COASTAL ENVIRONMENTAL IMPACTS BROUGHT ABOUT BY ALTERATIONS TO FRESH-WATER FLOW IN THE GULF-OF-MEXICO

Freshwater inflow is one of the most influential landscape processes a ffecting community structure and function in lagoons, estuaries, and d eltas of the world; nevertheless there are few reviews of coastal impa cts associated with altered freshwater inputs. A conceptual model of t he possible influences of freshwater inflows on biogeochemical and tro phic interactions was used to structure this review, evaluate dominant effects, and discuss tools for coastal management. Studies in the Gul f of Mexico were used to exemplify problems commonly encountered by co astal zone managers and scientists around the world. Landscape alterat ion, impacting the timing and volume of freshwater inflow, was found t o be the most common stress on estuarine systems. Poorly planned upstr eam landscape alterations can impact wetland and open-water salinity p atterns, nutrients, sediment fertility, bottom topography, dissolved o xygen, and concentrations of xenobiotics. These, in turn, influence pr oductivity, structure, and behavior of coastal plant and animal popula tions. Common biogeochemical impacts include excessive stratification, eutrophication, sediment deprivation, hypoxia, and contamination. Com mon biological impacts include reduction in livable habitats, promotio n of ''exotic'' species, and decreased diversity. New multiobjective s tatistical models and dynamic landscape simulations, used to conduct p olicy-relevant experiments and integrate a wide variety of coastal dat a for freshwater inflow management, assume that optimum estuarine prod uctivity and diversity is found somewhere between the stress associate d with altered freshwater flow and the subsidy associated with natural flow. These models attempt to maximize the area of spatial overlap wh ere favorable dynamic substrates, such as salinity, coincide with favo rable fixed substrates, such as bottom topography. Based upon this pri nciple of spatial overlap, a statistical performance model demonstrate s how population vitality measurements (growth, survival, and reproduc tion) can be used to define sediment, freshwater, and nutrient loading limits. Similarly, a spatially articulate landscape simulation model demonstrates how cumulative impacts and ecosystem processes can be pre dicted as a function of changes in freshwater, sediment, and nutrient inflows.