Freshwater Marshes: Community Description

Paynes Prairie Aerial

Freshwater marshes in the southeastern coastal plain can be found where local topography and impermeable soils prevent rapid runoff or infiltration, often in association with rivers and lakes or in basins or other depressions (Kushlan 1990). Freshwater marshes are most extensive in Florida (Frost 1995). Tidal freshwater marshes occur along the Atlantic and Gulf coasts in association with rivers just inland of salt and brackish marshes where tides cause rivers to back up and flood adjacent lands. These freshwater tidal marshes cover approximately 2 million acres in the southeast (Wade et al. 2000). Freshwater marsh systems in Florida cover many more acres than that covered by tidal freshwater marshes. (See also Isolated Herbaceous Wetlands for a discussion of smaller marshes imbedded within fire maintained landscapes.)

Freshwater marshes can be found in association with a variety of plant communities. Freshwater marshes that occur in large basins, often formerly occupied by lakes, may be associated with a variety of upland communities. Marshes within lake or river floodplains may be associated with various riverine/lake communities (FNAI 1990). Freshwater tidal marshes often grade into freshwater tidal swamp along a gradient going inland, and into brackish or salt marshes along a seaward gradient. Freshwater tidal marshes can also be bordered by various peatland or upland communities including fire maintained communities (Schafale and Weakley 1990).

Climate of freshwater marshes are variable ranging from temperate in the freshwater tidal marshes of the Atlantic coast to subtropical in the marshes of the Everglades where seasons tend to be as related to moisture regimes as to temperature. Seasonal changes related to temperature affect seasonal variation in plant abundance in some tidal freshwater marshes (Odum et al. 1984). Rainfall patterns also affect hydrologic patterns in marshes.

Soils of marshes can be organic, marls, or mineral soils depending on environmental variables and soil building processes, however, organic soils are common. Peat accumulates in deep, long hydroperiod marshes with little drying. Peats can be neutral or acidic depending on the source of plant material (Kushlan 1990). Marl accumulates under moderate hydroperiods and seasonal drying that oxidizes organic matter and where periphyton is present (an algal mat that precipitates carbonates as a byproduct of metabolic processes). Sand substrates occur where short hydroperiods and extensive seasonal drying do not favor organic matter accumulation (Kushlan 1990). Marsh soils other than peats tend to be neutral or slightly basic (Kushlan 1990). Tidal freshwater marshes have soils that are high in organic matter; however, some soils may be classified as mineral. The sedimentation process is important in marshes that regularly flood (Schafale and Weakley 1990). Where sedimentation rates are high, deposited clays and silts result in the development of mudflats. Irregularly flooded marshes tend to accumulate organic matter as plant matter decomposes. Organic matter build-up is favored where sediments accumulate in part due to low flushing rates. Deposits of peat occur in high marsh in association with some plant communities such as cattail (Odum 1984). Tidal freshwater marshes tend to have neutral or only slightly acidic soils (pH 6.0 to 6.5) (Odum 1984).

Though the hydrology of marshes is variable, periods of inundation sufficient to limit encroachment of many species of woody plants, relatively shallow water, and water level fluctuation are characteristic. Fire, however, also plays an important role in determining species composition. The frequency and duration of inundation influences plant communities within marshes (Kushlan 1990, Lowe 1986). Most non-tidal marshes experience seasonal fluctuation of water. This fluctuation is pronounced and relatively predictable in South Florida where rainfall is distinctly seasonal (See Everglades). On a gradient northward, rainfall becomes more evenly distributed throughout the year, thus water levels in marshes tend to be influenced by evapotranspiration and patterns of precipitation. Alternatively, tidal marshes experience daily fluctuation in water levels associated with tides, but generally have more stable water levels than inland marshes in terms of seasonal fluctuations (Mitsch and Gosselink 1983). Among tidal marshes, freshwater marshes include truly freshwater marshes with salinities less than 0.5 ppt and oligohaline marshes, those with salinities 0.5ppt to 5ppt (Odum et al. 1984).

Water quality of marshes depends on the water source of the marsh. Marshes that receive most water from rainfall or shallow groundwater tend to be low in nutrients (e.g.: Everglades, peripheral marshes away from river channels). Marshes receiving input from rivers or other surface waters tend to have higher nutrient levels (Kushlan 1990). Tidal marshes studied by Odum et al. (1984)

Productivity of marshes is variable. Eutrophic freshwater tidal marshes have high levels of biomass and net primary production (1000 to 3500 g/m²/yr) (Odum et al. 1984). However other marshes have lower values for production: flag marshes in Florida may only reach 320 - 688 g/m²/yr and some short hydroperiod marshes may only reach 150 g/m²/yr (reference within Kushlan 1990).

Vegetation of Non-tidal Marshes

Various plant associations occur in non-tidal freshwater marshes depending on hydroperiod, fire frequency and organic matter accumulation. Long hydroperiod marshes (flooding greater than 9 months per year) tend to be dominated by submerged and floating leaf herbaceous plants. These associations seldom dry completely. Representative species include naiad (Najas spp.), bladderwort (Utricularia spp.) and others.

Moderate hydroperiod marshes (flooded 6-9 months per year) can be dominated by broad-leaved herbaceous plants, often collectively called flag marshes, by sawgrass or by cattail. Flag marshes are often dominated by pickerelweed (Pontederia spp.), arrowhead (Sagittaria latifolia), spikerush (Eleocharis spp.), maidencane (Panicum hemitomon), and fire flag (Thalia geniculata). Cattail marshes and sawgrass marshes are often species poor and nearly monospecific. (See also Everglades for a discussion of sawgrass marshes). Cattail tends to grow on more nutrient rich sites than sawgrass. (Kushlan 1990)

Short hydroperiod marshes, sometimes called wet prairie, (flooded less than 6 months per year) are the most diverse of marsh associations. Maidencane, cordgrass (Spartina bakeri), beakrush (Rhynchospora spp.) among other marsh species are characteristic (Kushlan 1990). Wet prairies with their short hydroperiod may also support species often characteristic of uplands such as wiregrass (Aristida spp.), and toothache grass (Ctenium aromaticum) (FNAI 1990).

Vegetation of Tidal Marshes

Tidal freshwater marshes differ somewhat from inland marshes in species composition and plant groupings. Species composition differs on the Gulf and Atlantic coasts and along the Atlantic coast with latitude. Atlantic tidal freshwater marshes exhibit some zonation related to water depth. Common plants going from deeper water towards high marsh include spatterdock (Nuphar luteum), arrow arum (Peltandra virginica), pickerelweed (Pontederia cordata), wild rice (Zizania aquatica), cattail (Typha spp.), giant cutgrass (Zizaniopsis miliaceae), smartweed/tearthumb (Polygonum spp.), jewelweed (Impatiens capensis), burmarigold (Bidens spp.) rose mallow (Hibiscus spp.), and big cordgrass (Spartina cynosuroides) (Odum et al 1984). Along the Gulf coast arrowhead (Sagittaria spp.) replaces arrow arum and pickrelweed in low marshes and maidencane replaces many of the perennial species (Mitsch and Gosselink 1986). Other dominants in Gulf coast marshes include Eleocharis spp. and alligator weed (Alternanthera philoxeroides).

The vegetation of freshwater tidal marshes varies not only spatially within the marsh but also temporally over the course of the seasons. Mudflats are prominent in intertidal areas in winter; broadleaf plants emerge in spring but are overtopped by grasses such as wild rice and giant cutgrass by mid summer. The diversity of the marsh reaches its peak by summers end. Frosts kill vegetation, tidal flushing and decomposition remove vegetation renewing the cycle (Odum et al. 1984).

Exotic invasive plants are known to invade marshes. Several problematic plants include giant reed (Arundo donax), wild taro (Colocasia esculenta), and Chinese tallow, (Sapium sebiferum) (See exotic plants and fire).

Fauna of Marshes

Round Tailed Muskrat Nest

Conspicuous invertebrates of freshwater marshes include diptera (flies, mosquitoes). Dragonflies and mayflies are also diverse. Prawns, crayfish and various snails are also common. Insect communities associated with marsh plants are also present (often herbivores). Less conspicuous inhabitants include zooplankton and benthic invertebrates (Kushlan 1990, Odum et al. 1984, Mitsch and Gosselink 1986). Food chains in marshes are thought to be detritial based, however, little is known of invertebrate detritivores (Kushlan 1990, Odum et al. 1983, Mitsch and Gosselink 1986).

Marshes are important habitat for many fishes. Tidal freshwater marshes have fish populations dominated by freshwater species although estuarine and anadromous fishes also use freshwater tidal marshes. The most common fishes include cyprinids (minnows, shiners, and carp), centrarchids (sunfish, crappie and bass), and ictalurids (catfish). Freshwater tidal marshes are used as primary habitat by some fishes, as nursery grounds by others and as spawning grounds for yet others (Odem et al. 1984). Because water levels are not as stable in inland marshes as tidal marshes (having seasonal draw downs) size and relative abundance of fishes in inland marshes varies with hydropattern. In dry periods, fish depend on deeper water refugia to survive and repopulate the marsh. Small fish dominate in low water. Mosquito fishes (Gambusia affinis) are notably abundant. As water levels rise, fish production also rises. In periods of high, stable water, larger fishes such as centrarchids, gar (Lepisosteus platyrhincus), and catfish dominate (Kushlan 1990 and reference therein).

Numerous species of reptiles and amphibians also use marshes. Salamanders are generally rare or uncommon. Newts and sirens are present. Various species of frogs and toads are common. Freshwater turtles use marshes extensively. Water snakes (Nerodia spp.) are particularly common. Lizards are the least common reptile in marshes with the exception of the American alligator. The Federally listed American alligator serves an important ecological role in inland marshes that draw down. They excavate gator holes that serve as dry season habitat for aquatic organisms.

Marshes are used extensively by a variety of birds. Shorebirds utilize mudflats, wading birds use areas of shallow water, waterfowl are found in open water areas, densely vegetated marsh provides habitat for insectivorous and seed eating passerines, shrubs and trees are utilized by arboreal birds, and birds of prey utilize the entire marsh. Some birds are dependent on marsh habitat: bitterns, herons, egrets, rails, ibis, marsh wren, common yellowthroat, and red winged blackbird (Odum et al. 1983). Large populations of migrating waterfowl utilize Gulf coastal marshes (Gosselink 1984, Chabreck et al. 1989). Waterfowl use of marshes is less extensive in Florida where they use estuarine areas. Several endangered birds are known from marshes including the snail kite (Rostrhamus sociabilis plumbeus), wood stork (Mycteria americana), and Cape Sabal seaside sparrow (Ammodramus maritimus mirabilis).

Mammals that use marshes include common mammals of other habitats and mammals that specialize in non-forested wetlands. Common species include marsh rabbits (Sylvilagus palustris), opossum (Didelphis virginiana), raccoon (Procyon lotor), beaver (Castor canadensis), marsh rice rat (Oryzomys palustris), mink (Mustela vison), and white tailed deer (Odocoileus virginianus). Muskrats (Ondatra zibethica) are common in Gulf coastal and northern Atlantic coastal marshes but are largely absent in Southeastern Atlantic coastal marshes and in Florida. In Florida, the muskrat is replaced by the ecologically similar round tailed muskrat (Neofiber allenii). The introduced nutria (Myocastor coypus), is an important herbivore in Gulf coastal marshes (Odum et al. 1984, Kushlan 1990, Gosselink 1984).

For more in-depth reviews of Marshes the reader is referred to (Odum et al. 1984, Kushlan 1990, Gosselink 1984).