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Spatiotemporal chlorophyll-a dynamics on the Louisiana continental shelf derived from a dual satellite imagery algorithm
Citation:
Le, C., J. Lehrter, C. Hu, M. Murrell, AND L. Qi. Spatiotemporal chlorophyll-a dynamics on the Louisiana continental shelf derived from a dual satellite imagery algorithm. JOURNAL OF GEOPHYSICAL RESEARCH: OCEANS. American Geophysical Union, Washington, DC, 119(11):7449-7462, (2014).
Impact/Purpose:
Our goals in this study were to: (1) develop a hybrid algorithm to derive Chla time series from satellites on theLCS; (2) investigate factors on the river-dominated LCS that regulate satellite-observed Chla dynamics; and (3) apply the satellite-observed Chla to develop a hypoxia size prediction model.
Description:
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.
