A predictive model for satellite‐derived phytoplankton absorption over the Louisiana shelf hypoxic zone: Effects of nutrients and physical forcing

We investigated environmental forcing mechanisms of phytoplankton absorption near the Mississippi River delta using multiyear satellite data. An algorithm for the phytoplankton absorption coefficient (aph) was developed from in situ measurements and applied to ocean color imagery. We employed a suite of chemical and physical forcing variables, including surface currents. For satellite‐derived aph time series (2002–2004), correlation and stepwise regression analyses revealed the most important forcing variables of aph on the Louisiana shelf. Areally, Mississippi River discharge and nitrate concentration ([NO3]) were the two most important predictors of aph over the hypoxic zone (defined by its maximum extent). River discharge was important in a band stretching from the Mississippi River delta to the Louisiana‐Texas border. Riverine [NO3] and wind magnitude best predicted aph in nearshore waters, and solar radiation and SST were most important farther offshore over the hypoxic zone, indicating upwelled nutrient sources to phytoplankton. A multiple linear regression model performed well in resolving seasonal and interannual aph variability in model development years (2002–2004) (mean error of 18%, over all pixels and months) and in predicting shelf‐wide aph patterns in 2005 (mean error of 32%). Our results strongly suggest that in recent years, stratification and vertical mixing, in addition to riverine [NO3], play a primary role in regulating phytoplankton biomass over the hypoxic zone. As well, a springtime model experiment showed that aph over the hypoxic zone can differ by an average absolute 37% from its average scenario owing to changes solely in environmental variables other than NO3 flux.