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IMPACTS OF ENVIRONMENTAL CONDITIONS ON PHYTOPLANKTON ASSEMBLAGES IN LAKE LOUISE GEORGIA**

Abstract

Lakes are important aquatic ecosystems that are extensively influenced by seasonal variations. In this study we investigate the influence of environmental conditions on phytoplankton association in Lake Louise, a sinkhole lake located in southern Georgia. Seasonal changes in environmental conditions and their impacts on the phytoplankton of Lake Lousie have been investigated sporadically over the past 20 years, most recently in the aftermath of Hurricane Irma in 2017. In this paper we examine the current state of the phytoplankton in the lake five years after the passage of Irma and describe the impacts of Hurricanes Idalia, Debby, and Helene. Environmental variables measured include temperature, conductivity and dissolved oxygen. Additionally, chlorophyll concentrations and the structure of the phytoplankton association were determined at 1 m intervals from the surface to a depth of 5 m. The summer thermocline was found at depths of 1.2 to 3.5 meters, with a temperature drop of 12 °C recorded on approaching the hypolimnion. Oxygen concentrations were below 1 mL/L throughout the hypolimnion. Ion concentrations were higher in the hypolimnion than the epilimnion and increased over the course of the year. Both Debby and Helene caused increased mixing between the epilimnion and the upper parts of the hypolimnion, but the effects did not extend below 3 meters; this is similar to what was observed after Hurricane Irma. In contrast to Irma, we did not see an increase in chlorophyll or phytoplankton concentrations in the epilimnion in the aftermath of Debby and Helene. The heavy rainfalls associated with these two storms may have led to a higher proportion of the phytoplankton being washed out, while the lateness of the season reduced the regrowth of the association.

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