Regulation of eutrophication susceptibility in oligohaline regions of a northern Gulf of Mexico estuary, Mobile Bay, Alabama

The factors regulating the eutrophication susceptibility of seven oligohaline regions in the sub-estuaries of Mobile Bay, Alabama were examined in a comparative analysis. The oligohaline regions differed primarily by the dominant land-use of their upstream watersheds, with two of the regions being primarily urban, two being primarily agricultural, and three being primarily forested. A stepwise model selection procedure was used to determine a suite of multiple regression models describing eutrophication response, in terms of a chlorophyll a (chla) on a sampling event basis, in relation to estuarine mixing time scales, nutrient concentrations, light availability, and watershed delivery of freshwater and nutrients. The models indicated a strong positive relationship between chla and mixing time scales (i.e., residence time or freshwater flushing time). Mixing time scales longer than five days allowed maximum chla (64 μg l−1), while lowest chla (<1 μg l−1) occurred when mixing time scales were less than two days. Of the watershed inputs, chla exhibited opposing relationships with the components of freshwater load, having a negative relationship with discharge and a positive relationship with incoming freshwater nitrogen concentrations. Estuarine phosphorus concentrations and photosynthetically active radiation were also found to be good descriptors of chla. The comparative approach employed here allowed for the development of empirical models that were used to determine the nutrient concentration reductions required to achieve a trophic state of <20 μg l−1 chla. The average reductions in nitrogen and phosphorus needed to achieve this trophic state ranged from 0 to 32%.