My parents used to own a beachfront condo on Harbor Island, South Carolina. One year, I was surprised to find a freshwater marsh in front of the condo, complete with cattails, frogs, and red-winged blackbirds. The marsh was located behind a beach dune where it did not exist just a year earlier. This type of environment is known as an interdunal wetland, and they can form rapidly. The East Beach freshwater marsh on St. Simon’s Island formed in just 2 months. Rain washes beach wrack (dead plants and detritus) into the swales between dunes, creating enough topsoil for freshwater species of aquatic plants to take root. Heavy rains keep the marsh wet, and the dunes prevent drainage. The water table at sea level is often close to the surface, and because fresh water is lighter than sea water, it remains on top, providing habitat for fresh water species.
Whitney Lake on Cumberland Island, Georgia. Note the alligators. It’s a freshwater interdunal wetland with an outlet to the ocean. Storm surges of saltwater that kill woody plants keep it open. The state of Georgia has the least developed coast on the Atlantic side of North America. Most of Georgia’s barrier islands are not accessible via automobile.
Aerial photo of St. Catherine’s Island, Georgia. The brown represents upland maritime forests of live oak, loblolly pine, and palm trees. The green represents freshwater, brackish, and saltwater marshes. About 40,000 years ago sea level was near present day sea level and some of this island consists of sediment accumulated then. Then, as the ocean receded, that sediment underlay oak hammocks surrounded by miles of grassy savannahs. About 6,000 years ago, sea level rose to this approximate location again and sediment is currently accumulating and building on the old Pleistocene sediment.
Interdunal wetlands located behind beachfront dunes may form rapidly, but they can also be short-lived. Storm surges can breach beach dunes, resulting in an influx of salt water, followed by drainage. However, storm surges may play a role in maintaining older freshwater marshes located in the middle of a barrier island well away from the beach. The influx of salt water kills shrubs and trees. Eventually, rainwater reduces salinity and shade-intolerant aquatic vegetation becomes re-established.
Feral horse drinking water from Whitney Lake on Cumberland Island. Interdunal freshwater ponds acted as an oasis for Pleistocene megafauna surrounded by unpotable ocean water and salt marshes. Horses and hogs run wild on some of Georgia’s barrier islands today. The presence of megafauna on barrier islands during the Pleistocene wasn’t a problem because natural predators controlled their populations. Today, large herbivores can overgraze vegetation and cause harmful erosion unless human hunters or wranglers take action.
Freshwater ponds and marshes on barrier islands are like an oasis surrounded by undrinkable ocean water and vast salt marshes. During the Pleistocene these sources of fresh water would have attracted every species of megafauna living on a barrier island. Giant ground sloths, mammoths, mastodons, bison, horses, tapirs, llamas, peccaries, deer, capybara, and giant beaver were all drawn to the drinkable water and highly edible plants that grew in the marshes. Predators, such as dire wolves and big cats, would have lain in wait at these water holes. The location of interdunal marshes may explain the abundance of Pleistocene fossil sites in this region. There are 9 above sea level, Pleistocene-aged fossil sites near the Georgia coast (Fossilosa, Isle of Hope, Mayfair, Porter’s Pit, Savannah River dredgings, Turtle River dredgings, Clark’s Quarry, and Watkin’s Quarry) compared to 3 sites in the Ridge and Valley Region and just 1 site in the Piedmont Region. Animals attracted to freshwater marshes occasionally died there and were buried by sediment carried by storm surges. This combination of factors explains why there are more Pleistocene fossils found near the coast than anywhere else in the state, though there are other depositional origins for some of the sites here as well.
Today, megafaunal species living on Georgia’s barrier islands include white-tail deer, feral horses and hogs, some exotic species of introduced deer, and alligators. Paleontologists have noticed the relative lack of alligator fossils found in the Pleistocene sites near the Georgia coast, where they would be expected to be abundant. Apparently, alligators did range here during the Pleistocene, but it may be that egg-eating mammals kept their populations in check. Jaguars probably directly preyed upon adults. Interdunal wetlands also host marsh rabbits, raccoons, mink, otters, wading birds, ducks, and songbirds that would otherwise be absent due to the lack of freshwater.
Mosquito fish (Gambusia holbrooki). It’s well adapted for life in interdunal wetlands. They are related to those tropical aquarium favorites–guppies.
Killifish, mosquito fish, top-nosed minnows, mullet, and gars thrive in interdunal ponds. Mosquito fish are well adapted for this type of environment. They eat their own weight in mosquito larva everyday, giving them the energy to reproduce rapidly–a population of 7,000 can increase to 120,000 is just 5 months. This allows them to quickly colonize newly created marshes and improves their chances of survival during storm surges. Mosquito fish can endure high temperatures and high levels of salinity that would prove fatal to most other species of fresh water fish. Bream are also found in some interdunal ponds, but scientists don’t know whether they occur naturally or were introduced by man.
Botanists recognize 4 successional stages of interdunal wetlands. The first is an open water stage with floating plants such as duckweed and emergent vegetation including cattails, pickerel weed, and arrow-arum. The second stage has less open water and is dominated by cattails and pickerel weed. The third stage is a grassy shrub stage with saw grass (of Everglades fame), cordgrass, sedges, buttonbush, rose mallow, and wax myrtle. The final stage is a woods of red maple, Carolina willow, tupelo, and water oak. A storm surge of saltwater can kill the trees and return the marsh to the open water stage. Below are some photos of some shrubs commonly found in interdunal wetlands.
Buttonbush (Cephalanthus occidentalis) is common on the edges of interdunal wetlands. A study of fossil mastodon dung found buttonbush to be a common item in their diet in Florida. Mastodon foraging faciliated the spread of the seed-filled buttonballs which float. A mastodon tearing apart a buttonbush was beneficial to the plant species’s long term survival.
Carolina willow (Salix caroliniana) with egret nests.
Wax myrtle (Myrica cerifera). The wax rendered from the berries is used to make candles. Euell Gibbons states that the leaves can be used for seasoning soups and stews, like bay leaves.
Rose mallow (Hibiscus moscheutos), a common and beautiful plant found growing in interdunal wetlands. The roots were used to make marshmallows. All modern marshmallows sold in grocery stores are made artificially without mallow roots.
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