A Threshold Sea‐Surface Temperature at 14°C for Phytoplankton Nonlinear Responses to Ocean Warming

Abstract Marine phytoplankton play a central role in supporting life in the oceans and profoundly affect global biogeochemical cycles. Previous studies have revealed positive effects of sea‐surface temperature (SST) on phytoplankton in terms of chlorophyll a concentrations (Chla) in high latitude oceans, while negative effects prevail in tropical and midlatitude oceans as well as under stratified summer conditions at higher latitudes. Based on a global analysis of 20 years of ocean Chla and SST data, we first investigated how interannual variability in SST is associated with Chla for each month of the season for every ocean province. We then quantified how the SST‐Chla relationships varied with the long‐term average (baseline) SST. We found significant season‐dependent SST effects on Chla in most ocean provinces. The signs and magnitudes of these effects were consistently associated with the baseline SST, with a shift from positive to negative effects of SST on Chla around 14°C. Based on field observations and literature data, we also estimated the interaction between nitrate limitation and temperature on the SST‐Chla relationship. Our findings suggest that the ocean warming effects on Chla depend consistently on the baseline temperature, both with regard to seasonal effects within regions and regional effects between high and low latitude provinces. Our analysis further suggests that the monthly 14°C isotherms can be used as a first approximation to separate areas and seasons where warming has opposite signed effects.