We are linking a suite of well-established models to quantify fish and shrimp population responses to combinations of nutrient loadings and planned river diversions. Our scenario analyses include different land-use and agricultural practices in the watershed and alternative river diversions. The linked model system informs and supports management decisions by estimating how reduced nutrients and diversion operations affect hypoxia and key living resources.

Why We Care
The northern portion of the Gulf of Mexico experiences an annual summer hypoxia event when over-enrichment of nutrients in the water causes very low levels of dissolved oxygen. The dead zone of low oxygen in the northern Gulf of Mexico, occurring from Texas to Louisiana, is one of the most intractable environmental issues facing the United States. Far-reaching corrective measures implemented since hypoxia developed in the early 1980s have not yet resulted in hypoxia mitigation. Year after year, the dead zone varies in size but persists, seriously degrading the ecological health of the region.

The current goal of our Northern Gulf of Mexico Ecosystems and Hypoxia Assessment (NGOMEX) program is to conduct research that will improve existing models or develop new quantitative models to determine population- to ecosystem-level effects of Gulf of Mexico hypoxia, both spatially and temporally, on ecologically and commercially important aquatic species. The overall objective is to quantify, through multidisciplinary ecosystem models or other methods, the ecological and socioeconomic impacts of hypoxia, including an evaluation of the effects of alternative management strategies on ecosystem function and living resource populations.

What We Are Doing
There are several models available related to hypoxia in the Northern Gulf of Mexico, with most emphasizing how watershed activities affect riverine inputs and how riverine inputs to the continental shelf interact with the physical and biogeochemical processes to produce hypoxia.

The DLEM (Dynamic Land Ecosystem Model) has been applied to the Mississippi River watershed and used to examine how land use and agricultural practices affect loadings to the Northern Gulf of Mexico. Multiple statistical models have been developed relating the annual areal extent of July hypoxia to nutrient loadings and other explanatory variables. Three-dimensional, hydrodynamics–water quality models have also been developed and forced by riverine discharge and nutrient loadings to simulate the movement of water and biogeochemical processes in the cells of their spatial grids. A next logical step in the modeling of hypoxia for the Northern Gulf of Mexico is to inform management by combining the various models to generate defensible quantitative predictions of how watershed and river management actions would affect ecologically and economically important populations of fish and shellfish.

We intend to link the various models to provide the most defensible way to go from watershed and river management actions to fish and shellfish responses. Such a linked-model system would support management decisions by providing quantitative estimates of how reduced riverine nutrient loadings and diversion operations affect hypoxia, and how those changes in hypoxia affect key living resources. Such information on ecological effects can be combined with socioeconomic analyses to provide managers with quantitative and ecological bases for evaluating the effectiveness and efficiency of watershed management actions.

The project team consists of Dr. Kenneth Rose (Principal Investigator from Louisiana State University) and Dr. Kevin Craig (Application Principal Investigator with NOAA’s National Marine Fisheries Service), whose role is to ensure the outputs and outcomes of the project are effectively transferred to the management community. Co-investigators are Drs. Haosheng Huang, Dubravko Justic, and Zuo (George) Xue of Louisiana State University; Dr. Ehab Meselhe of The Water Institute of the Gulf; and Dr. Hanqin Tian of Auburn University.
Benefits of Our Work
Our modeling results will provide managers with quantitative information about how nutrient reductions will affect fish populations, and how the combination of river diversions and hypoxia will affect fish and shrimp populations. This is a change from the current information used by managers. To date, such information has been mostly anecdotal or necessarily inferred from other studies not specifically designed and linked to generate this information.

A second outcome will be the availability of a new modeling tool that links existing models in a way that provides consistent and defensible predictions, from the watershed to the living resources. In the longer term, we anticipate that the results will contribute to management decisions about watersheds and diversions that reduce the extent and severity of the hypoxic zone, decisions at least partially based on fishery population-level responses.

Population-level endpoints provide a sound scientific, economic, and political basis for reducing nutrient loadings and hypoxia. Knowing how hypoxia will affect fish and shrimp populations could also contribute to the improved sustainability of some key fisheries. Information on why fish populations vary and how much of this variability can be attributed to factors other than harvest increase the effectiveness of fisheries management and land-use in the watershed and river management for diversions to fish and shrimp populations.