Salinity, Nutrient, and Sediment Dynamics in the Pontchartrain Estuary.

Algal blooms have been recorded in the Pontchartrain Estuary when elevated nutrient loads combine with other ambient conditions, e.g., low salinity and low turbidity. Nutrient and sediment loads were quantified and incorporated into a dynamic mass balance model for the upper Pontchartrain Estuary to determine the timing of conditions that can lead to algal blooms. General nutrient and sediment loading relationships were developed for the tributaries of the Pontchartrain Estuary where data were available; these relationships were extended to estimate the loading for ungauged areas. The annual water yield for the drainage basin was found to be 500 mm. The annual nutrient loadings from all sources were found to be 21,000 t (13 g m-2 y-1) of total nitrogen and 2700 t (1.6 g m-2 y-1) of phosphorus for Lakes Maurepas, Pontchartrain, and Borgne. The basin sediment yield was calculated to be 42 t km-2. Nutrient and sediment concentrations in the Mississippi River were combined with the estimated leakage through the Bonnet Carré Spillway to obtain nutrient loads related to the river stage. The mass-balance model was applied to assess the occurrence of algal blooms for the period 1990-2008; the model predicted all five observed algal blooms but also indicated a potential for two more that were not documented. The strongest potential for algal blooms was in the northwest quadrant of Lake Pontchartrain although the highest dissolved inorganic nitrogen concentrations were predicted for the southwest quadrant in connection with spillage from the Bonnet Carré Spillway. The lower than expected response of the southwest quadrant may be associated with the higher turbidity due to the Spillway plume. [ABSTRACT FROM AUTHOR]