Abstract
Human impacts on coastal marshes are considerable, with 1-2% of coastal marshes lost per year leading to subsequent losses in ecosystem services like nutrient filtering and carbon sequestration. Currently, marsh construction is being used to mitigate losses of marsh cover and services resulting from human impacts in coastal areas. Though marsh structure can be recovered shortly after construction, there are often long-term temporal lags in the recovery of ecosystem functions in constructed marshes. We conducted a year-long field study comparing productivity, denitrification and dissimilatory nitrate reduction to ammonium (DNRA) between two 30-year old constructed marshes (CON-1, CON-2) and a nearby natural reference marsh (NAT). Additionally, we compared porewater nutrient stocks and above- and belowground biomass stocks in each marsh to identify potential drivers of functional differences between marshes. We found that CON-1 and CON-2 were structurally similar to NAT (i.e., plant biomass was similar). Likewise, gross ecosystem productivity (GEP), ecosystem respiration (ER) and net ecosystem exchange (NEE) were similar across all marshes. Further, DNRA and denitrification were fully recovered in the constructed marshes; in fact, denitrification was greater in CON-2 when compared to NAT. While ammonium porewater concentrations were similar across all marshes, phosphate, nitrate + nitrite, and hydrogen sulfide concentrations were greater in NAT than CON-1 and CON-2. This work suggest that current marsh construction practices could be a suitable tool for replacing lost marsh function and cover. However, the lag in recovery of porewater nutrient stocks may also suggests that there are other biogeochemical functions not observed in this study that may lost or not fully restored in constructed marshes.
