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publications > paper > PP 1011 > ecosystems > freshwater and terrestrial > ponds and sloughs
Ecosystems of south Florida
Freshwater and terrestrial ecosystems
Ponds and sloughs
Depressions occupied by ponds and sloughs may be caused by solution of the limestone by organic acids. Solution holes of the coastal ridge, as deep as 4.5 m (15 ft), were formed during the last ice age when sea level was much lower than the present level and water percolated downward to dissolve areas of limestone (Craighead, 1971).
If organic matter accumulates in ponds and sloughs faster than it is removed or faster than the underlying bedrock is dissolved, depressions will be filled. Under these conditions the general successional trend is from pond to marsh to swamp to hardwood hammock (fig. 8). Various processes may slow or change this succession. Fires during the dry season, for example, may remove accumulated sediment and deepen ponds and sloughs. Alligators also help check this succession by digging and removing sediment and plants from ponds (Craighead, 1971).
Sloughs are important watercourses in south Florida. They deliver freshwater to bays and estuaries, a function similar to that of rivers in other areas. Flow through a slough, however, is slower than through a river. Because of this, water is subject to evapotranspiration and chemical changes for longer periods in a slough than in a river. Also, the shallowness and biological character of sloughs favor alteration of water quality before the water enters the estuarine environment. Some processes of alteration are particularly important in maintaining good quality water in the estuaries. Removal of pollutants and excess nutrients as water moves through the slough, for example, is necessary to prevent degradation of the receiving estuaries.
Life in ponds and sloughs is a diverse assemblage of aquatic plants, animals, and microorganisms. A conspicuous component of the system is the emergent marsh vegetation such as cattail, arrowhead, pickerelweed, fire flag, water rush, and spikerush. Other important vascular plants include immersed species, such as bladderwort and niads, and floating species such as waterlettuce, duckweed, and water hyacinth. Characteristic nonvascular plants include green and blue-green algae, flagellates, and diatoms.
Periphyton, an assemblage of algae, other microorganisms, and small animals, is an important component of the pond-slough and the wet prairie systems. It thrives in shallow sunlit water where it adheres to a substrate or floats. It often forms dense calcareous mats several inches thick that are attached to the bottom or to vascular plants. Because of its concentration of microorganisms, which require oxygen and other nutrients, periphyton has a pronounced effect on water chemistry. Filamentous algae, a major group in the periphyton mat, release oxygen through photosynthesis and precipitate calcite.
The larger, predatory animals that feed in ponds and sloughs require a large biomass of smaller aquatic animals to sustain them, The production of this biomass is related to the extent of water inundation. The availability of the biomass, on the other hand, is related to seasonal water-level fluctuations. Small animals live and reproduce over a wide area of wetland during high water. As water levels recede in the dry season, these small animals are forced into the remaining deep slough and pond areas that still contain water. Alligators, wading birds, and otters congregate at the sloughs and ponds at this time to feed. Wading birds often feed in numerous ponds scattered over many square kilometers. In some species, such as the wood ibis, reproductive success is related to the availability of food in the dry season.
During the dry season, ponds and sloughs serve as feeding areas and survival holes. Because predators are not completely efficient, some small aquatic animals, which would die without water, survive and are able to repopulate the wetlands when water levels rise.
The pond-slough ecosystem has been altered by drainage, particularly in the northern Everglades; about 2,530 ha (6,300 acres) have been destroyed in Broward County alone (Birnhak and Crowder, 1974). In that part of the system that remains, the period of inundation has been altered by drainage or water impoundment. Perhaps the best indication of change is seen in the populations of index species; today there are about 150,000 wading birds, but there were 2.5 million in south Florida in 1870 (Crowder, 1974c; Natl. Audubon Soc., 1973).
U.S. Department of the Interior, U.S. Geological Survey
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Last updated: 04 September, 2013 @ 02:04 PM (KP)
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