to project SOM responses to environmental change.

Detection of pore space in CT soil images using artificial neural networks

2010-08-16T00:00:00+02:00

Detection of pore space in CT soil images using artificial neural networks

Biogeosciences Discussions, 7, 6173-6205, 2010

Author(s): M. G. Cortina-Januchs, J. Quintanilla-Dominguez, A. Vega-Corona, A. M. Tarquis, and D. Andina

Computed Tomography (CT) images provide a non-invasive alternative for observing soil structures, particularly pore space. Pore space in soil data indicates empty or free space in the sense that no material is present there except fluids such as air, water, and gas. Fluid transport depends on where pore spaces are located in the soil, and for this reason, it is important to identify pore zones. The low contrast between soil and pore space in CT images presents a problem with respect to pore quantification. In this paper, we present a methodology that integrates image processing, clustering techniques and artificial neural networks, in order to classify pore space in soil images. Image processing was used for the feature extraction of images. Three clustering algorithms were implemented (K-means, fuzzy C-means, and self organizing maps) to segment images. The objective of clustering process is to find pixel groups of a similar grey level intensity and to organise them into more or less homogeneous groups. The segmented images are used for test a classifier. An artificial neural network is characterised by a great degree of modularity and flexibility, and it is very efficient for large-scale and generic pattern recognition applications. For these reasons, an artificial neural network was used to classify soil images into two classes (pore space and solid soil). Our methodology shows an alternative way to detect solid soil and pore space in CT images. The percentages of correct classifications of pore space of the total number of classifications among the tested images were 97.01%, 96.47% and 96.12%.

Quantifying wetland methane emissions with process-based models of different complexities

2010-08-16T00:00:00+02:00

Quantifying wetland methane emissions with process-based models of different complexities

Biogeosciences Discussions, 7, 6121-6171, 2010

Author(s): J. Tang, Q. Zhuang, R. D. Shannon, and J. R. White

Bubbling is an important pathway of methane emissions from wetland ecosystems; however the concentration-based threshold function approach in current biogeochemistry models of methane is not sufficient to represent the complex ebullition process. Here we revise an extant process-based biogeochemistry model, the Terrestrial Ecosystem Model into a multi-substance model (CH₄, O₂, CO₂ and N₂) to simulate methane production, oxidation, and transport (particularly ebullition) with different model complexities. When ebullition is modeled with a concentration-based threshold function and if the inhibition effect of oxygen on methane production and the competition for oxygen between methanotrophy and heterotrophic respiration are retained, the model is a two-substance system. Ignoring the role of oxygen, while still modeling ebullition with a concentration-based threshold function, reduces the model to a one-substance system. These models were tested through a group of sensitivity analyses at two temperate peatland sites in Michigan. We demonstrate that only the four-substance model with a pressure-based ebullition algorithm is able to capture the episodic emissions induced by a sudden decrease in atmospheric pressure. All models captured the retardation effect on methane efflux from an increase in surface standing water which results from the inhibition of diffusion and the increase in rhizospheric oxidation. We conclude that to more accurately account for the effects of atmospheric pressure dynamics and standing water on methane effluxes, the multi-substance model with a pressure-based ebullition algorithm should be used in the future to quantify global wetland CH₄ emissions. Further, to more accurately simulate the pore water gas concentrations and different pathways of methane transport, an exponential root distribution function should be used and the phase-related parameters should be treated as temperature dependent.

Impact of seasonal oxygen deficiency on the phosphorous geochemistry of surface sediments along the Western Continental Shelf of India

2010-08-13T00:00:00+02:00

Impact of seasonal oxygen deficiency on the phosphorous geochemistry of surface sediments along the Western Continental Shelf of India

Biogeosciences Discussions, 7, 6089-6119, 2010

Author(s): Josia Jacob, Prosenjit Ghosh, K. K. Balchandran, and Rejomon George

The intensification of the natural coastal hypoxic zone over the western Indian shelf in the recent years and its impact on the biogeochemistry and marine life is a matter of concern. This study examines the influence of the seasonal oxygen deficiency on the phosphorus geochemistry of the surface sediments along the western continental shelf of India (WCSI). Speciation of phosphorus along with the geochemical characteristics (total organic carbon – TOC, total nitrogen – TN, and total phosphorus – TP) of the surface sediments and the hydrography of the western continental shelf of India (WCSI) were studied, during late summer monsoon (LSM) and spring intermonsoon (SIM). The hydrography of the WCSI revealed upwelling and associated seasonal oxygen deficiency with denitrifying suboxic conditions along the inner shelf and hypoxic conditions along the outer shelf. High concentrations of dissolved phosphate (PO₄) and dissolved Iron (Fe) were also observed in the subsurface water of the inner shelf during LSM. The shelf water of the WCSI was oligotrophic and oxygen rich during SIM. A latitudinal enrichment of TOC, TN and TP in the surface sediments was observed at 13–17° N, along the WCSI during LSM, where seasonal suboxia was intense. Authigenic apatite bound phosphorus (P_aut) was the major phosphorus species along the WCSI during LSM whereas detrital flourapatite bound phosphorus (P_det) was the major species during SIM. Substantial depletion of reactive iron(III)-bound phosphorus (ΔP_Fe) was observed in the surface sediments of the WCSI during LSM which showed significant correlation with the enrichment of PO₄ (ΔPO₄) in the overlying water during LSM compared to SIM. PO₄ diffusing into the water column from the sediments by reductive dissolution of PP_Fe probably leads to high dissolved PO₄ along the inner shelf water during LSM which agrees with the existing hypothesis. Hence, phosphorus geochemistry of the surface sediments plays a major role in the biogeochemical cycling of phosphorus during periods of seasonal oxygen deficiency along the WCSI. Similar studies carried out along the eastern continental shelf of India (ECSI), where any kind of seasonal oxygen deficiency has not been reported yet, showed an abundance of P_det (~50% of TP) and P_org (~32% of TP) in the surface sediments. The characteristic hydrographical features of the region such as high terrigeneous input, low production in the surface euphotic layers and greater preservation of labile organic matter in the sediments is also reflected in the phosphorus geochemistry of the surface sediments along ECSI.

Seasonal variations in nitrate isotope composition of three rivers draining into the North Sea

2010-08-13T00:00:00+02:00

Seasonal variations in nitrate isotope composition of three rivers draining into the North Sea

Biogeosciences Discussions, 7, 6051-6088, 2010

Author(s): A. Deek, K. Emeis, and U. Struck

Nitrate loading of coastal ecosystems by rivers that drain industrialised catchments continues to be a problem in the South Eastern North Sea, in spite of significant mitigation efforts over the last 2 decades. To identify nitrate sources, sinks, and turnover in three German rivers that discharge into the German Bight, we determined δ ¹⁵N-NO₃^- and δ¹⁸O- NO₃^- in nitrate and δ ¹⁵N of particulate nitrogen for the period 2006–2009 (biweekly samples). The nitrate loads of Rhine, Weser and Ems varied seasonally in magnitude and δ ¹⁵N-NO₃^- (6.5–21‰), whereas the δ ¹⁸O-NO₃^- (-0.3–5.9‰) and δ ¹⁵N-PN (4–14‰) were less variable. Overall temporal patterns in nitrate mass fluxes and isotopic composition suggest that a combination of nitrate delivery from nitrification of soil ammonia in the catchment and assimilation of nitrate in the rivers control the isotopic composition of nitrate. Nitrification in soils as a source is indicated by low δ ¹⁸O-NO₃^- in winter, which traces the δ ¹⁸O of river water. Mean values of δ ¹⁸O-H₂O were between –9.4‰ and –7.3‰; combined in a ratio of 2:1 with the atmospheric oxygen δ ¹⁸O of 23.5‰ agrees with the found δ ¹⁸O of nitrate in the rivers.

Parallel variations of δ ¹⁵N-NO₃^- and δ ¹⁸O-NO₃^- within each individual river are caused by isotope effects associated with nitrate assimilation in the water column, the extent of which is determined by residence time in the river. Assimilation is furthermore to some extent mirrored both by the δ ¹⁵N of nitrate and particulate N. Although δ ¹⁵-NO₃^- observed in Rhine, Weser and Ems are reflected in high average δ ¹⁵N-PN (between 6‰ and 9‰, both are uncorrelated in the time series due to lateral and temporal mixing of PN. That a larger enrichment was consistently seen in δ ¹⁵N-NO₃^- relative to δ ¹⁸O-NO₃^- is attributed to constant additional diffuse nitrate inputs deriving from soil nitrification in the catchment area. A statistically significant inverse correlation exists between increasing δ ¹⁵N-NO₃^- values and decreasing NO₃^- concentrations. This inverse relationship – observed in each seasonal cycle – together with a robust relationship between human dominated land use and δ ¹⁵N-NO₃^- values demonstrates a strong influence of human activities and riverine nitrate consumption efficiency on the isotopic composition of riverine nitrate.

The greenhouse gas balance of European grasslands

2010-08-13T00:00:00+02:00

The greenhouse gas balance of European grasslands

Biogeosciences Discussions, 7, 5997-6050, 2010

Author(s): P. Ciais, J. F. Soussana, N. Vuichard, S. Luyssaert, A. Don, I. A. Janssens, S. L. Piao, R. Dechow, J. Lathière, F. Maignan, M. Wattenbach, P. Smith, C. Ammann, A. Freibauer, E. D. Schulze, and the CARBOEUROPE Synthesis Team

The long-term carbon balance (NBP) of grasslands is estimated by combining scarce multi-year eddy-covariance observations at ecosystem observation sites where information on carbon inputs and harvesting removals is available. Following accounting for carbon leached to rivers, we estimated grasslands to be net carbon sinks of 74±10 g C m⁻² yr⁻¹. Uncertainties arise from the small number of sites and the short measurement period. Only 11 sites, out of a total of 20 grassland sites in Europe where eddy covariance systems are installed, were set-up for estimating NBP. These 11 selected sites are representative of intensive management practice and we lack information on disturbance history, such as plowing. This suggests that the grassland NBP estimate is likely biased towards overestimating the sink, compared to the European average. Direct measurements of Net Primary Productivity (NPP) are not possible in grasslands given permanent biomass removal by grazing and mowing, uncertainties in rhizodeposition and production of volatile organic carbon compounds lost to the atmosphere. Therefore, the grassland process-based ecosystem model PASIM was used to estimate the spatial-temporal distribution of NPP, providing a European average value of 750±150 g C across extensively grazed, intensively grazed pastures, and forage production systems. In Europe the NPP of grasslands seems higher than that of croplands and forests. The carbon sequestration efficiency of grasslands, defined as the ratio of NBP to NPP, amounts to 0.09±0.10. Therefore, per unit of carbon input, grasslands sequester 3–4 times more carbon in the soil than forests do, making them a good candidate for managing onsite carbon sinks. When using the 100 yr greenhouse warming potential for CH₄ and N₂O, their emissions due to management of grasslands together offset roughly 70–80% of the carbon sink. Uncertainties on the European grassland greenhouse gas balance, including CO₂, CH₄ and N₂O fluxes are likely to be reduced in the near future, with data being collected from more sites, and improved up-scaling methods.

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