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phytoplankton biomass, ocean color, Louisiana continental shelf, satellite imagery, algorithm, hypoxia

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A monthly time series of remotely sensed chlorophyll-a (Chlars) 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 Chlars, tuned separately for clear and turbid waters, was developed using field-observed chlorophyll-a (Chlaobs) collected during 12 cruises from 2002 to 2007. The new algorithm reproduced Chlaobs, 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 Chlars estimates at 1 km resolution across the LCS. Correlation and multiple stepwise regression analyses were used to relate the Chlars estimates to key environmental drivers expected to influence phytoplankton variability. The Chlars time series covaried with river discharge and nutrient concentration, PAR, and wind speed, and there were spatial differences in how these environmental drivers influenced Chlars. The main axis of spatial variability occurred in a cross-shelf direction with highest Chlars observed on the inner shelf. Both inner (<10 m depth) and middle-shelf (10–50 m depth) Chlars were observed to covary with interannual variations in the size of the hypoxic (O2 < 63 mmol m−3) area, and they explained ∼70 and ∼50% variability in interannual hypoxia size, respectively.

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Journal of Geophysical Research: Oceans, v. 119, issue 11, p. 7449-7462

©2014. American Geophysical Union. All Rights Reserved.

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