Abstract
We recently documented that during times of extreme shallow depth, there are severe effects on the water quality of one of the largest shallow lakes in the southeastern USA—Lake Apopka. During those times, total phosphorus (TP), total nitrogen (TN), chlorophyll-a (Chl-a) and toxic cyanobacteria blooms increase, and Secchi transparency (SD) declines. The lake recovers when water levels rise in subsequent years. In this paper, we determined whether extreme shallow depth events, particularly when they re-occur frequently, can stop the long-term recovery of a shallow eutrophic lake undergoing nutrient reduction programs. Apopka is an ideal location for this case study because the State of Florida has spent over 200 million USD in order to reduce the inputs of P to the lake, to build large filter marshes to treat the water, and to remove large quantities of benthivorous fish that contribute to internal P loading. We obtained data from 1985 to 2018, a period that had relatively stable water levels for nearly 15 years, and then three successive periods of extreme shallow depth, and we examined the long-term trends in TP, TN, Chl-a, and SD. There were significant decreasing trends in all of these water quality variables, and even though water quality deteriorated during periods of extreme shallow depth, and reduced the slope of the long-term trends, it did not stop the recovery. However, in the future, if climate change leads to more frequent shallow depth events, which in lakes such as Apopka, result in the concentration of water and nutrients, it is unclear whether the resilience we document here will continue, vs. the lake not responding to further nutrient input reductions.
Highlights
It is well-established that droughts, and associated periods of extreme shallow depth, can cause deteriorated water quality in eutrophic lakes, when there is a substantive reduction in water volume
Reported an increase in nutrients and toxic Microcystis blooms in a Korean river during a severe drought, and Nõges et al [5] found that the phytoplankton shifted from Lyngbya with no blooms to Anabaena and intense blooms in Lake Võrtsjärv, Estonia in a drought period that was linked to a particular phase of the North Atlantic Oscillation (NAO)
Documented between and 2018 might be a result of the restoration projects conducted by the state around and in the lake, and 2018 might be a result of the restoration projects conducted by the state around and in the lake, including a large-scale purchase of farmland north of the lake and conversion to wetland including a large-scale purchase of farmland north of the lake and conversion to wetland conservation conservation land [16] and harvesting of omnivorous fish to reduce internal P cycling from the land sediments
Summary
It is well-established that droughts, and associated periods of extreme shallow depth, can cause deteriorated water quality in eutrophic lakes, when there is a substantive reduction in water volume. Bouvy et al [1,2] observed elevated concentrations of total nitrogen (TN), total phosphorus (TP), and chlorophyll-a (Chl-a), along with blooms of toxin-producing. Water 2019, 11, 538 were reviewed by Reichwald and Ghadouani [6], who identified a number of reasons why extreme shallow depth stimulates blooms of cyanobacteria, including warmer water, the concentration of nutrients in the reduced volume, stagnation, and greater nutrient transport from the lake sediments. We recently found that in one of Florida’s largest lakes, Lake Apopka, multi-year oscillations in depth were the major factor affecting TP, TN, Chl-a, and Secchi disk transparency (SD) in this lake over the period of time from 1999 to 2016 [8]
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