Seasonal dynamics of plankton and aquatic environment in farmland ponds around Erhai Lake

Farmland pounds around plateau lake play a critical role in maintaining regional ecological health. However, agricultural intensification has resulted in substantial inputs of nitrogen and phosphorus into ponds and adjacent lakes through surface runoff, thereby exerting significant impacts on plankton community structure. In this study, plankton communities in agricultural ponds surrounding Lake Erhai were investigated to characterize seasonal variations in water quality and plankton assemblages between winter and spring. The results showed pronounced seasonal differences in water quality, particularly in total nitrogen (TN) concentration. TN levels were significantly higher in winter than in spring and exceeded the threshold for Class V surface water quality standards. Plankton diversity was higher in spring than in winter. The dominant phytoplankton species were Cyclotella meneghiniana in winter and Cyclotella spp. in spring, while the dominant zooplankton species were Asplanchna spp. in spring and Brachionus calyciflorus in winter. Linear discriminant analysis effect size (LEfSe) identified additional season-specific taxa beyond the dominant species, indicating clear seasonal shifts in plankton community composition. In winter ponds, the phytoplankton taxa Navicula spp. and Cyclotella meneghiniana, together with copepods and cladocerans among zooplankton, were predominant and may serve as potential indicator taxa of TN pollution. Mantel tests revealed significant positive correlations between plankton community diversity and TN concentration, electrical conductivity, and pH. Redundancy analysis (RDA) further demonstrated that community structure and its dominant environmental drivers varied seasonally, with stronger environmental filtering effects occurring during winter. These findings provide scientific evidence for season-specific pond management, nitrogen load control, and water quality conservation in agricultural pond ecosystems surrounding plateau lakes.