Evaluating potential effects of the Mid-Breton Diversion on living marine resources in the Mississippi Sound and bight using a coupled modeling framework

Objectives

1. To develop a coupled modeling framework that encompasses the northern Gulf Coast from the Mississippi River Delta, including the planned Mid-Breton Sediment Diversion, to Mobile Bay, and can simulate processes from physics to fish in Louisiana, Mississippi and Alabama waters.
2. To use the newly developed modeling framework to test effects of proposed Mid-Breton Sediment Diversion flow regimes on water circulation, salinity, temperature, biomass and distribution of living marine resources in the Mississippi Sound and Bight.
3. To disseminate our results in academic and non-academic settings and help our partners use our output for management decisions.

Methodology

The research team will develop a coupled modeling framework consisting of a ROMS model, habitat suitability models, and an ecosystem model develop with Ecopath with Ecosim. The Regional Ocean Modeling System (ROMS) will be used as the circulation model at the core of an application of the Coupled Ocean Atmosphere Wave Sediment Transport (COAWST) modeling system to the Mississippi Bight. The Mid-Breton Sediment Diversion is incorporated into the model to allow for the flow of freshwater discharge into Breton Sound within the model domain.

The temperature and salinity output from the ROMS model will be used to create habitat suitability indices (HSI) as a means of quantifying habitat suitability for eastern oyster, blue crab and brown shrimp. Particle distribution as part of the ROMS model will be used to determine where oysters will settle. The team will adapt an existing Mississippi River Delta Ecospace model to include the Mississippi Sound and Bight. Maps of either HSI output (oysters, blue crab, brown shrimp) or temperature and salinity output from the ROMS model (all other species) in combination with habitat features will determine the habitat capacity of each grid cell for each model time step for each species. Lab experiments will augment the team's knowledge on tolerance ranges of local species to water quality parameters, which will be included as response curves in the model.

Trophic interactions as part of the food web model will further influence the biomass and distribution of each species in each area. Field collections of environmental and water quality parameters such as temperature and salinity will be planned five times per year to calibrate and validate the models. The coupled modeling framework will be used to determine biomass and distribution of 41 species in the Mississippi Sound and Bight over decadal model runs.

Rationale

A combination of subsidence, erosion and sea level rise is resulting in substantial wetland loss in the Mississippi River Delta. Restricting overbank flooding by a series of levees accelerates this loss, since subsiding wetlands are not nourished by an inflow of sediment and nutrients. Louisiana is currently proposing to develop two large sediment diversions that would be used to divert freshwater containing sediment and nutrients into the subsiding estuaries of Breton Sound and Barataria Bay. Because of the size and proposed location, the mid-Breton sediment diversion has attracted considerable public attention regarding its potential to affect the ecology of the Mississippi Sound and Bight. Some of this attention may be related to increased awareness of the potential for broad ecological effects from actions in adjoining states such as the opening of the Bonnet Carré Spillway (BCS) can have in the Mississippi Sound.

Using freshwater release from the BCS as a proxy is not sufficiently informative, because the mid-Breton diversion discharges in a basin that is located differently in relation to the Mississippi Sound, and has a different salinity profile. It is therefore timely and imperative to develop a modeling framework that can help anticipate potential positive or negative effects of major actions, such as the mid-Breton diversion, on the hydrology, water quality and ecology of the Mississippi Sound and Bight. Once developed, the modeling framework will be a strategic tool to help anticipate effects of environmental changes on living marine resources at the Mississippi Gulf Coast, and can be expanded to evaluate impacts of factors such as climate change, harmful algal blooms and overfishing.