The emergence of top-down controls on marine biomass in the 21st Century

Uncertainty in the fate of marine phytoplankton biomass in a warming ocean has typically been attributed to uncertainty in how more light but less nutrients will impact growth conditions. However, across 11 CMIP6 models forced with high emissions, 50% of ensemble biomass moves in the opposite direction of cell-division rates. This indicates that regardless of the balance of bottom-up controls, biomass change is often determined by top-down driven changes to loss processes. Importantly, when biomass is modified from the top-down, a large fraction becomes decoupled from changes in primary (62%), secondary (65%) and export (46%) production, carrying critical implications for how to interpret satellite-based NPP trends, force fisheries models and predict ocean-climate feedbacks. We diagnostically decompose the drivers of all loss terms and find that the vertical redistribution of biomass is the largest driver of top-down control, but that at least 4 distinct pathways can decouple biomass from growth conditions.