Oligohaline marshes are biologically diverse ecosystems upstream from tidal salt marshes and downstream from tidal freshwater marshes. Such marshes provide food and habitat for numerous bird species. Their tidal creeks provide forage habitat, nursery zones, spawning grounds and migratory corridors for aquatic invertebrate and fish species.


But oligohaline marshes—with their very low salinity—face big challenges from sea–level rise and its attendant saltwater intrusion at Savannah National Wildlife Refuge and other coastal refuges. The salinity in oligohaline marshes ranges from 0.5 to 5 parts per thousand (ppt), whereas full–strength seawater is 35 ppt.


As sea–level rise occurs, marshes are inundated, often resulting in significant habitat conversion or loss. To learn more about the impact of sea–level rise, the Southeast Region Inventory and Monitoring (I&M) Network is monitoring elevation in oligohaline marsh, salt marsh and pocosin wetlands at 18 coastal refuges in the South Atlantic Landscape Conservation Cooperative (LCC).


“This project will monitor rates of wetland elevation change and relative sea–level rise, and using this information, we can forecast the longevity of these habitats on refuges and actions we need to undertake to sustain natural resources,” says Southeast Region I&M Network coordinator Laurel Barnhill.


As a coastal ecologist with the Southeast Region I&M Network, I have been working with refuges and scientists from Atkins Global, an engineering consulting firm, to install rod surface elevation table (RSET) benchmarks at coastal refuges from North Carolina to Florida. It’s tough work that involves standing on a platform above marsh grasses and driving stainless steel rods 40 to 100 feet deep to reach a hard–soil subsurface.


We started last spring at Savannah Refuge, which includes low–lying wetlands along the South Carolina–Georgia border. In addition to saltwater intrusion, development and harbor dredging threaten the marsh. The refuge once contained more than 6,000 acres of oligohaline and tidal freshwater marshes. In part because of past Savannah River dredging, the refuge has less than 3,000 acres of marsh today. Those marshes and their tidal creeks benefit king rail, least bittern, American coot, ducks, herons, white and brown shrimp, blue crabs, Atlantic croaker, bay anchovy, red drum and Atlantic menhaden, to name a few.


Information from the marsh elevation monitoring project “will be essential for us to determine how to proactively respond to habitat changes that are due to factors largely outside of our control,” says Savannah Refuge supervisory wildlife biologist Chuck Hayes.


Starting this fall, marsh surface elevation is being measured from each RSET benchmark. Height measurements (in millimeters) are being taken by attaching the RSET instrument to the permanent benchmark and lowering pins to the marsh surface. During the first year, measurements will be collected quarterly by refuge biologists; biannually the second and third years; and annually thereafter. Each RSET benchmark is also being surveyed so marsh surface elevation can be linked to local sea level. The information will be entered into a regional database for storage and analysis, and will be made available to biologists, planners and researchers.


In addition to the 18 refuges and Atkins Global, the Southeast I&M Network is partnering with the U.S. Geological Survey, The Nature Conservancy, the National Park Service, the South Atlantic LCC, the National Estuarine Research Reserve System and the National Geodetic Survey to develop and share data about landscape–scale changes resulting from sea–level rise.


“Working together is the only way we will be able to compile information to help answer questions at multiple scales,” says Barnhill. “We are leveraging multiple agency efforts in order to sustain our natural landscapes.”


Nicole Rankin is a coastal ecologist based at Cape Romain National Wildlife Refuge, SC.