Assessing recovery of ecosystem structure and function in restored tidal marshes of the Mississippi-Alabama Gulf Coast

Objectives

• To compare plant community composition, plant biomass, soil carbon and nitrogen content, and denitrification in restored coastal wetlands of different ages and relative to reference wetlands.
• To estimate recovery trajectories for ecosystem structure and functions in restored wetlands and the time to equivalency with reference wetlands.
• To evaluate the relative effectiveness of different habitat restoration projects based on comparisons among restored wetlands and relative to reference wetlands.

Methodology

We will inventory structural and functional attributes in 16 coastal wetlands along the Mississippi-Alabama Coast, including four reference wetlands at Grand Bay National Estuarine Research Reserve (NERR), Weeks Bay NERR, Fowl River, and Dauphin Island and 12 created or restored wetlands ranging in age from 3 to 30 years. Restored sites will include living shorelines, wetlands created using beneficial use of dredged material and marshes constructed from coastal forest for mitigation purposes. All sites will be tidal saline marshes comprised, in part, of Juncus roemerianus and/or Sporobolus alternflorus (formerly Spartina alterniflora), two common marsh plants in the region.

At all sites, we will randomly establish three sampling plots (1 m² each) from which we will subsample the plant community to quantify plant species composition, rooting depth, above- and below-ground plant biomass, and plant carbon and nitrogen content. We also will collect duplicate, 20cm deep soil cores from each plot to characterize edaphic conditions (e.g., bulk density, organic matter and soil carbon and nitrogen content) and to determine denitrification rates using the isotope pairing technique. We will collect porewater to measure salinity, pH and sulfides. All variables will be sampled once per year for comparisons among sites and between years. Means for all of these variables will be compared across sites and along an age gradient, from which we can then estimate time post-restoration to structural or functional equivalency with reference sites. 

Rationale

Coastal wetland restoration practices are increasingly utilized to offset wetland loss and degradation and to recover ecosystem services, making it important to evaluate the relative effectiveness and times to functional equivalence of different restoration strategies. Assessment of wetland restoration and creation success have historically been based on structural measurements, yet restoration of wetland structure may not correspond to recovery of ecosystem function or the provision of important ecosystem services.

If restored or created wetlands fail to function similarly to their natural counterparts, they may provide fewer ecosystem services and be less resilient to continued environmental pressures. Two critically important services provided by coastal wetlands are carbon storage and nitrogen removal. The accumulation and burial of organic carbon not only mitigates climate change, but promotes marsh resilience by contributing to vertical accretion. In addition, coastal wetlands serve as hotspots of biogeochemical cycling, including denitrification, which helps mitigate nitrogen loading to coastal waters, thereby reducing drivers of harmful algal blooms or dead zones. By restoring or creating wetlands, it is possible to recover these functions and services, thereby promoting more resilient coastal watersheds. To assess restoration success, we propose to evaluate ecosystem structure and function in habitat restoration and creation projects of different ages.

We will compare plant community structure, carbon storage and nitrogen removal capacity among restored wetlands and relative to reference wetlands along the Mississippi-Alabama Gulf Coast. By assessing recovery of ecosystem structure and function across a range of restored wetlands, we will address the Healthy Coastal Ecosystems focus area of this RFP and provide feedback to managers and practitioners employing restoration as a tool to recover ecosystem services. Based on our results, we can evaluate the relative effectiveness of different habitat restoration projects along the Mississippi-Alabama Gulf Coast, which can be used to improve techniques or inform best practices.