Coastal wetlands are important sites of nitrogen removal, a critical ecosystem service in highly eutrophic environments. They are also subject to increasing threat from eutrophication, sea-level rise, and coastal development. In Jamaica Bay, over 92% of historic wetland area has been lost over the past century. Despite considerable efforts to restore wetlands, few studies have examined the value of ecosystem services that newly constructed wetlands provide, and little is known about the ecological mechanisms contributing to the success or failure of reconstruction. Past and ongoing restoration efforts in Jamaica Bay provide a unique opportunity to study nitrogen-removal ecosystem services in extant and restored wetlands in an urban, eutrophic environment. In an ongoing collaboration with researchers from several institutions, we use a chronosequence of marsh restorations (restored 2003-2012) to assess how marsh vegetation, sediment characteristics, and key processes of the nitrogen cycle develop over time following restoration. The goal of our project is to determine the restoration age and environmental conditions under which salt-marsh restoration will effectively provide ecosystem services such as nitrogen removal. We employ a combination of flow-through incubations, field surveys, and experimental methods to identify the key biological and abiotic factors limiting nitrogen-removal services in restored marshes.
Results from continuous flow-through core incubations indicated that restored marshes remove nitrogen via denitrification. We found that nitrogen removal generally increased with marsh age. The highest denitrification rates were observed in an unrestored marsh that has remained relatively stable. Denitrification rates in young restored marshes were seasonally limited by the availability of nitrate and organic carbon. Nitrate limitation was not detected in the oldest restored marsh nor in unrestored marshes. In surveys conducted across the restoration chronosequence, we detected increases in total organic matter and belowground plant biomass, suggesting that restored marshes are likely to become more stable and that the influence of marsh plants on sediment chemistry and microbial processes is likely to increase as restored marshes age. In field experiments, marsh plants were found to increase the temporal and spatial variability of sediment oxygen concentrations and to increase denitrification rates by as much as 5-fold in young restored marshes. Our results provide strong support that restored marshes are providing nitrogen-removal services for which they were constructed and that these services are likely to increase in value as the marshes age. Marsh plants were also found to play a significant role in enhancing nitrogen-removal services in young restored marshes.