Spatiotemporal Chlorophyll-a Dynamics on the Louisiana Continental Shelf Derived from a Dual Satellite Imagery Algorithm

A monthly time series of remotely sensed chlorophyll- a (Chla _rs ) over the Louisiana continental shelf (LCS) was developed and examined for its relationship to river discharge, nitrate concentration, total phosphorus concentration, photosynthetically available radiation (PAR), wind speed, and interannual variation in hypoxic area size. A new algorithm for Chla _rs , tuned separately for clear and turbid waters, was developed using field-observed chlorophyll- a (Chla _obs ) collected during 12 cruises from 2002 to 2007. The new algorithm reproduced Chla _obs , with ∼40% and ∼60% uncertainties at satellite pixel level for clear offshore waters and turbid nearshore waters, respectively. The algorithm was then applied to SeaWiFS and MODIS images to calculate long-term (1998–2013) monthly mean Chla _rs estimates at 1 km resolution across the LCS. Correlation and multiple stepwise regression analyses were used to relate the Chla _rs estimates to key environmental drivers expected to influence phytoplankton variability. The Chla _rs time series covaried with river discharge and nutrient concentration, PAR, and wind speed, and there were spatial differences in how these environmental drivers influenced Chla _rs . The main axis of spatial variability occurred in a cross-shelf direction with highest Chla _rs observed on the inner shelf. Both inner (<10 m depth) and middle-shelf (10–50 m depth) Chla _rs were observed to covary with interannual variations in the size of the hypoxic (O ₂  < 63 mmol m ⁻³ ) area, and they explained ∼70 and ∼50% variability in interannual hypoxia size, respectively.