Written by Taylor Shook (excerpted from 2018 Conservationist)
Georgia’s tidal marshes, more numerous than those of any other state on the East Coast, are valued as wildlife habitat and for the crucial role they play in reducing flooding and protecting against coastal erosion. Its salt marshes, covering an estimated 378,000 acres, are known for their quiet beauty and claim most of the glory. Yet, the state’s freshwater and brackish marshes, though much smaller in number, are equally important and deserving of conservation protection. They also can be equally beautiful. Tidal flow is the most important environmental factor shaping tidal marshes. Incoming tides deliver nutrients, planktonic organisms, and various other materials to the marsh, while outgoing tides remove detritus, planktonic organisms, and assorted others from it. The bidirectional flow of materials is a large part of why tidal marshes are so prolific.
Although salt, brackish and freshwater tidal marshes differ significantly based on their level of salinity, they are all highly productive systems. Research has shown that both salt and brackish marshes are among the most fertile ecosystems in the world. Although there are numerous studies proving the productivity of salt and brackish marshes, freshwater marshes are equally productive or even more fruitful than the salty marshes downstream. Even though the research done on tidal freshwater marsh is not as extensive as its salty counterpart, the above-ground primary production is overall comparable among them.
In fact, Georgia contains 47,000 acres of freshwater tidal marsh. These marshes occur as a result of the interaction between river waters and tidal influences from the ocean. Tide has an effect as far as 45 miles upriver in the Savannah River. Although tides dominate the rise and fall of the water in freshwater tidal marshes, the wetlands themselves are not salty except in the case of extreme drought.
Salinity is a stressor for most plants. Vascular plant diversity in salt marshes is remarkably low, being almost entirely a monoculture of smooth cordgrass. Smooth cordgrass, Spartina alterniflora, has two competitive advantages; being salt-tolerant and flood-tolerant. Smooth cordgrass would prefer to grow in low-salinity conditions but does not compete well against other plants in these environments. Saltwort, glasswort, and salt grass dominate smooth cordgrass in areas of very
In comparison, the lack of salt stress in freshwater marsh allows for a greater diversity of plants to thrive. As many as 60 plant species can be found at a single location in freshwater marsh, and the vegetation varies from season to season. Research has shown that there is an increase in plant species richness, height, and nitrogen content associated with a decrease in salinity from salt to brackish to fresh tidal marsh sites.
Animal diversity, on the other hand, will increase from the most part from fresh to brackish to saltier sites. Benthic algal production (source of food, energy, and cover for many organisms) is much higher in saltier sites compared to freshwater, resulting in a higher diversity of fish and shellfish than in fresher sites. But the diversity of reptiles, amphibians, waterfowl, and furbearer species increases with a decrease in salinity. The increase in diversity of these groups of animals most likely increases with decreased salinity due to the higher variety of plants and food value.
Freshwater tidal marshes are relatively rare natural communities in Georgia because much of the area has been lost to saltwater intrusion, hydrologic changes due to development and agriculture, and other landscape modifications. Rising sea levels due to climate change will eventually impact the distribution of all types of tidal marshes. Sea-level rise is currently advancing salinity gradients upstream, leading to shifts in vegetation composition and the conversion of some marshes from freshwater to brackish and, eventually, to salt.
Because of their productivity, rarity, and vulnerability, Georgia’s freshwater and brackish marshes deserve equal, if not greater, recognition of their importance in the overall goal of protecting Georgia’s tidal marshes for present and future generations.