COASTAL DUNE LAKES

Guide to the Natural Communities of Florida; FNAI and FDNR, 1990

G2 = Imperiled globally because of rarity (6-20 occurrences or very little remaining area, e.g., <10,000 acres) or because of some factor(s) making it very vulnerable to extinction throughout its range; S1 = Critically imperiled in state because of extreme rarity (5 or fewer occurrences or very little remaining area) or because of some factor(s) making it especially vulnerable to extinction; Coastal Dune Lake - (synonyms: lagoon, sand-bottomed lake, silt-bottomed lake, oligotrophic lake, coastal lake). Coastal Dune Lakes are generally characterized as shallow irregularly shaped or elliptic depressions occurring in coastal communities. They are generally permanent water bodies, although water levels may fluctuate substantially. They are typically lentic water bodies without significant surface inflows or outflows. Instead, water is largely derived from lateral ground water seepage through the surrounding well-drained coastal sands. Storms occasionally provide large inputs of salt water and salinities vary dramatically over the long term.

Vegetation may be largely restricted to a narrow band along the shore, composed of hydrophytic grasses and herbs or a dense shrub thicket, depending on fire frequency and/or water 46 fluctuations. Shallow, gradually sloping shorelines may have much broader bands of emergent vegetation with submersed aquatic plants occasionally dominating much of the surface. Typical plants include rushes, sedges, marsh pennywort, cattail, sawgrass, water lilies, water shield, royal fern, marsh fleabane, marsh elder, salt myrtle, and black willow. Typical animals include mosquitofish, sailfin molly, alligator, mud turtle, saltmarsh snake, little blue heron, coot, and otter.

The substrate of Coastal Dune Lakes is primarily composed of sands with organic deposits increasing with water depth. Coastal Dune Lakes characteristically have slightly acidic, hard water with high mineral content, predominately sodium and chloride. Salinity levels often vary greatly, depending on local rainfall and storms. They are generally oligotrophic with low nutrient levels.

Coastal Dune Lakes develop from various coastal processes. They most commonly begin as a tidally influenced basin or lagoon that becomes closed by sand filling its inlet. Once isolated from the direct effects of tides, the water may become hypersaline, as salt water intrusion occurs readily through the sandy substrates, and the surface water evaporates rapidly. With further isolation from subsurface and overwash saltwater intrusion, the water gradually becomes less saline, but still varies considerably with local weather conditions.

Coastal Dune Lakes are very unusual coastal features, being relatively short-lived and likely to disappear rapidly. They are important breeding areas for many insects that form the base of numerous food chains. They may also be important watering holes for many mammals and birds inhabiting the surrounding xeric and coastal communities. Wading birds and ducks may also use these lakes as feeding and resting areas.

Coastal Dune Lakes are extremely vulnerable to hydrological manipulations. Excessive withdrawals of ground water could lower local water tables or increase salt water intrusion and, thus, induce successional responses in the lake basin. Groundwater pollution, especially from misapplications of chemicals on the surrounding coastal communities, could significantly alter the nutrient balance and produce devastating effects on the fauna and flora.