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publications > paper > PP 1011 > ecosystems > freshwater and terrestrial > canals and lakes
Ecosystems of south Florida
Freshwater and terrestrial ecosystems
Canals and lakes
Lake Okeechobee is fed from the north mainly by the Kissimmee River and by a system of pumping stations along its south shore, a system which, during wet periods, pumps surplus water from the north part of the Everglades agricultural area into the lake. Generally, during dry periods, water is released from the lake to sustain the agricultural areas just south of the lake and to supply water to the urban lower east coast. When levels in the lake exceed scheduled elevations during some rainy seasons, releases are made through the St. Lucie Canal and the Caloosahatchee River (Klein and others, 1973).
Canals form an extensive network to distribute water and to discharge seasonal excesses into estuaries. Before the 1930's, canals were constructed primarily to provide the drainage and flood protection that were essential to land development. These canals were connected directly with the estuaries and during dry seasons often allowed saltwater to penetrate inland. The drought of 1943-45 created conditions that demonstrated the need for control structures to prevent the inland migration of saltwater, and such structures were subsequently placed in most canals near the coast.
Canals are biologically productive systems that support a variety of aquatic plants, animals, and microorganisms, many of which also thrive in ponds, sloughs, and marshes. Canals retain water throughout the year, whereas sloughs and marshes become dry for several months during most years. Canals, therefore, provide an environment for the year-round growth of aquatic plants and survival of aquatic organisms during drought. Fish and other aquatic animals retreat to canals as adjacent marshes dry.
The quality of canal water is often degraded by the inflow of pollutants, particularly in urban and agricultural areas. The pollutants may be transported in solution or associated with sediment. Biological uptake may be restricted because of poor light conditions, deep water (which hinders interaction with the bottom sediment), rapid water movement, and other factors.
Canals that drain agricultural or urban land often have relatively high concentrations of nutrients. These high concentrations in stagnant canals favor the growth of nuisance aquatic plants. Two troublesome plants, water hyacinth and hydrilla, often form dense stands that clog the canals, hinder the flow, and adversely affect the recreational use of the canals. Algae also degrade canals with dense blooms and scums, both of which detract from the recreational and aesthetic value of the canals.
Dense growth of aquatic plants may contribute to low concentrations of oxygen in canals. In addition to producing oxygen through photosynthesis, plants consume oxygen through respiration. Thus at night when photosynthesis ceases, plants deplete oxygen. Plant materials lower oxygen concentration when they are broken down by bacteria. Concentrations of oxygen tend to be low near the bottom of canals where dead plants and other organic materials settle and bacterial respiration is greatest. A large accumulation of dead plants, particularly under low-flow conditions, may result in anaerobic conditions throughout the canal. Floating plants reduce oxygen levels by blocking out the sunlight necessary for photosynthesis. Anaerobic conditions can result in the death of fish, noxious odors, and health hazards.
South Florida's lakes and canals provide recreation such as sport fishing and boating. Fish production abounds in these systems and their peripheral marshes during the wet season; fish are later concentrated in the canals and lakes as water levels decline and marshes dry.
The major freshwater commercial fishery in south Florida is in Lake Okeechobee. On the average more than 1.8 million kg (4 million lb) of catfish and turtles are taken annually, and a record 3.2 million kg (7 million lb) was taken in 1971 (Natl. Marine Fisheries Service, Miami, unpub. data). Review of the Okeechobee commercial fisheries suggests that this fishery is underexploited and that restrictions could be relaxed to allow a higher sustainable yield. The removal of more fish is not expected to adversely affect the sport fishery (Crowder, 1974g) and could aid the overall condition of the lake by removing nutrients (in the form of fish flesh) (Marshall and others, 1972).
Lake Okeechobee provides more than 140,000 fisherman trips annually (Ager, 1970). But the lake began in the early 1970's to exhibit increasingly noticeable eutrophication caused by the inflow of domestic, industrial, and agricultural wastes. Nutrient-enriched, highly mineralized water from about 283,000 ha (700,000 acres) of agricultural land either flows or is pumped into the lake and surrounding conservation areas. With extensive fishkills becoming common in the Okeechobee rim ditch because waters low in dissolved oxygen flowed into the lake (Crowder, 1974g), the State in 1973 funded the Special Project to Prevent the Eutrophication of Lake Okeechobee. The project was to conduct and integrate research aimed at turning around the processes which were degrading the lake with overenrichment (eutrophication). Preliminary conclusions, in a May 1975 interim report (Written communication. 1975. Florida Division of State Planning, interim report on the Special Project to Prevent the Eutrophication of Lake Okeechobee), acknowledged "the South Florida ecosystem is presently under a high degree of stress as a result of man's disordering of the system's evolved patterns of coping. . ." [italics from the report], but that "catastrophic trends appear to be reversible, provided effective management for optimum water storage and quality is implemented in the very near future."
A May 1976 draft report (Written communication, 1976. Curry Hutchinson, Dale D. Walker, Stephen E. Gatewood, and Rotha MacGill, Florida Division of State Planning) on management plans promises early publication of findings and recommendations which, it stated, "should be considered as one of the first comprehensive steps in the state's planning process to develop optimum management programs. State level planning," it continued, "will lead to the Water Use Plan in 1978, the Water Quality Plan in 1978, and ultimately the Florida Comprehensive Plan in 1979."
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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