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Salt marshes have ecological and economic value, but shoreline development, the increasing rate of sea-level rise, and other human impacts have caused significant loss of salt marshes. As a result, restoration of these ecosystems is widespread. For restoration and management to be effective, it is imperative to improve our understanding of marsh-building plants that serve as the ecological foundation of these habitats. Given the observed differences in characteristics between populations of smooth cordgrass, Spartina alterniflora, restoration plantings may impact the biodiversity and resilience of restored ecosystems. Understanding differences in the structural and functional outcomes of active planting of restoration sites will enable the long-term success of restoration efforts to be improved. Natural and restored salt marshes in Long Island Sound were studied in 2021–2022 for S. alterniflora genetics, biomass, stem morphology, and faunal community composition. The average genotypic diversity of S. alterniflora was more than 4 times higher in restored than in natural marshes, and differentiation between each restored site and natural sites decreased with time. No difference was observed in live S. alterniflora belowground biomass; however, mean dead belowground biomass in natural marshes was more than 3 times greater than in restored marshes. Marsh platform invertebrates differed between the restored and natural sites, with natural marsh edge habitats having 9 times higher density of Geukensia demissa and 3 times as many crab burrows than in restored marshes, but there was no detected difference in species richness or abundance of nekton at high tide. With restoration practitioners seeking resilient, self-sustaining ecosystems, it is important to evaluate whether restored marsh characteristics are consistent with those goals and modify restoration planning accordingly to incorporate genetics, structure, and function. [ABSTRACT FROM AUTHOR]

Understanding the threat to ecosystems from excess nitrogen in coastal waters is a priority issue in scientific research and natural resource management. Previous field studies have demonstrated that high nitrogen loading can decrease the health and resiliency of salt marshes through shifting biomass allocation, increasing decomposition, and causing creek bank instability, all of which can lead to increased marsh loss with sea-level rise. However, other studies have shown relatively little impact of increasing nitrogen on the structure and function of these systems. Due to the long history of eutrophication in Long Island Sound, aggressive nitrogen reduction strategies have been enacted in this region, but detrimental nutrient inputs persist at variable levels throughout the watershed. Here, the extent of nitrogen-linked salt marsh change under varying levels of nutrient stress was measured, testing the hypothesis that salt marsh resilience (as measured by Spartina alterniflora belowground biomass and marsh edge stability) decreases with increasing nitrogen loading. S. alterniflora growth (stem height, stem density, and biomass) and within-marsh creek area were quantified in 10 salt marshes along a nitrogen-loading gradient. Increasing nitrogen loading showed a significant negative relationship with dead belowground biomass in S. alterniflora; the loss of this belowground biomass in higher nitrogen systems may decrease salt marshes' ability to keep pace with sea-level rise. Neither shifts in live biomass allocation nor a positive relationship between aboveground biomass or stem height and increasing nitrogen was observed that might promote additional sediment capture, but higher stem density could play a role in promoting sedimentation on the marsh surface in more sediment-rich systems. Aerial photography analysis revealed marsh creek expansion since 1934 at 90% of the marshes studied, but unlike findings from prior experimental enrichment studies, the rate of marsh loss did not increase with increasing nitrogen loading. Given the importance of these ecosystems and the potential of nitrogen to decrease their resiliency, understanding the impacts of eutrophication on salt marshes is critical. However, these results show that the relative importance of nitrogen in driving salt marsh loss in Long Island Sound may be less than studies from other regions have suggested. [ABSTRACT FROM AUTHOR]