Predicting euphotic-depth-integrated chlorophyll-a from discrete-depth and satellite-observable chlorophyll-a off central California

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-a concentrations (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-a profiles 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-a from Chl-a measured 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-a profiles obtained from an autonomous underwater vehicle. Surprisingly, the relationship between surface Chl-a and depth-integrated Chl-a holds 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.