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 fine-scale 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 underlying mechanisms and how to diagnose patchy distributions. 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 sub-euphotic zone nitrate concentrations, which also increase with salinity. Therefore, we can define maximum potential chlorophyll as a function of salinity and 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 using salinity observations.