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Biogeosciences An interactive open-access journal of the European Geosciences Union
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the Creative Commons Attribution 4.0 License.
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 26 Nov 2018

Research article | 26 Nov 2018

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This discussion paper is a preprint. It is a manuscript under review for the journal Biogeosciences (BG).

Phytoplankton calcifiers control nitrate cycling and the pace of transition in warming icehouse and cooling greenhouse climates

Karin F. Kvale1, Katherine E. Turner1,2, Angela Landolfi1, and Katrin J. Meissner3,4 Karin F. Kvale et al.
  • 1GEOMAR Helmholtz Centre for Ocean Research, West shore campus, Duesternbrooker Way 20, 24105 Kiel, Germany
  • 2Dept. of Earth, Ocean, and Ecological Sciences, Univ. of Liverpool, Nicholson Building, 4 Brownlow St., Liverpool Merseyside, L69 3GP, UK
  • 3Climate Change Research Centre, Level 4 Mathews Building, UNSW, Sydney, NSW, Australia
  • 4ARC Centre of Excellence for Climate Extremes, Australia

Abstract. Phytoplankton calcifiers contribute to global carbon cycling through their dual formation of calcium carbonate and particulate organic carbon (POC). The carbonate might provide an efficient export pathway for the associated POC to the deep ocean, reducing the particles' exposure to biological degradation in the upper ocean and increasing the particle settling rate. Previous work has suggested ballasting of POC by carbonate might increase in a warming climate, in spite of increasing carbonate dissolution rates, because calcifiers benefit from the widespread nutrient limitation arising from stratification. We compare the biogeochemical responses of three models containing 1) a single mixed phytoplankton class, 2) additional explicit small phytoplankton and calcifiers, and 3) additional explicit small phytoplankton and calcifiers with a prognostic carbonate ballast model, to two rapid changes in atmospheric CO2. The first CO2 scenario represents a rapid (150 year) transition from a stable icehouse climate (285ppm) into a greenhouse climate (1257ppm); the second represents a symmetric rapid transition from a stable greenhouse climate into an icehouse climate. We identify a slope change in the global net primary production response with a transition point at about 3.5°C global mean sea surface temperature change in all models, driven by a combination of physics and biology. We also find that in both warming and cooling scenarios, the application of a prognostic carbonate ballast model moderates changes in carbon export production, suboxic volume, and nitrate sources and sinks, reducing the long-term model response to about one-third that of the calcifier model without ballast. Explicit small phytoplankton and calcifiers, and carbonate ballasting, increase the physical separation of nitrate sources and sinks through a combination of phytoplankton competition and lengthened remineralization profile, resulting in a significantly higher global nitrate inventory in this model compared to the single phytoplankton type model (15% and 32% higher, for icehouse and greenhouse climates). Higher nitrate inventory alleviates nitrate limitation, increasing phytoplankton sensitivity to changes in physical limitation factors (light and temperature). This larger sensitivity to physical forcing produces stronger shifts in ocean phosphate storage between icehouse and greenhouse climates. The greenhouse climate is found to hold phosphate and nitrate deeper in the ocean, despite a shorter remineralization length scale than the icehouse climate, because of the longer residence times of the deep water masses. We conclude the global biogeochemical impact of calcifiers extends beyond their role in global carbon cycling, and that the ecological composition of the global ocean can affect how ocean biogeochemistry responds to climate forcing.

Karin F. Kvale et al.
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Karin F. Kvale et al.
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Short summary
Drivers motivating the evolution of calcifying phytoplankton are poorly understood. We explore differences in global ocean chemistry with and without calcifiers during rapid climate changes. We find the presence of phytoplankton calcifiers stabilizes the volume of low oxygen regions and consequently stabilizes the concentration of nitrate, which is an important nutrient required for photosynthesis. By stabilizing nitrate concentrations, calcifiers improve their growth conditions.
Drivers motivating the evolution of calcifying phytoplankton are poorly understood. We explore ...