Ecological processes in the hydropower plant reservoir under multiple anthropogenic pressures

Abstract

Aquatic ecosystems are often subject to multiple anthropogenic stressors that can significantly alter the processes within them. Due to the occasional observance of algal blooms in the reservoir Moste of the hydropower plant (HPP) on the Sava River in recent years, the present research was conducted to identify the possible anthropogenic drivers that significantly impact the ecological processes in the reservoir. The first hypothesis was that the tributaries and wastewater treatment plant (WWTP) and steel factory effluents are the main sources of nutrients and/or thermal pollution due to the inflow of nutrient-rich and/or warmer water into the reservoir. To assess the anthropological pressures on selected water bodies, the ecological conditions in the reservoir and its tributaries/effluents were evaluated using standard methods for assessing trophic status according to the OECD criteria, and additionally by total protein content (TPC) and electron transport system activity (ETSA) in sediments as alternative in water quality assessment. We hypothesised that TPC as a proxy of microbial biomass (i.e. structural measure) and ETSA of microbial communities as a functional measure of sediment processes will show an overall impact of anthropogenic stressors, including their interactions, over time. To test the hypotheses, the selected environmental indicators were measured in water and sediments from the reservoir and its main tributaries and effluents. According to the OECD criteria, the HPP Moste reservoir was categorised as a meso-eutrophic system. Relatively high water temperatures of some tributaries and effluents compared to the Sava River contributed to the warming and thermal stratification in the reservoir. Moreover, high nutrient inputs, especially nitrogen, likely stimulated the growth of primary producers in the reservoir. These results corroborate the first hypothesis that the processes in the reservoir are driven by the temperature regime, and increased concentrations of nutrients, which are all substantially modified by the hydropower plant operation and other human activities in the catchment. Moreover, the investigation of the microbial biomass and respiratory potential activity in the sediments as indicators of the trophic conditions revealed significant increase as a response to increased nutrient input and/or increased temperature.