Zooplankton as Indicator of Ecological Status in the Streževo Reservoir (North Macedonia)

This study examined the ecological status of the Streževo Reservoir in North Macedonia, focusing on zooplankton as an indicator of water quality. Built in 1982, the Streževo Reservoir serves several purposes, including irrigation, water supply, and hydropower generation. The research project investigated the seasonal and vertical variation in zooplankton abundance and biomass as well as the influence of environmental factors. Sampling was conducted seasonally (spring, summer, and autumn) in 2010 and 2011 across the longitudinal profile (epilimnion, metalimnion, and hypolimnion) of the reservoir at three sampling stations: the inflow of the Šemnica River, a central station in open water, and a site near the dam. The Streževo Reservoir is characterized by significantly pronounced seasonal and vertical temperature stratifications. The species diversity of the zooplankton was low, with only 21 taxa identified. Seasonal oscillations in abundance were statistically significant, with maximum values in the summer period and minimum values in spring. The Shannon diversity index displayed the lowest diversity values in the autumn, in the hypolimnion, and the highest values in the summer, in the metalimnion. The RDA analysis showed that temperature was the most important predictor of zooplankton abundance distribution, followed by Chl a concentration and TN. According to the Zooplankton Index of Quality Assessment (Zoo-IQ), during the investigated period the reservoir had good water quality in all three studied seasons, as well as through the whole profile. Overall, the study highlights the importance of zooplankton as an indicator of water quality and provides valuable insights into the ecological status of the Streževo Reservoir. The novelty of this study lies in its comprehensive examination of the interconnected dynamics affecting reservoir ecology, particularly as the present study is the first to perform such an analysis for the Streževo Reservoir. It highlights the impacts of thermal stratification on biochemical processes, the seasonal variations in dissolved oxygen and phosphorus levels due to phytoplankton activity, and the influences of temperature on zooplankton diversity and abundance. Furthermore, it introduces the Zoo-IQ index as an innovative tool for assessing water quality through zooplankton analysis, emphasizing its relevance as an early indicator of ecological changes in freshwater systems. Moreover, this multi-faceted approach underscores the complexity of reservoir ecosystems and the importance of proactive management strategies to the mitigation of water quality fluctuations. This study underlines the need for continuous monitoring and proactive management strategies to address the aging of reservoirs.