Phytoplankton diversity and abundance of Oguta Lake and its trophic state

The abundance and composition of phytoplankton in Oguta Lake were studied. Oguta Lake is strategic as the largest natural lake in the Imo River Basin of South-Eastern Nigeria. A plankton net of 2-μm mesh size and 30 cm diameter was used to obtain the phytoplankton. Samplings were carried out according to study stations for one year. Phytoplankton samples collected were preserved in 4% formalin before laboratory analysis. Using a compound microscope, the phytoplankton were identified to species level using relevant taxonomic keys. 80 species of phytoplankton belonging to 29 taxonomic groups and three thousand seven hundred and twenty-two (3722) individuals were recorded. The different phyla recorded in order of abundance were Chlorophyta>Charophyta>Bacillariophyta>Euglenophyta>Cyanophyta>Chrysophyta>Xanthophyta>Rhodophyta. The four sampling stations located at Osemoto, Utu, Njaba, and Orashi had different relative abundances of 25.52%, 25.23%, 22.60%, and 26.65% of phytoplankton, respectively. August recorded the highest phytoplankton abundance of 16.36%, while the least abundance occurred in February (2.39%). Stichococcus bacillaris recorded the highest relative abundance in August (7.22%). Seasonal mean values of 19 species showed a significant difference at p<0.05, while 61 species showed no significant difference at p>0.05. Margalef’s index of species richness, Simpson’s index of species dominance, and Jaccard’s similarity index showed minor variations among the study stations. The trophic state of Oguta Lake can be described as moderately eutrophic, as indicated by the presence of species belonging to the genera Nitella, Chaetophora, Oscillatoria, Chlorella, and Ulothrix.