Impact of tidal restoration on green house gas fluxes and carbon storage in salt marsh ecosystems

Background/Question/Methods: Approximately 50% of U.S. wetlands have been lost since 1900, with some of the highest loss rates from 1970-1990. Restoration efforts largely focus on restoring ecosystem function and services, including fish and bird habitat, shoreline protection and nutrient filtering. With the recognition of the value of Blue Carbon, an additional benefit to wetland restoration lies within their ability to store carbon. Additionally, the climate implications of wetland restoration extend to fluxes of other green house gases, including CH₄, which is over 30 times more potent than CO₂. However, carbon burial and GHG flux are rarely evaluated in salt marshes post restoration. In this study, we evaluate carbon burial and GHG fluxes in salt marshes where tidal exchange has been restored. In the past these marshes were drained and diked to prevent tidal exchange, drastically altering soil conditions in favor of organic matter degradation, which can result in a large release of CO₂ to the atmosphere. At the same time, fresh water conditions may have favored CH₄ production. Typically, vegetation response to marsh restoration is rapid, however, little is known about the time scale over which sediment geochemistry is restored to natural values. A two-pronged experimental design was used to account for both temporal and spatial variability that may occur in the restored salt marshes. To compare restored marshes to natural ones, two sites at each marsh were identified: one upstream and one downstream of the previous location of the tidal restriction. Down stream sites are considered “natural”, since their hydrology was not altered by the restriction, while the corresponding upstream site is considered “restored” from fresh wetlands back to salt marsh. Secondly, to determine the time frame over which carbon and GHG dynamics return, the salt marshes studied here represent a chronosequence of restoration from pre-restoration to 4 to 14 years post restoration.

Results/Conclusions: At marshes where tidal exchange was restored, soil salinity increased and pH decreased to pre-disturbance values and has largely resulted in the return of salt marsh vegetation. During years when the marshes were restricted, most sites experienced a decrease in soil carbon content as well as a reduction in accretion rate. Post restoration, soil carbon content is near natural marsh values, while accretion rates remain variable. Green house gas fluxes post restoration are comparable to natural marshes at the same salinity. Restoration of tidal exchange appears to positively influence the climate impact of restored salt marshes.