The Importance of Pulsed Physical Events for Sustainability of Louisiana Coastal Forested Wetlands

A number of freshwater diversions from the Mississippi River into Louisiana’s coastal wetlands are currently in operation or in the planning stage. These diversions have multiple objectives including maintaining a desirable salinity gradient, restoring deteriorating wetlands, and enhancing fisheries. The extensive freshwater forested wetlands surrounding the western end of Lake Pontchartrain receive little or no sediment input and are currently deteriorating due to continuous flooding. Diverting nutrient-rich water through wetlands can lead to substantial nutrient removal and to enhanced accretion. The objective of this paper is to compare the impacts on freshwater wetland ecology, accretion, and water quality of several scenarios for diverting freshwater from the Mississippi River. Accretion will be increased so that the area will become progressively less flooded, enhancing productivity and seedling recruitment. With overland flow, about 73% of N and 48% of P will be retained in the wetland, while other alternatives are estimated to remove 0-33% of N and 0-40% of P. Productivity of the forested wetland will decrease by at least 20-50% over the next 50 years if nothing is done, but there would be a substantial increase in productivity if river water is diverted into the area. We recommend that a 45-55 m3/s diversion be placed in the southern portion of the swamps of Lake Maurepas and that the diversion be operated annually from winter to early spring. There are indications that a diversion in late summer and fall would be beneficial, as that is when salt pulses typically occur