Original paper

The sulfate budget of a shallow subtropical lake

James, R.Thomas; V.McCormick, Paul


Sulfate biochemistry is of particular interest in south Florida, USA, due to its role in mediating mercury-methylation rates in the Florida Everglades. Discharges from Lake Okeechobee - a subtropical, polymictic, and eutrophic lake in south Florida - are a source of sulfate to the Everglades, but it remains unclear whether this large shallow lake is simply a passive reservoir for watershed sulfate loads or an active sink or source. We evaluate hydrologic, chloride and sulfate budgets to answer this question and to clarify Lake Okeechobee's role in sulfate loading for south Florida. Evaporative losses accounted for 62% of the water removal from Lake Okeechobee and explain the 40% higher sulfate concentrations within the lake compared to inflowing waters. Three lines of evidence suggest the lake is a small net sink for sulfate: 1) the in-lake ratio of sulfate to chloride is lower than the in-flow ratio (0.6 vs. 0.7), 2) average sulfate budget residuals (an estimate of missing sources or sinks) represent 9% of the in-lake mass, 3) the best fit for a simple sulfate model included a removal rate of 0.061 per year. Sulfate in the lake has declined significantly over the past three decades, a result of declining sulfate loads from the Everglades Agricultural Area (EAA) and Kissimmee subwatersheds. This suggests that sulfate within the lake can be further reduced through management of the loads to the lake. In the past five years, the average net-load of sulfate discharged to the EAA from Lake Okeechobee was approximately 13,000 t year-1 representing between 16 and 20% of the EAA sulfate budget as compared to previous estimates of 31 to 50%. Lake Okeechobee is a much smaller contributor of sulfate to the EAA and thus to the Everglades than previously determined.


sulfatechloridehydrologybudgetlake okeechobeemercury (hg)