Assessing the Effectiveness of Green Infrastructure
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Large scale wetland restoration efforts are common worldwide, and often motivated towards regaining lost ecosystem services such as nitrogen (N) removal and carbon (C) sequestration. Despite large monetary investment in wetland construction, research on the capacity for restored wetlands to retain C and remove N in eutrophic environments lags far behind. It is critical, however, to document these ecosystem services to help justify costs of restoration. In 2015-16 NYC Parks completed an assessment of 22 restored tidal wetlands in NYC that included measurements of habitat value and marsh structure but did not quantify N assimilation, N removal, or C sequestration. We measured C and N pools, and N fluxes including denitrification at four restored tidal wetland sites in the Harlem River. We expected differences among sites due to varying ages (4 years to 15 years post-restoration) and associated wetland community development. The results, however, show that there were few differences in wetland structure and function across sites. Rates of denitrification were high at all sites and averaged 465 μmol N m-2 h-1. These rates are higher than other reported values for natural and restored marshes suggesting that restored wetlands are hot spots for N removal in the Harlem River. The wetlands were also a sink for dissolved inorganic N (N retained rather than recycled), however, two of the four sites were sources of reactive P to the ecosystem likely due to reduced sediment. Overall, nitrogen removal and retention at these sites is very high and should be considered an important ecosystem service provided by these restored habitats. We found high above and belowground biomass of the salt marsh plant, Spartina alterniflora, at all sites. The high belowground biomass and accumulation of sediment carbon led to significant C sequestration at each of the sites. The wetlands sequestered an average of 50 metric tons of C per acre which would have an economic value of $2,000 per acre. The C sequestered at Harlem River wetlands is similar to values reported in natural wetlands suggesting this is also an important ecosystem service provided by the restored habitats. Future studies should consider seasonal differences in N cycling to better resolve estimates of annual N removal. Ongoing efforts aim to integrate the data into indices of marsh health with New York City Parks.