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Abstract Phytoplankton stoichiometry modulates the interaction between carbon, nitrogen and phosphorus cycles. Environmentally driven variations in phytoplankton C:N:P can alter biogeochemical cycling compared to expectations under fixed ratios. In fact, the assumption of fixed C:N:P has been linked to Earth System Model (ESM) biases and potential misrepresentation of responses to future change. Here we integrate key elements of the Adaptive Trait Optimization Model (ATOM) for phytoplankton stoichiometry with the Carbon, Ocean Biogeochemistry and Lower Trophics (COBALT) ocean biogeochemical model. Within a series of global ocean‐ice‐ecosystem retrospective simulations, ATOM‐COBALT reproduced observations of phytoplankton N:P, and compared to static ratios, exhibited reduced phytoplankton P‐limitation, enhanced N‐fixation, and increased low‐latitude export, improving consistency with observations and highlighting the biogeochemical implications of dynamic N:P. We applied ATOM‐COBALT to explore the impacts of different physiological mechanisms hypothesized to underlie N:P variation, finding that two mechanisms together drove the observed patterns: proportionality of P‐rich ribosomes in phytoplankton cells to growth rates and reductions in P‐storage during scarcity. A third mechanism which linked temperature with phytoplankton biomass allocations to non‐ribosomal proteins, led only to relatively modest impacts because this mechanism decreased the temperature dependence of phytoplankton growth rates, compensating for changes in N:P. We find that there are quantitative response differences that associate distinctive biogeochemical footprints with each mechanism, which are most apparent in highly productive low‐latitude regions. These results suggest that variable phytoplankton N:P makes phytoplankton productivity and export resilient to environmental changes, and support further research on the physiological and environmental drivers of phytoplankton stoichiometry and biogeochemical role.
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Shifts in regional production as a driver of future global ocean production stoichiometry
Abstract Using a global ocean biogeochemistry model, we examined three drivers of global ocean production C:N:P ratio: flexible phytoplankton stoichiometry, phytoplankton community composition, and regional production shifts. For a middle-of-the-road warming scenario (SSP2), the model predicts a substantial increase in the global export C:P ratio from 113:1 to 119:1 by the year 2100. The most important physiological driver of this stoichiometric change is the effect of the worldwide warming on cyanobacteria, followed by the effect of phosphate depletion on eukaryotes in the Southern Ocean. Also, there is a modest global shift in the phytoplankton community in favor of cyanobacteria at the expense of eukaryotes with a minimal effect on the global production stoichiometry. We find that shifts in the regional production, even in the absence of any change in phytoplankton stoichiometry or taxonomy, can change the global production C:N:P ratio. For example, enhancing the production in the polar waters, which typically have low C:N:P ratios, will have the effect of lowering the global ratio. In our model, the retreat of Antarctic sea ice has this very effect but is offset by production changes downstream and elsewhere. This study thus provides an understanding of how regional production changes can affect the global production C:N:P ratio. However, the current literature indicates substantial uncertainty in the future projections of regional production changes, so it is unclear at this time what their net effect is on the global production C:N:P ratio. Finally, our model predicts that the overall increase in the carbon content of organic matter due to flexible C:N:P ratio helps to stabilize carbon export in the face of reduced nutrient export (i.e. the decrease in C export is ~30% smaller than expected from the decrease in P export by 2100) but has a minimal effect on atmospheric CO2uptake (~1%).
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- Award ID(s):
- 1827948
- PAR ID:
- 10204454
- Publisher / Repository:
- IOP Publishing
- Date Published:
- Journal Name:
- Environmental Research Letters
- Volume:
- 15
- Issue:
- 12
- ISSN:
- 1748-9326
- Page Range / eLocation ID:
- Article No. 124027
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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