Modeling the Factors Controlling Phytoplankton in the St. Louis Bay Estuary, Mississippi and Evaluating Estuarine Responses to Nutrient Load Modifications

Several biochemical and physical factors regulate phytoplankton primary productivity and algal bloom events in estuarine environments. Some of the most important factors include nitrogen, phosphorus and silica availability, light availability, and estuarine flushing potential. A better understanding of these processes is necessary to support sound management strategies that take into account the hydrological, hydraulic, and biochemical connectivity between estuaries and their watersheds. In this paper the factors controlling phytoplankton productivity in a tributary estuary of the northern Gulf of Mexico, the St. Louis Bay estuary, Mississippi, and the system responses to nutrient load alterations are studied. For this purpose a coupled hydrodynamic and water quality model based on U.S. EPA computer models was implemented. The writers present an evaluation of the model predictive capacity, and its implementation to study the processes controlling phytoplankton dynamics, nutrient cycling, and oxygen availability. The results suggest that primary productivity is limited by nitrogen availability. Under current nutrient load conditions the concentrations of nitrogen fall below 0.025 mg/L, and the rates of primary productivity vary between 0.5 and 1.8 gO/m2/day with a mean of 0.65 gO/m2/day. The simulation of four different nutrient load scenarios suggests a low sensitivity of the estuary to the evaluated conditions in the watershed. The results also indicate that the coupled model is a cost-effective strategy to investigate the impacts of different management strategies. At a regional scale, the implemented approach can contribute to the design of integrated strategies to reduce hypoxia and algal bloom problems in the Gulf of Mexico.