Ocean warming is associated with a decline in the global oxygen (O₂) inventory, but the contribution of biological activity to O₂ trends is poorly understood. We analyzed historical variability in temperature (T), surface nitrate (NO₃), and deep O₂ in hydrographic observations and model simulations of critical deep water formation regions in the polar North Atlantic and Southern Ocean. In both regions, interannual to decadal fluctuations of surface NO₃ are strongly correlated with temperature: warmer conditions are associated with reduced photic zone nutrients.  In a hindcast model simulation of the North Atlantic, biologically mediated drawdown of surface nutrients and associated respiration in deeper water account for a 35% enhancement of thermocline O₂ depletion during warm years. Across multiple Earth system models of the Southern Ocean, the magnitude of future NO₃ depletion and associated O₂ loss are strongly correlated to the historical relationship between surface nutrient and temperature anomalies, providing an emergent constraint on likely nutrient response to warming in the polar Southern Ocean. The increased nutrient consumption in warm years appears to be driven by an early start of the phytoplankton growing season and faster phytoplankton growth rates at higher temperatures. These results highlight an important role for phytoplankton T-dependent growth rates in amplifying ocean O₂ loss.

Climate change,hypoxia,nutrients,Earth System Models