At the Water's Edge: The Salt Marsh | Changing Seas

Salt Marshes: Ecology and Importance

Salt marshes develop along sheltered coastlines with soft sediment and limited wave action. Their tidal creeks and shifting mix of water and land support abundant wildlife, including commercially important shrimp, crabs, and fish, as well as wading birds and mud-dwelling organisms.

Marshes provide important ecological services: they filter nutrients and contaminants, reduce wave energy and storm impacts, and store carbon. Coastal wetlands can bury carbon at rates estimated in the program to be 10–100 times higher than upland forests.

Georgia has extensive marsh habitat, supported in part by its large tidal range, which averages about seven feet. Researchers have studied the region for decades, including through a long-term project monitoring marsh conditions and plant productivity.

Marsh Plants, Wildlife, and Carbon

Smooth cordgrass (Spartina alterniflora) is the dominant marsh grass along much of the Southeast and Gulf coasts. Its growth varies with freshwater flows and temperature: freshwater can reduce salt stress, while hot years can reduce plant growth.

Marsh invertebrates also affect vegetation and sediment:

• Periwinkle snails can damage plants.
• Fiddler-crab burrows improve the movement of nutrients through the soil.
• Marsh mussels filter water and add sediment and nutrients.

Scientists are investigating how carbon moves among marsh plants, animals, soil, water, and the atmosphere. Measurements show differences among tall, medium, and short cordgrass: taller forms can have less below-ground biomass than shorter forms. Researchers have also found evidence of below-ground biomass loss, which could weaken plant growth and the marsh’s ability to hold sediment.

Sediment and Sea-Level Rise

Georgia marshes are accumulating sediment at roughly 1–3 millimeters per year, while the average sea-level rise rate over the past 80 years was about 3.5 millimeters per year. Marsh elevation provides some buffer, but current sediment accumulation may not keep pace. Models cited in the program project rapid marsh loss by 2100.

Sea-level rise also threatens communities on barrier islands, where flooding along marsh edges is difficult to prevent.

Fish Research in Florida Salt Marshes

Research in Florida’s salt marshes focuses on several fish:

• Salt marsh topminnows prefer low-to-moderate salinity and marsh edges or tidal creeks. The Florida population appears stable, but researchers identified five genetically distinct groups in Pensacola Bay. Threats to each group need further study.
• Alligator gar tolerate fresh, brackish, and salt water. Researchers are tracking their movements to estimate populations and identify spawning rivers.
• Gulf sturgeon, federally threatened since 1991, migrate between freshwater spawning areas and marine habitats. Juveniles are strongly associated with low-salinity salt-marsh habitat, and poor survival during early life stages may be limiting population recovery.

Research Methods

Researchers use a range of methods to monitor marsh ecosystems and study fish:

• Conduct annual fall surveys at 10 Georgia marsh sites to monitor ecological change and plant productivity.
• Count marsh invertebrates and assess how they influence plants and sediment.
• Use a flux tower to measure environmental conditions and carbon exchange between the marsh and atmosphere.
• Collect monthly cordgrass cores, wash and dry plant material, and weigh it to estimate above- and below-ground biomass. Combine habitat maps and satellite observations to assess patterns across the marsh.
• Take sediment cores and analyze naturally occurring radionuclides to estimate sediment accumulation over roughly the past century.
• Survey topminnows at randomly selected sites using alternating trap types. Identify and count fish; record their length, reproductive status, and habitat; and take fin clips for DNA analysis.
• Capture alligator gar with large-mesh gill nets, collect biological data, and implant acoustic tags. Receivers distributed across Pensacola Bay detect tagged fish and track their movements.
• Use acoustic receivers to study Gulf sturgeon movements and identify habitats used by adults and juveniles.

Researchers and Organizations Featured

Researchers

• Dr. Merryl Alber — leads the Georgia Coastal Ecosystems Long-Term Ecological Research Project.
• Dr. John Teal — early salt-marsh ecologist who studied productivity around Sapelo Island beginning in the late 1950s.
• Dr. Clark Alexander — sedimentary geologist working with University of Georgia scientists and the City of Tybee Island on marsh sediment and flooding.
• Chelsea Miles-McBurney — studies salt marsh topminnows at the Garcon Point Lab.

Organizations and Research Teams

• University of Georgia Marine Institute
• Georgia Coastal Ecosystems Long-Term Ecological Research Project
• Florida Fish and Wildlife Research Institute, Garcon Point Lab
• U.S. Fish and Wildlife Service
• Florida Fish and Wildlife Conservation Commission
• City of Tybee Island
• A larger, multi-partner research team studying juvenile Gulf sturgeon