Multiyear oscillations in depth affect water quality in Lake Apopka

Abstract

We evaluated the effects of multiyear oscillations in depth on water quality in Lake Apopka, a shallow hypereutrophic lake on the Florida peninsula. A 17-year record of monthly data on rainfall, mean depth, total phosphorus (TP), total nitrogen (TN), chlorophyll a (Chl-a), and Secchi disk (SD) transparency was used to quantify relationships. We also looked for long-term trends because the lake has been the subject of major watershed and in-lake programs to reduce concentrations of TP and Chl-a. The dataset (1999 to 2016) included 4 high-water events and 3 drought events. We found no long-term trends in TP or SD and only minor long-term increases in Chl-a and TN. By contrast, all of the water quality attributes were significantly related to mean depth (p < 0.001). Water quality deteriorated with each drought and improved with each high-water period. The results illustrate how variation in climate can control water quality in shallow lakes with legacy nutrients in the sediments. When depth and volume are reduced during droughts in Lake Apopka, the likely scenario is a concentration of nutrients and solutes in the water column as well as greater net effects of benthivorous fish in mobilizing nutrients from the sediments and in creating turbidity. Because shallow lakes are more sensitive to changes in depth than deeper lakes, they can serve as early warning sites for effects of climate change on lake ecosystems.