Assessment of Dissolved Heavy Metal Pollution and Ecological Risk in the Waters of Lake Kotokel (Eastern Cisbaikalia, Russia)

Safeguarding the ecological integrity of aquatic ecosystems is a national priority. Lake Kotokel, one of the largest lakes in the Baikal region, is hydrologically connected to Lake Baikal through a system of channels and holds considerable socio-economic significance. This study presents the first detailed seasonal analysis of dissolved heavy metal (HM) concentrations in the lake. Their relative abundance followed the order: Fe > Mn > Zn > Ni > Cr > Cu > Pb > Cd. Clear patterns of seasonal dynamics and spatial distribution were identified. The highest concentrations of manganese, zinc, and copper occurred during the under-ice period, driven by mobilization from silty bottom sediments and groundwater. In contrast, during the open-water period, elevated concentrations of iron, chromium, and nickel were associated with meltwater and rainfall inputs, which transport weathering products of rocks and soil organic matter from the watershed into the lake. To investigate the relationships between HM concentrations and water parameters—including physicochemical characteristics, nutrient levels, chlorophyll-a (chl-a), and total organic carbon (TOC)—Spearman correlation and principal component analyses (PCA) were applied. The results demonstrated that seasonal variations in physicochemical conditions and the intensity of biological processes exert a decisive influence on HM dynamics. For the first time, HM pollution and the associated ecological risk were assessed for Lake Kotokel. The pollution indices—Heavy Metal Evaluation Index (HEI), Nemerov Pollution Index (NPI), as well as Ecological Risk Index (ERI) indicated that HM pollution originates primarily from natural sources, with iron and manganese as the dominant contributors. However, the detection of cadmium in isolated samples near settlements elevated the potential ecological risk for aquatic organisms, underscoring the need for continuous monitoring of local pollution sources.