In addition to being surrounded by water on three sides, Florida is also home to some 10,400 freshwater lakes, many of which are located in northern Florida. Of these, more than 7800 are larger than 0.4 hectares, covering a total of 9270 square kilometers, more than six-percent of Florida’s landscape (Myers & Ewel, 1990). Lakes are a common feature of the landscape in some areas of Florida due in part to the abundance of rainfall and the flat irregular surface that characterizes the State. Many of Florida’s lakes are highly diverse in their flora and fauna; for example, approximately 40 species of native fishes and 20 species of nonnative fishes inhabit these systems. Florida lakes are unusual in that underground tunnels often connect them; however, they are not as "systemic" as riverine and canal systems (Alden et al., 1998). Because of this, invasive exotics have not been as successful invading lake ecosystems. In addition to fish, many different species of invertebrates, amphibians, reptiles, and water birds may be found in association with these lakes. Lakes not only harbor great numbers of plants and animals, but they also they also mitigate the surrounding microclimate. Extended, gradual heat release by lakes helps protect surrounding crops from freezing.
"Freshwater life zones", as they are often referred to, have dissolved salt concentrations less than one-percent (Miller, 2000). The rainfall that provides for these lakes also contributes to new lake formation. Each year new lakes form as Florida’s naturally acidic rain percolates down through the soil, slowly dissolving the underlying limestone bedrock (Alden et al, 1998). Aquifer drawdown also leads to sinkhole formation, followed by water filling. Water bodies formed in this way are known as seepage lakes. Although most Florida lakes are seepage lakes, some are anthropogenic in origin. These include quarry pits, large storm-water retention ponds, and basins resulting from phosphate mining (Myers & Ewel, 1990).
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Odum, Odum, and Brown (1998) contend that many Florida lakes "…are as shallow ponds." This is due to a somewhat unclear difference between the definition of lakes and ponds (Horne & Goldman, 1994). Some definitions focus on the presence of higher aquatic plants, while others use mixing as a defining characteristic. Miller (2000) stated "…large natural bodies of standing freshwater- formed when precipitation, runoff, or ground water seepage fills depressions in the Earth’s surface- are called lakes." Often integrated with that definition is the presence of a barren wave-swept shore denoting the body as a lake. Although lake designation is somewhat unclear, lake formation is a better known process. Over millions of years, rainwater has percolated through wetlands, dissolving underlying limestone, thereby creating water-filled depressions and sinkholes. Some lakes also formed among dunes when seas retreated (Odum et al., 1998).
The slow process of dissolution is the reason that Florida’s lakes are shallow. In fact, three-quarters of Florida’s lakes are less than five meters (m) in depth, including Lake Okeechobee, the second largest lake by surface area in the lower 48 states (Florida Conservation Foundation, 1993). Collier County in South Florida contains one of the State’s deepest lakes, Deep Lake, at 29 meters. A large surface area, coupled with year-round warm temperatures, results in high evaporation rates from Florida lakes. That which does not evaporate percolates downward into the aquifer (Myers & Ewel, 1990). Typically unstratified, shallow lakes can be highly productive (Myers & Ewel, 1990). An unstratified lake is subject to mixing by wind, distributing nutrients throughout the water column. An example of this is Lake Tohopekaliga, the waters of which are less than six inches deep, and yet contain an estimated 20 million small fishes and other animals per acre (Florida Conservation Foundation, 1993). Deeper lakes undergo thermal stratification in spring and summer months; the warm upper region is called the epilimnion, while the deeper, colder region is the hypolimnion.
Florida lakes are fed primarily from rainfall and from streams that drain the surrounding watershed; they receive nutrients from these same sources. Nutrient levels, along with the associated levels of primary production, are used to classify individual lakes as eutrophic, mesotrophic, of oligotrophic (Miller, 2000). The natural process of gradual nutrient accumulation is called eutrophication. A lake that is high in nutrients is eutrophic. In contrast, low nutrient levels are oligotrophic. Most lakes in Florida are eutrophic due in large part to farm runoff, wastewater release, and industrial discharges (Odum, et al., 1998). Runoff from adjacent ecosystems supplies lakes with both nutrients and organic matter-often in overabundance. Nutrients are optimal when in particular concentrations: the Redfield Ratio of 116 parts carbon: 16 parts nitrogen: 1 part phosphorus is often used as a measure of optimality and to identify nutrient limitation. For example, the lakes on the FGCU campus have N:P ratios of approximately 30, suggesting that phosphorous is the nutrient that most limits aquatic plant growth.
Lakes have three groups of primary producers: phytoplankton, benthic algae, and emergent plants. These producers provide food, refuge, oxygen, and protection for zooplankton, other invertebrates, fishes, amphibians, reptiles, and wading birds (Odum et al., 1998). Emergent plants also act as sponges for incoming runoff and the nutrients it brings. Some emergent plants such as water hyacinth (Eichhornia) and hydrilla may actually alter temperatures by blocking out the sun, alter mixing by blocking the wind, and alter gas exchange by reducing surface area. A few common plants found in Floridian lakes are listed below.
Freshwater covers a mere one-percent of the Earth’s surface, however it contains 41% of known fish species (Miller, 2000). Fishes found in lakes on the Florida Gulf Coast University campus are are provded in the species list, as are reptiles, wading birds and birds of prey.
There are four pronounced zones that can be identified in lakes: the littoral zone, the limnetic zone, the profundal zone, and the benthic zone. The littoral zone is the shallow zone near shore, to a depth at which rooted plants do not grow. It is the most productive zone, high in biodiversity, sunlight, and nutrients (mainly due to runoff). Here you may find cattails, waterlillies, muskgrass, duckweed, decomposers, frogs, insects, and fish. The limnetic zone is the sunlit open water away from the shore. Phytoplankton, zooplankton, small and large fishes are present, but are usually less abundant than in the littoral. The profundal zone is deep open water that is devoid of light; as a result, photosynthesis does not occur here. The benthic region is the bottom of the lake, where infaunal and epifaunal organisms are present. Decomposers such as bacteria and fungi, along with detritus-feeding clams, worm-like insect larvae, and catfish dominate this region (Miller, 2000). The littoral and limnetic zones receive oxygen via photosynthesis. Oxygen concentrations decline at night as photosynthesis shuts down but respiration continues, or in association with increasing water temperatures.
Pollution has a variety of impacts on ecosystems and often ends up in lakes and other water bodies. Anthropogenic inputs of nitrates and sulfates to the atmosphere ultimately form nitric and sulfuric acid and further enhance the already acidic nature of Florida’s water and soil. Acid rain is less impacting in Florida than in northern regions, because of the underlying limestone. Deforestation, mining, construction, road building, channelization of waterways, and pumping of groundwater also contribute to decreasing water quality (Myers & Ewel, 1990). The conversion of land use in surrounding wetlands and other regions of the watershed have also exposed lakes to increased pollution and sedimentation (Florida Conservation Foundation, 1993).
Freshwater lakes are diverse, complex systems that provide a wide array of benefits for ecosystems and organisms that lie near or are connected to them. As a result, lakes influence adjacent terrestrial ecosystems to the same degree that surrounding ecosystems influence lake ecology.