EFFECTS OF CLIMATE CHANGE AND DROUGHT ON TRACE METAL DISTRIBUTION IN SEDIMENTS OF TWO SHALLOW FLORIDA LAKES

One predicted direct effect of climate change on Florida is altered rainfall patterns, specifically longer droughts and more intense wet periods. Not as well constrained are indirect effects, those caused by interactions between climate change and prevailing environmental conditions. We investigated the impact of drought on trace metal distributions in Florida lake sediments to address how changing hydrologic patterns will alter metal fate and transport.

Of Florida’s 7,800 lakes, ~75% have maximum depths <5 m and 70% are seepage basins, losing water only to evapotranspiration and groundwater. The combined effects of increasing drought intensity, greater groundwater use during dry episodes, and the growing water demand of an expanding human populatin on lowers water tables and lake levels, often exposing large areas of lake bed.

Previous paleolimnological studies in north Florida show increasing concentrations of Pb, Cu, Ni, Sn, Zn, V, Sb, Bi, and Cr in sediments since 1900, with highest concentrations found near the sediment surface. Lake sediments are sinks for trace metals, which typically remain undisturbed and buried. However, we hypothesize that lower lake levels, driven by more frequent and intense droughts, will alter sediment distribution and result in higher concentrations of metals at the oxidized sediment surface, thereby resulting in a potential source of legacy metal pollution.

We assessed the effects of the 2012 winter drought on trace metals in the sediment of two, shallow (<5 m deep) closed-basin lakes in north Florida, Little Lake Johnson and Geneva. During the extreme water-level low, we collected surface sediment along transects on the exposed lake bottom. We measured total metal concentrations, grain size, and Pb isotopes in the samples. Fine-grain organic-rich sediments were concentrated in topographic lows. Trace metals adsorbed to these fine-grain sediments are enriched 2-6 times relative to deposits collected near the former lakeshore. Lead isotopes indicate that high amounts of Pb come from leaded gasoline. Our study illustrates how climate change, e.g. drought, can have indirect effects on the environment, in this case, altering trace metal distributions in lake sediments. Such changes may have environmental management implications, given predicted climate change scenarios.