Predicting euphotic‐depth‐integrated chlorophyll‐afrom discrete‐depth and satellite‐observable chlorophyll‐a off central California

Abstract

Predicting water column integrated phytoplankton biomass from near‐surface measurements has been an important effort in marine ecological research, particularly since the advent of satellite remote sensing of ocean color. Quantitative relationships between chlorophyll‐aconcentrations (Chl‐a) at the surface and its depth‐integrated magnitude have thus far only been developed for open‐ocean waters. Here we develop and test for the first time an extension of open‐ocean relationships into ocean‐margin waters, specifically the highly productive and variable eastern boundary upwelling ecosystem off the central California coast. This region was chosen because of the unique availability of a 30‐year record of ship‐based Chl‐aprofiles measured using consistent methods. The extended relationship allows accurate prediction of integrated biomass from surface measurements. Further, we develop a new set of relationships for predicting the depth‐integrated Chl‐afrom Chl‐ameasured over a range of discrete depths (i.e., as measured by fluorometers on moorings). The newly developed relationships are tested against 15,000 fluorometric Chl‐aprofiles obtained from an autonomous underwater vehicle. Surprisingly, the relationship between surface Chl‐aand depth‐integrated Chl‐aholds for profiles with high concentrations of Chl‐a in persistent subsurface thin phytoplankton layers (layers <3 m thick and located below the first optical depth). The results have implications for monitoring of algal blooms and for quantifying ocean primary productivity from satellite observations of ocean color.