Seasonal dynamics of microcystins in Yanghe Reservoir, Northern China, and their responses to environmental factors

<title>Abstract</title> <p>Although laboratory studies on Microcystis toxins are extensive, they often fail to fully explain the high variability observed in natural cyanobacterial blooms. This study investigated the seasonal dynamics of microcystins (MCs) and their environmental drivers in Yanghe Reservoir, a critical drinking water source in Northern China. From January to December 2023, water samples from five sites were analyzed for MC-LR, MC-RR, and MC-YR variants, alongside comprehensive physicochemical and biological parameters. The results demonstrated that total MC concentrations ranged from 0.24 to 22.61 µg/L, peaking during the summer months. Spatially, elevated toxin levels were predominantly observed in the western estuary. Dissolved MCs exhibited positive correlations with phosphorus and chlorophyll-a, but correlated negatively with nitrogen forms. Notably, the dissolved peaks of MC-LR and MC-YR preceded their intracellular maxima, a phenomenon that deviates from traditional laboratory-based models. With maximum dissolved concentrations (22.61 µg/L) significantly exceeding the World Health Organization (WHO) provisional guideline of 1.0 µg/L, this study highlights a severe public health risk. These findings underscore the urgent necessity for targeted nutrient control and enhanced, high-frequency monitoring strategies in this critical reservoir ecosystem.</p>