Dynamics of zooplankton in a lake undergoing restoration

This study investigated the zooplankton community structure in Gołdap Lake, located in the northeastern part of Poland. Due to the many years of uncontrolled inflows of pollutants, the ecological condition of the lake has deteriorated. Despite many remedial actions in the catchment, the internal nutrient load of the lake was too great to improve its condition. This goal required additional in-situ activities. The restoration of Gołdap Lake started in 2017. The substance which was used for restoration purposes was Phoslock®. It is lanthanum-modified clay that can bind phosphate into insoluble rhabdophane-(La). Water and zooplankton samples were collected from September 2017 to August 2021. A total of 70 species, including 44 rotifers, 16 copepods, and 10 cladocerans species, were detected during the research period. The whole zooplankton community was divided into 7 functional groups: rotifer filter feeders (RF), rotifer carnivores (RC), small crustacean filter feeders (SCF), mid-sized crustacean filter feeders (MCF), mid-sized crustacean carnivores (MSC), large crustacean filter feeders (LCF) and large crustacean carnivores (LCC). The most abundant were species from the rotifer filter feeder functional group. This study showed a significant correlation between dominant zooplankton groups and environmental variables such as nitrite nitrogen, water temperature, dissolved oxygen, conductivity, biological oxygen demand, and dissolved organic carbon. According to the values of zooplankton-based indices, the ecological state of Gołdap Lake hadn't changed during the research period. The major driving factors which affect the structure of zooplankton were: poor quality of food base, top-down pressure of planktivorous fish, and competition between zooplankton groups.