BOGO restoration: Can seagrass and native hard clams benefit one another?
In early 2023, Gulf Shellfish Institute (GSI) scientists launched a pilot study in Sarasota Bay to test whether native southern hard clams and seagrass mutually benefit one another. With ~26% of Sarasota Bay’s seagrass habitat lost between 2016 and 2022, novel approaches to seagrass recovery, such as the use of bivalve co-plantings to accelerate seagrass expansion, are timely. Hard clams are filter feeding bivalves that graze phytoplankton and transfer nutrients to the seafloor which can help seagrass grow. Additionally, the physical structure that seagrass creates in both above- (leaves) and below-ground (roots and rhizomes) could protect hard clams from predators, potentially increasing their survival in seagrass habitats. Facilitation between hard clams and seagrass might be useful to accelerate seagrass recovery, improve restoration success, and make seagrass habitats more resilient to future stressors.
Forty-eight experimental plots were established within a recovering seagrass bed that experienced a major die-off in 2018. Plots varied in initial seagrass coverage and half were randomly assigned hard clam plantings. With the help of a local clam farmer, 46,080 adult hard clams were released into plots at a density of 30 clams per meter squared. Monitoring of the project is on-going and includes sediment characterization, sediment nutrient sampling, seagrass cover assessments, and mark-recapture of hard clams biannually. Plots will be monitored for two years with the help of multiple scientific collaborators, including Florida Fish and Wildlife Research Institute (FWRI) and the University of Florida (UF).
After one seagrass growing season, preliminary findings suggest that hard clams might benefit more from seagrass than seagrass benefits from hard clams. Although seagrass coverage increased across the site, plots with hard clams did not significantly enhance seagrass expansion. However there was high hard clam mortality (~80%) from mark-recapture of tethers. The lack of seagrass response to hard clams could be a result of poor hard clam survival and an insufficient number of bivalves to fertilize sediments.
Although we did not find a benefit of hard clams for seagrass after one growing season, preliminary findings suggest hard clam survival is dependent upon a minimum threshold of seagrass cover. Hard clam survivorship from mark-recapture of tethers significantly increased in plots with higher seagrass coverage. Plots with greater than 70% occurrence of seagrass averaged 40% hard clam survival, while plots with <70% seagrass averaged only 8% clam survival.
The multi-partner research team will continue to monitor plots for another seagrass growing season and hope that the surviving hard clams are abundant enough to benefit seagrass in the final summer. GSI will further examine causes for the high hard clam mortality and conduct a final bivalve sampling in fall 2024 to estimate remaining hard clam densities and growth.
