Seasonal variation in water quality, plankton diversity and microbial load of tropical freshwater lakes in Nigeria

Seasonal changes significantly affect tropical ecosystems; hence, verification of how these changes affect water quality is important for waterbodies that serve as water and food sources, particular as such changes are often associated with shifts in plankton diversity and microbial loads. This study assessed the seasonal changes in water quality, plankton diversity and microbial load in four lakes serving as sources of drinking water. Temperature, hardness and phosphate concentration were elevated in the dry season, and pH, biochemical oxygen demand, transparency, turbidity, total dissolved solids, conductivity, alkalinity and nitrate concentrations were elevated in the wet season. A phytoplankton analysis revealed that Chlorophyta, Bacillariophyta, Cyanophyta and Dinophyta were most common, with the families Desmidiaceae, Microcystaceae and Euglenaceae (phylum Euglenophyta) displaying dominance. For zooplankton, Rotifera was most common, with Branchionidae dominating the lakes in both seasons. In the wet season, Chlorophyta, Cyanophyta, Bacillariophyta and Dinophyta dominated, with Aphanizomenonaceae and Microcystaceae the most diverse families. Disease-causing pathogens, Ascaridae, Trichuridae and Ancylostomatidae (phylum Nematoda), were detected during periods of high rainfall. The waters in the dry season had higher microbial loads than in the wet season, ranging from 1.50 to 233.50 CFU g(-1) (p < 0.05). This demonstrates the seasonal variations in risk to users and underlies the importance of regular assessment of water quality particularly given the threat of seasonal changes elated to climate change.