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Extensive salt marsh restoration is expected in the northern Gulf of Mexico over the next several decades, funded in part by settlements from the 2010 Deepwater Horizon oil spill. Understanding the ecological benefits of restored marshes over time is integral to setting appropriate restoration targets and performance criteria and in determining the restoration area needed to achieve desired restoration goals and offset quantified natural resource injuries. We present a method for quantifying anticipated ecological benefits associated with marsh restoration projects, particularly marsh creation or enhancement through the placement of dredged material, in the northern Gulf of Mexico. Using salt marsh vegetation (percent cover, aboveground biomass, and belowground biomass) and indicator faunal species (periwinkle snails and amphipods) as representative marsh community components, we used resource equivalency analysis (REA) to model projected ecological benefits over time and quantified total net project benefits for a hypothetical marsh creation project in Barataria Bay, Louisiana. Sensitivity analysis of the resulting model suggests that the recovery trajectories for each marsh component were the most important drivers of modeled restoration benefits and that model uncertainty was greatest for marsh fauna, which has limited data availability compared to marsh vegetation and high natural variability. Longer-term monitoring at restored restoration sites and/or targeted monitoring of older restoration projects would reduce variability in the recovery trajectories for the marsh community components examined in this case study and improve the reliability of the REA model for projecting benefits associated with salt marsh restoration.

Recovery following salt marsh restoration in the northern Gulf of Mexico is investigated using meta-analysis for two salt marsh indicator invertebrates, the periwinkle snail (Littoraria irrorata) and amphipod crustaceans (Amphipoda). These invertebrates serve key marsh ecosystem functions including facilitating nutrient cycling and serving as prey for larger ecologically and economically important species. Recovery of periwinkles in restored marshes compared to reference sites is quantified by progression in population density and, because the species is long-lived (~ 10 years), in terms of biomass added per unit area each year following restoration. Amphipods are shorter-lived with high annual turnover; thus, recovery through time is estimated by the density of individuals rather than by biomass. The results of the analyses indicate progressive periwinkle recovery to equivalence with reference systems by year 4 in terms of density and year 6 with respect to annual biomass addition, while amphipod densities do not fully recover in the first 20 years following restoration. Although periwinkle recovery in terms of annual biomass addition reaches equivalence by year 6, the development of an age class structure characteristic of reference marshes would likely take longer because of the relatively long lifespan for this species. In addition to providing insight into the benefits of salt marsh restoration in the northern Gulf of Mexico, the approach described can be applied more generally to restoration scaling in a natural resource damage assessment context.

Deepwater Horizon was the largest marine oil spill in U.S. waters, oiling large expanses of coastal wetland shorelines. We compared marsh periwinkle (Littoraria irrorata) density and shell length at salt marsh sites with heavy oiling to reference conditions ∼16 months after oiling. We also compared periwinkle density and size among oiled sites with and without shoreline cleanup treatments. Densities of periwinkles were reduced by 80-90% at the oiled marsh edge and by 50% in the oiled marsh interior (∼9 m inland) compared to reference, with greatest numerical losses of periwinkles in the marsh interior, where densities were naturally higher. Shoreline cleanup further reduced adult snail density as well as snail size. Based on the size of adult periwinkles observed coupled with age and growth information, population recovery is projected to take several years once oiling and habitat conditions in affected areas are suitable to support normal periwinkle life-history functions. Where heavily oiled marshes have experienced accelerated erosion as a result of the spill, these habitat impacts would represent additional losses of periwinkles. Losses of marsh periwinkles would likely affect other ecosystem processes and attributes, including organic matter and nutrient cycling, marsh-estuarine food chains, and multiple species that prey on periwinkles.