Emergence of Top-down Controls on Biomass in the 21st Century
Tyler Rohr
1. Accurately predicting climate-driven changes to phytoplankton biomass is essential to protecting ecosystems, managing fisheries and understanding ocean-climate feedbacks on anthropogenic CO2 uptake/CDR. Past modelling work has almost exclusively approached this question from the bottom up: Will relieved light & temperature or exacerbated nutrient stress win out in driving changes to phytoplankton growth rates? However, changes in loss rates are rarely considered and almost never saved in CMIP projections
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Here I have diagnostically recomputed loss rates (to grazing and non-grazing mortality) across 11 CMIP6 simulations (SSP585) to determine how top-down changes in loss rates help control, and in many cases dominate, projected changes in marine biomass
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On average, roughly 50% of the ocean simulates decreasing phytoplankton biomass despite improving growth conditions. This paradox is only possible if increasing phytoplankton loss rates become decoupled from and outpace increasing division rates. Several distinct, model-specific, mechanisms are identified and quantified to explain this emergence of top-down controls, ranging from changes in community structure to bio-physical coupling. Importantly, regions of the ocean that see the same amount of decreasing biomass through top-down pathways have dramatically different implications for export production (carbon storage) and trophic transfer (to fisheries).
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BGC modellers, Ecosystem Modellers, Physical ocean models interested in bio-physical coupling
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Ocean-climate Feedbacks, biogeochemistry, carbon storages, ecosystem modelling
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