Relationships among Satellite Chlorophyll a, River Inputs, and Hypoxia on the Louisiana Continental Shelf, Gulf of Mexico.

Walker, Nan. D and Nancy N. Rabalais

SeaWiFS ocean color measurements were used to investigate interannual, monthly, and weekly variations in chlorophyll a (chl a) on the Louisiana shelf and to assess relationships with river discharge, nitrate load, and hypoxia. During the study period (2000-2003), interannual changes in shelf-wide chl a concentrations averaged over January-July ranged from +57% to -33% of the 4-yr average, in accord with freshwater discharge changes of +20% to -29% and nitrate load changes of +20% to -35% from the Mississippi and Atchafalaya Rivers. Chl a variations were largest on the shelf between the Mississippi and Atchafalaya Deltas. Within this region, which corresponds spatially to the area of most frequent hypoxia, lowest January-July mean chl a concentrations (5.5 mg m^-3 over 7,000 km^2) occurred during 2000, the year of lowest freshwater discharge (16,136 m^3 s^-1) and nitrate load (55,738 MT N d^-1) onto the shelf. Highest January-July mean chl a concentrations (13 mg m^-3 over 7,000 km^2) were measured in 2002, when freshwater discharge (27,440 m^3 s^-1) and nitrate load (101,761 MT N d^-1) were highest and second highest, respectively. Positive correlations (R^2 = 0.4-0.5) were found between chl a and both freshwater and nitrate loads with 0 to 1 month lags, with the strongest relationships just west of the Mississippi Delta. In 2001, unusually clear skies allowed the identification of distinct spring and summer chl a blooms west of the Mississippi Delta 4-5 wk after peaks in river discharge. East of the delta, the chl a concentrations peaked in June and July, following the seasonal reversal in the coastal current. A clear linkage was not detected between satellite-measured chl a and hypoxia during the 4-yr period, based on a time series of bottom oxygen concentrations at one station within the area of most frequent hypoxia. Clear relationships are confounded by the interaction of physical processes (wind stress effects) with the seasonal cycle of nutrient-enhanced productivity and are influenced by the prior year's nitrate load and carbon accumulation at the seabed.

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