Salinity is diagnostic of maximum potential chlorophyll and phytoplankton community structure in an Eastern Boundary Upwelling System

Coastal upwelling ecosystems associated with strong physical stirring exhibit significant mesoscale hydrographic and biological patchiness. Though many studies have found broad correlations between hydrographic properties (e.g., temperature and salinity) and phytoplankton biomass, we lack a detailed understanding of the mechanisms underlying these correlations. Here, using observational data from coastal waters in the California Current System, we demonstrate that the maximum observed chlorophyll in a water parcel increases with salinity—a conservative water‐mass tracer. This relationship arises from the correlations of vertical salinity and sub‐euphotic zone nitrate profiles. This allows us to define the maximum potential chlorophyll as a function of salinity, and thus nitrate. We show that variations in salinity explain patterns in phytoplankton community structure, and discuss how growth, grazing, and light and micronutrient limitation can generate chlorophyll values below the maximum potential. Our mechanistic explanation provides a novel framework for diagnosing biological patchiness solely through salinity observations.