Coastal wetland loss is an ongoing problem across the northern Gulf of Mexico, with Mississippi having lost 10,000 acres of this habitat in the past 70 years. In 2005 and 2016, two separate restoration projects strived to mitigate land loss to Deer Island, a small barrier island just south of Biloxi, by creating new wetland habitats adjacent to the island using sediments from nearby dredging projects.
A team of researchers led by Patrick Biber, Zachary Darnell and Kevin Dillon at The University of Southern Mississippi recently compared those sites to a nearby natural marsh area to gauge the projects’ outcomes. They found that neither of the two restored sites are functioning exactly like natural wetlands at this time.
“Even 15 years after construction,” Biber said, “it looks like a natural marsh, but in terms of its functionality and the ecosystem services it provides, it’s not working like a natural marsh — yet.”
The researchers approached the data from a number of angles to reach that conclusion. In addition to field sampling to determine soil composition, vegetation, invertebrates and small fishes at the three sites, the team used stable isotope analyses of carbon and nitrogen to track development of food webs at each site.
The first finding the team noted was that black needlerush, a native marsh grass in decline in the region as wetlands have disappeared, did not thrive where it had been planted in the restored marshes. The restored sites also had a smaller variety of animal life than the natural site, or what scientists call lower diversity.
Biber said the cause of the observed shortcomings in the restored habitat might be in part their design. The projects are an example of beneficial use of dredge materials for restoration. Navigation channels must be dredged on a regular basis to ensure that larger vessels can traverse the area without grounding. Rather than haul the mud dug out from the dredging project a long distance to dispose of it, engineers repurpose it to create these emergent marsh habitats. To do this, they build a sand containment berm adjacent to an existing island and then pump in mud that is retained in place until planted vegetation can grow in enough to further stabilize and anchor the fine sediments.
“The low diversity issues probably stem from that,” Biber said, noting that the containment berm limits the regular water exchanges found in a natural system. “Marine organisms can’t get into the rebuilt marsh area through tidal channels like they could in a natural site.”
The good news is that despite these limitations, the stable isotope analysis of carbon markers in the food web showed that the restored sites supported a variety of primary producers, such as plants, phytoplankton and algae, that the fishes and invertebrates can find enough food to live on. Nitrogen stable isotopes, a natural marker used to show evidence of fertilizer or sewage runoff, showed minimal human influence, and only in the more recent restoration project. Biber said he is expecting the nitrogen isotope values will drop with time based on the results at the other sites.
So how could the findings from this study inform future restoration efforts? Biber said that biologists, natural resource managers and engineers should work together to figure out how to promote regular tidal water exchange into these sites, which would promote diversity in the animal populations that live there, to make them more successful. It may come through rethinking the placement of the sites themselves or reconfiguring the way they are built, based on best practices in restoration engineering.