Modeling the impacts of pulsed riverine inflows on hydrodynamics and water quality in the Barataria Bay estuary

Abstract

Eutrophication and coastal wetland loss are the major environmental problems affecting estuaries around the world. In Louisiana, controlled diversions of the Mississippi River water back into coastal wetlands are thought to be an important engineering solution that could reverse coastal land loss. There are concerns, however, that freshwater diversions may increase nutrient inputs and create severe eutrophication problems in estuaries and wetlands adjacent to the diversion sites. My dissertation research concerns modeling the effects of the observed and hypothetical freshwater diversion discharges on the hydrodynamics, salinity and water quality in the Barataria estuary, a deltaic estuary in south Louisiana. This estuary receives freshwater and nutrient discharges from the Davis Pond diversion, the world’s largest freshwater diversion project. I have implemented two Barataria Bay simulation models of differing complexity, a simple 6-box mass-balance model and a high resolution two-dimensional (2-D) coupled hydrology-hydrodynamic- water quality model. Model results have shown that the Barataria estuary imports nitrogen and exports carbon to the coastal ocean. Compared to the lower Mississippi River, the Barataria estuary appears to be a very small source of total organic carbon for the northern Gulf of Mexico and is unlikely to have a significant influence on the development of the Gulf’s hypoxia. Model simulations pointed out that the effects of different diversion discharges on salinity are most apparent in the middle and lower sections of the Barataria estuary. Further, tracer simulation experiments have shown that residence times differ markedly at different locations within the same water body due to differences in small scale hydrodynamics. Model simulations clearly demonstrated the importance of residence times for the overall functioning of the estuary. Model simulations also pointed out the differences in spatial patterns in phytoplankton response to distributed freshwater and nutrient inflows, reflecting the near-field control of nutrients and far-field control of residence times on phytoplankton standing stock. The models reiterate the fact that there are significant tradeoffs in using freshwater diversions in coastal restoration efforts, namely tradeoffs between hydrologic restoration and water quality effects.