Estimating the Benefits of Freshwater Introduction into Coastal Wetland Ecosystems in Louisiana: Nutrient and Sediment Analyses

Abstract

Models have become a key tool for effectively forecasting the benefits of restoration projects, yet they are often too complicated and inaccessible to resource managers for practical use. A desktop numeric model was developed for resources managers to estimate the benefits of nutrients and sediments introduced into coastal marshes in Louisiana and to improve the predictability of coastal restoration alternatives. In the model, nutrient benefits are based on the cumulative volume of water introduced by an average annual flow rate, the average total nitrogen and phosphorus concentration of the source water, the nutrient requirements of the plants based upon the annual plant production rate per unit area, and the proportion of nutrients retained in the system. Sediment benefits are based upon the cumulative volume of water introduced by the average annual flow, total suspended solid concentration, bulk density of the receiving marsh area, average depth of the receiver area, and the retention of material introduced into the system. The sum of nutrient and sediment benefits represents a gross change that is then applied to adjust the annual land change rate. Model applications indicate that volume, concentration, and retention of materials tend to be the most important factors in determining the efficiency of marsh building. Sites closest to the main flow of a river, where material concentrations are highest, are capable of forming land at higher rates; however, off-river sites with more than 50% preexisting marsh vegetation can potentially rebuild at comparable rates because of higher retention efficiencies. Lowest rates of land development are in off-river sites with highly degraded marsh, and new delta formations in deep open water.