An Evaluation of Phytoplankton and Chlorophyll Productivity in Oguta Lake, Imo State, Southeast Nigeria

Abstract

Assessing phytoplankton and chlorophyll concentrations is important because these biological and chemical components provide an integrated picture of the ecological condition, productivity, trophic status, and environmental health of a lake. This study investigated the phytoplankton and chlorophyll productivity of Oguta Lake, Imo State, Southeast Nigeria, with emphasis on their spatial, monthly and seasonal distribution and diversity. Water and phytoplankton samples were collected from 15 sampling stations between March and September 2025. Phytoplankton were identified and quantified, while chlorophyll a, b and c concentrations were also determined. The Shannon–Wiener diversity index (H′), Pielou's evenness index (J′) and Margalef richness index (d) were used to evaluate the diversity of phytoplankton community. A total of 7,810 cells/L of identified phytoplankton, comprising 11 taxa across four divisions, namely Bacillariophyta, Chlorophyta, Cyanophyta and Euglenophyta, were recorded. Chlorophyta was the dominant group, with Chlorella vulgaris having the highest abundance (1,838 cells/L), followed by Stichococcus bacillaris (1,696 cells/L). Phytoplankton abundance varied spatially from 324 cells/L at SP13 to 611 cells/L at SP1, while monthly abundance increased from 597 cells/L in March to 1,691 cells/L in August, before declining to 1,543 cells/L in September. Seasonal abundance was substantially higher during the wet season (7,213 cells/L) than the dry season (597 cells/L). Phytoplankton Shannon diversity ranged from 1.95 to 2.10, indicating moderate diversity across the sampling stations. Chlorophyll concentrations ranged from 38.6 to 44.8 µg/L for chlorophyll a, 8.9 to 10.4 µg/L for chlorophyll b and 3.1 to 3.7 µg/L for chlorophyll c. The study indicates that Oguta Lake is a biologically productive and relatively diverse freshwater ecosystem, with pronounced spatial and seasonal variations in phytoplankton communities and supports continuous ecological monitoring, control of nutrient and pollutant inputs, protection of riparian habitats and sustainable management of the lake to maintain its ecological integrity and fisheries productivity.