Research Product

Eutrophication susceptibility of 7 oligohaline tidal river regions was examined in a comparative empirical analysis. Eutrophication response, chlorophyll a (ch la), was related to estuarine mixing time scales, nutrient concentrations, and watershed delivery of freshwater and nutrients. Mixing time scales (i.e. residence time or freshwater flushing time) longer than 10 days resulted in maximum chla (64 ug 1-1), while lowest chla (< I ug 1-1) occurred when mixing time scales were less than 2 days. During periods of long mixing times and low discharges, chla was regulated by the nitrogen concentration delivered by the watershed and tidal river phosphorus concentration. A stepwise model selection procedure was used to determine the best empirical models describing chla in relation to these factors. The models were subsequently used to estimate the nutrient reductions required to achieve a desired trophic level (<20 ug 1-1chla ). Average reductions in nitrogen and phosphorus ranged from 0 to 70%.