Ecology

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The Kissimmee Prairie lies within the Eastern Flatwoods District of Florida and is composed of nearly level Pleistocene marine terrace deposits (Webb, 1990). Typical of a marine terrace deposit, the substrate is generally composed of well-sorted fine quartz sands. These marine sands can be drought-prone, yet poorly drained due to the presence of an organic and/or calcareous pan which can impede vertical water movement (McCollum and Pendleton, 1971; Pettry et al., 1965).

In shallow surface waters, water chemistry is strongly influenced by soil composition (Wetzel, 1983). As water moves through the soil there is presumably an exchange of solutes. Marine terrace deposits in Florida constitute a geomorphic surface that favors the formation of the Spodosol soil order (Brown et al., 1990). Spodosols are typified by an eluvial horizon from which material has been leached. Below this layer is an illuvial horizon enriched in the leachate from above (McKeague et al., 1983). The illuvial portion of the soil profile is referred to as the spodic horizon. This horizon consists primarily of sequioxides of iron and aluminum, and organocomplexes (Allen and Fanning, 1983). The spodic horizon in Florida soils generally follows the contour of the ground and is related to water table depth (Pettry et al.1965; Yuan, 1966). Spodic horizon development is thought to be inhibited by conditions of permanent soil saturation (McKeague et al., 1983). As organic matter fills the interstitial areas in the soil, hydraulic conductivity can be significantly retarded. However, once the soil is saturated, hydraulic conductivity can be restored (Pettry et al., 1965). It is reasonable to expect the spodic horizon to influence pond hydrology and chemistry.

The major vegetation communities surrounding the ponds were wiregrass/palmetto prairie and wet prairie (Duever, unpublished report). The palmetto prairie area is found on Immokalee and Myakka fine sands. These are deep acidic sands with a spodic horizon at a depth of 70 to 90 cm (McCollum and Pendleton, 1971). Both soils are strongly acidic (pH < 5) to a depth of > 1.5 m. Wet prairie is an ecotone between the palmetto prairie and the marsh/pond communities. The spodic horizon can extend beneath the wet prairie to the edge of the ponds (Duever, unpublished report). The principal soil of the wet prairie is Charlotte fine sand. This soil is only slightly acidic at the surface, and neutral at a depth of 15 to 48 cm (McCollum and Pendleton, 1971). Below 48 cm the soil is mildly alkaline. Proximity of this mildly alkaline soil to pond water can have an effect on water pH. The marshes and ponds are underlain by white sands with varying layers of peat and muck on the surface. When water stands above the soil surface for a sufficient amount of time organic matter can accumulate due to the slower decomposition rates in the oxygen limited environment (Mitsch and Gosselink, 1993). Duever (unpublished report) found as much as 25 cm of peat in the centers of Kissimmee Prairie ponds. The larger ponds (> 1 ha) have firm sand bottoms without any organic accumulation in wide zones.
 

Vegetation

Ponds within the Kissimmee Prairie usually exhibited a concentric zonation of vegetation, presumably related to hydroperiod (Figure 2). In the three study ponds the vegetation grows to a density that hides the water from view. The center of most ponds is inhabited by a monotypic stand of Pontederia cordata (pickerelweed) (Figures 3 and 4). The pickerelweed zone is not well defined in pond 1. Surrounding this zone is a dense growth of Panicum hemitomon (maidencane) that grades into a zone of Hypericum fasciculatum (St. John's-wort) accompanied by Utricularia spp., Nymphoides aquatica, Eleocharis spp., Carex spp., Ranunculus spp., and Rhynchospora spp. Dense mats of filamentous green algae form within the pond as water levels recede and vascular vegetation senesces, releasing nutrients.

In the wet prairie transitional zone between the pond and the wiregrass/palmetto prairie is the vegetation community comprised of Drosera spp., Sphagnum spp., Eriocaulon spp., Dichromena spp., Xyris spp., Aristida beyrichiana, Carex spp., Polygonum spp., Rhyncospora spp. etc. Some ponds have a crescent of woody vegetation at the edge such as Persea spp., Myrica cerifera, and Quercus spp. This crescent marks a fire-shadow indicating the direction of the last fire to move through the area (Paul Gray, personal communication). Hypericum fasciculatum and Panicum hemitomon are fire tolerant (VanArman and Goodrick, 1979; Vogl, 1973). Kirkman et al. (1998) found higher species richness for plants in the transitional zone between the longleaf pine/wiregrass community and small seasonal ponds. Winchester et al. (1985) studied the vegetation and substrate of 24 shallow ponds and wet prairie areas in southwestern Florida and found a similar zonation. Several ponds they studied had an accumulation of peat in the center. They believed that fire played an important role in the maintenance of vegetation zonation. They found that the St. John's-wort zone consisted of an inner zone of dead plants during conditions of higher water. This dead vegetation zone was evident in the Kissimmee Prairie ponds after several years of wet conditions. LaClaire (1995) studied the vegetation of 13 seasonal ponds in north, and north-central Florida. She found hydrologic zonation in each pond, and noted the presence of fire-shadows.

The plant species LaClaire (1995) found were also common in the KPS ponds. She made no mention of matting by filamentous algae. She reported that species diversity was higher in ponds that had recently dried out. Greening and Gerritsen (1987) monitored changes in four marsh plant communities under different hydrologic regimes in the Okefenokee Swamp. They found that the highest macrophyte diversity and greatest variability in biomass dynamics were correlated with unpredictable drawdowns. Aquatic vegetation can affect the thermal regime of littoral communities through shading, and reduced water movements (Kushlan and Hunt, 1979), which can lead to thermal stratification. In addition, high water color can increase the rate of heating of shallow water exposed to sunlight (Wetzel, 1985).

Water Chemistry

In a wetland or lake, water chemistry represents the result of a series of physicochemical and biological processes that determine the concentrations of solutes within the body of water (Dunson et al., 1997). Factors such as pH and specific conductance have an influence on the organisms inhabiting a body of water. For example, due to its greater tolerance of low pH, the pine woods treefrog (Hyla femoralis) out-competes the barking treefrog (Hyla gratiosa) in acidic ponds, in spite of its smaller size (Warner et al., 1993). Calcium ion is important in maintaining low epithelial permeability in fish (Dunson et al., 1997). Alkalinity determines the resistance of water to changes in pH (Wetzel, 1983). Nutrient concentrations influence the species composition of the plant communities inhabiting a body of water.

Organic staining is a variable property of Florida lakes (Shannon and Brezonik, 1972). Water color can be produced in the water and/or imported via drainage, and is considered to be an important component of the metabolism of a body of water (Rich and Wetzel, 1978). Tannins and lignins can inhibit microbial decomposition, especially when combined with low pH (Rich and Wetzel, 1978). The ratios of nitrogen and phosphorus to carbon in aquatic humus are highly variable (Gjessing, 1976; Wetzel, 1983). Humic substances are thought to have a net negative charge, and possess amphiprotic properties, becoming positively charged at low pH (Gjessing, 1976). Humic material has an affinity for mercury and can reduce metals (Zimmerman, 1981). When complexed with aluminum, organic matter is thought to reduce the surface tension of water and thus affect gaseous diffusion and invertebrates that drift in or stride on the water (Hall et al., 1985).

Dissolved organic substances are a source of reduced carbon, and thus energy. Cummins et al. (1972) reported that the microbial community in an artificial lotic system was able to rapidly process leaf leachate. Ultraviolet light breaks apart the more refractory humic compounds, making them available to the biota (Suberkropp et al., 1976; Rich and Wetzel, 1978). Ultraviolet light reduces the color of water, but not necessarily the organic carbon content (Manny et al. 1971). Ultraviolet light liberates nitrite and nitrate from ringed carbon structures. Ultraviolet light can photo-oxidize dissolved organic matter to produce carbon monoxide and carbon dioxide (Zuo and Jones, 1997).

Klosowski (1992) found that some chemical characteristics of water in a littoral zone in a lake in Poland varied significantly within different vegetation zones. For example, pH varied significantly between zones but not within each zone. Klosowski reported (1992) that pH was lowest near shore and increased with water depth. Concentrations of orthophosphate and nitrate decreased towards open water. Dunson et al. (1997) studied the water chemistry and occurrence of fish in southwestern Florida pine flatwoods ponds and found many of the ponds contained soft water. They suggested that differing specific conductances within the ponds were a result of differing degrees of isolation of ponds from surface water inputs.

Nutrient concentrations in pollution-free areas of the Everglades are very low. For example, orthophosphate concentrations are generally expected to be less than 5 mg/l, and nitrate less than 100 mg/l in these waters (Doren et al. 1997). Invasion of wet prairie by Typha spp. is correlated with increases in nutrients, particularly phosphorus (Kushlan, 1990; Gunderson, 1994). In unpolluted wetlands, soil contact is probably the major source of nitrogen and phosphorus for vascular plant growth (Davis, 1991; Rattray et al., 1991). Sequences of drought and flooding provide a greater input of nitrogen and phosphorus to rooted vegetation than is available under continuously flooded conditions (Bayley et al., 1985).

Literature Cited