Florida

Abstract

Florida lakes are extremely diverse. Although many are influenced by agriculture and other aspects of cultural land use, Florida also contains more acidic (ANC ≤ 0) lakes than any other region in the United States. Approximately 460 lakes are acidic (22% of the Eastern Lake Survey - Phase I population) despite receiving only moderate rates of wet deposition of H⁺ and SO₄²⁻. About half the acidic lakes appear to be naturally so because of organic acids. The remaining clearwater systems derive their acidity from mineral acids. As in the Upper Midwest and Maine, the occurrence of acidic lakes is strongly related to hydrologic type; over 80% of the acidic lakes in Florida are seepage lakes.

Unlike other case study regions, neutralization of acidic deposition in dilute seepage lakes is due largely to anion retention (SO₄²⁻ and NO₃⁻). Base cation supply accounts for only 12% of ANC generation. Low base cation supply reflects the depleted base status of the Plio-Pleistocene sands mantling the karst terrane characteristic of Florida seepage lake districts. Within a particular subregion, differences in ANC among undisturbed, soft-water seepage lakes reflect hydrologic factors, and the proximity of clay and carbonate deposits to the lake bed.

Major ion chemistry varies longitudinally across the state, primarily because of differences in relative amounts of precipitation and evaporation. Regionally, precipitation: evaporation ratios appear to exert competitive effects on lakewater ANC through (1) direct evapoconcentration of atmospheric deposition, and (2) controls on the relative importance of groundwater inseepage on hydrologic and mineral budgets of seepage lakes. The most sensitive lakes are located in the Panhandle, where precipitation: evaporation ratios are high and inseepage fluxes of ANC are believed to be small.

Diatom reconstructions of historical pH in the Trail Ridge lake district indicate that some lakes have become more acidic since the 1950s. Chemical evidence indicative of recent acidification is more ambiguous. Increased acidic deposition and changes in groundwater hydrology (with associated changes in base cation supply) have been hypothesized to account for the paleolimnological record. Regional increases in SO₂ emissions and progressive declines in the artesian aquifer potentiometric surface near the Trail Ridge have been documented since 1940; model hindcasting suggests that both phenomena may account for observed changes in pH. Depending on the mechanism, different responses in other major ion components are predicted and insufficient data are available to distinguish between the two hypotheses.