Assessment of the Species Composition and Abundance of Plankton in Oguta Lake, Imo State

Phytoplankton and zooplankton communities in freshwater lakes serve as foundational indicators of ecosystem productivity and water quality, yet comprehensive plankton surveys of Oguta Lake in Imo State, South East Nigeria, are limited in the recent literature. This study assesses the species composition, abundance, and seasonal variation of phytoplankton and zooplankton in Oguta Lake over a six-month period spanning the dry and wet seasons. Water samples for plankton analysis were collected at four stations distributed across the lake at monthly intervals. Plankton were preserved in Lugol's iodine solution and identified and counted using standard taxonomic keys at 400x magnification under a light microscope. Physicochemical parameters including temperature, pH, conductivity, transparency, and nutrient concentrations were measured concurrently. A total of 47 phytoplankton taxa were identified, dominated by Chlorophyceae, Bacillariophyceae, and Cyanophyceae. Zooplankton diversity was lower, with 23 taxa recorded, predominantly rotifers and cladocerans. Total plankton abundance was significantly higher during the dry season, when nutrient concentrations were elevated and water transparency was greater. A strong positive correlation was observed between phosphate concentration and phytoplankton abundance. Indicator taxa associated with organic enrichment were detected at the station nearest to human settlement. The findings provide a baseline plankton biodiversity dataset for Oguta Lake and highlight the need for pollution monitoring and catchment management to preserve the lake's ecological integrity and aquaculture potential.

Keywords: phytoplankton, zooplankton, Oguta Lake, species composition, seasonal variation