Multiple stressor effects on the phyto- and zooplankton communities in a mining lake affected by acid mine drainage

Before 1925, an artificial lake (TR-17) was created in a pit excavation in the area of Łuk Mużakowa (southwestern Poland). During the nineteenth century, this area was an active lignite mine, which means that its location was within the range of acid mine drainage (AMD). Between 1928 and 1943, the lake was transformed into a municipal bathing area with developed infrastructure. In the 1970s, a pig farm and meat-processing plant were in operation. Sewage resulting from pig farm and meat production was released directly into the surrounding fields causing eutrophication. Together with surface and ground water, nutrients and toxic metals were delivered to the lake, impacting the phyto- and zooplankton communities. Changes in the lake trophic state and pollution were inferred from diatom, Cladocera, and chrysophyte–stomatocyst assemblages, supported by geochemical analyses, including the C:N ratio and concentrations of heavy metals such as Cr, Co, Cu, Ni, and Zn. After the closure of the meat-processing plant, the lake began to recover from both eutrophication and heavy metal pollution. During that time, other factors, such as liming and fish stocking, affected the development of phyto- and zooplankton. However, the presence of AMD around the lake, along with processes releasing sulfuric acid and heavy metals as a result of the oxidation of sulfide minerals, have caused the reacidification of the lake since 2009. Heavy rainfalls in 2010 contributed to an increase in weathered materials entering the lake. The results show that various stressors significantly affect the conditions of the lake ecosystem. At the same time, we found that it is often difficult to clearly determine which factor had the greatest impact on the lake’s current state. In the case of Lake TR-17, AMD appears to have been the dominant influence, except during the 20 year period of the pig farm’s operation, when the inflow of domestic sewage from animal husbandry played a more significant role.