Wetlands, Marshes and Swamps

Tidal Creek in Salt Marsh-Rattlesnake Island
A tidal creek meanders through a salt marsh on Rattlesnake Island

NPS Photo

It is said that the only constant in nature is change. Perhaps no other ecosystem exemplifies this maxim better than the salt marsh. In northeast Florida, twice per day the salt marsh is inundated by waters pushed inland by the high tides, and twice per day those waters recede, exposing the marsh to relatively dry conditions. It is because salt marshes occur along the coast, in the zone between low and high tides, that they form a transitional plant community between dry uplands and open water. But their exposure to environmental extremes does not end there. They are also subject to both salt water from the ocean and fresh water from upland rivers and streams. In fact, they exist where these water sources mix in a brackish brine called the estuary.

Not many plants can survive in such a variety of extreme conditions, but there are a few that have been honed by natural selection into the hardy stalwarts of the salt marsh. Chief among these are species of cordgrass, interspersed with a rush called black rush. All these plants are hydrophytic emergents, meaning their extensive root systems can grow in saturated mud, while their green tops are able to withstand being submerged twice a day. They are able to deal with the salty water taken up by their roots by secreting the salt out through specialized glands on their leaves and stems.

Though once looked upon as wastelands ripe for draining and filling (over half the marshes in the United States have been converted for other uses- primarily development and agriculture), they are now recognized for the many benefits they provide. For example, marshes attenuate the energy of storms and help prevent inland areas from flooding. They are also one of nature’s best filtration systems. Cyanobacteria (bacteria that liberate oxygen through their metabolic process) exist in the marsh, and one of their chemical products is an enzyme (or organic catalyst) called hydrogenase. This enzyme helps break down dead matter, releasing the elements of nitrogen and sulphur as by-products, which can then be re-used by living plants. Likewise, the cyanobacteria also break down toxins into safer chemical components, thus making the environment an overall safer and cleaner place. (However, excessive levels of pollution can overwhelm and damage marshes.)

Surprisingly, even though the salt marsh is low in terms of plant species diversity, it is still an incredibly productive ecosystem. This is due to the nutrient-rich detritus, or the muck on the surface of the marsh. Most marsh plants are not consumed by animals, but rather die and fall to the marsh floor where they partially decompose and become mixed in with organic particles. This resulting detritus is spread throughout the marsh by tidal creeks, waves, and storms. Worms, fish, shrimp, crabs, fungi, and bacteria all eat the detritus, excrete the remains, and thus re-nourish the marsh.

Because of all decomposition going on, the marsh sometimes takes on a somewhat “organic” odor. This is aided and abetted by the hydric soils present- finely packed sediments that result in the chemical reduction of elements, meaning a loss of oxygen and the addition of hydrogen. Anaerobic (chemosynthetic) bacteria living in the mud then release hydrogen sulfide as they metabolize dead tissue. Thus the slight smell of rotten eggs which sometimes emanates from the marsh! But keep in mind, that is the smell of a healthy salt marsh doing what it naturally does.

Another beneficial aspect of salt marshes for both humans and nature is their role as a “nursery” for many species of young fish, shellfish, and crustaceans. (In fact, up to 70% of them.) Although larger fish are transient predators that can come into the marsh during times of high tide, they are excluded at all other times, giving the vulnerable young of many species time to hide and grow. Several of the seafood species which people love to eat absolutely rely on salt marshes when they are in their developmental stages, and would likely not survive to adulthood without them.

Can salt marshes come in handy during Scrabble? They can, if you remember they possess “aufwuchs”. Aufwuchs are communities of algae, bacteria, protozoa, and tiny animals that live on the stems of marsh plants. They trap and decompose particles swept in by the in-coming tides. The aufwuchs themselves are then preyed upon by periwinkle and olive nerite snails, grass shrimp, hermit crabs, and amphipods. Other animal communities in the marsh also filter out water-borne particles and contribute to water quality and clarity, including mussels, barnacles, byrozoans, clams, and particularly oysters.

More mobile animals can be seen in the marsh, such as fiddler crabs feeding on micro-algae during low tides. Larger blue crabs can crush the shells of their prey, though they also eat small fish and carrion. The sometimes copious amount of insect life in the marsh provides a reliable food source for both birds and fish. Herons and egrets are often seen stalking their prey of fish, shrimp, and crabs in the marsh. Sometimes heard but seldom seen are clapper rails that form hidden roosting areas on the marsh surface. Reptiles are not known for being marsh residents, with the possible exception of the diamondback terrapin turtle. Mammals adapted to the marsh include the marsh rice rat, the herbivorous marsh rabbit, and the omnivorous raccoon.

On the east coast of Florida, salt marshes are primarily found from Daytona Beach north. South of there more tropical mangrove forests begin to predominate. Fort Matanzas exists at the northern end of the transition zone where mangroves begin to reach their northern limit, but this “boundary” may creep farther north as the effects of climate change are felt. (Already the more cold intolerant red mangrove has joined the commonly found black mangrove in the salt marshes of the park.) About 75 acres of salt marsh are protected within the boundaries of Fort Matanzas, and it will be interesting to see how this vital biotic community responds in years to come to warmer conditions, higher tides, and more intense storms.

Last updated: April 14, 2015

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