Detectability of Phytoplankton Biomass Extremes Using Simulated Satellite Chlorophyll Observations

Abstract Extreme open‐ocean phytoplankton events can influence marine ecosystems, yet their global occurrence, drivers, and consequences remain poorly understood. Most large‐scale studies rely on satellite chlorophyll, which provides only a surface view, is affected by physiological variability, and is often missing due to clouds and low sunlight. Here, we use an Earth system model with a satellite chlorophyll simulator to test when and where vertically integrated phytoplankton biomass extremes align with satellite‐detected chlorophyll extremes. Globally, about 10% of low and 19% of high phytoplankton biomass extremes are detected. The detection rate is the result of the combined impacts of missing data and extreme misalignment: only 34% of low and 56% of high detected chlorophyll extremes correspond with true biomass extremes, with the largest discrepancies occurring in the subtropical gyres. These findings highlight the need for caution when interpreting satellite chlorophyll as a proxy for phytoplankton biomass extremes.