Evaluation of physicochemical parameters and phytoplankton assemblage of Lake Asa, Ilorin, Nigeria

Abstract

A limnological investigation of Lake Asa was conducted from November 2023 to October 2024, encompassing three sampling stations, to assess its physicochemical properties and phytoplankton composition. Measured values of physicochemical parameters were evaluated against established water quality standards. The lake's water quality was assessed through three analytical approaches: (1) analysis of phytoplankton taxonomic structure; (2) multimetric assessment using diversity indices; and (3) multivariate ordination of taxonomic group distribution to environmental variables. Thirteen physicochemical parameters were analyzed. Comparative analysis revealed that nine parameters met acceptable limits for domestic water supply and aquaculture, while four parameters were deemed unsuitable. Specifically, transparency, total nitrogen, organic matter, total organic carbon, and total phosphorus concentrations indicated a hypereutrophic state across all sampling stations, a hallmark of eutrophication. However, taxonomic analysis suggested a mesotrophic status for the lake. Phytoplankton community composition consisted of six taxonomic groups, with Cyanophyta dominating (83.20%). Other groups present, in descending order of abundance, were Bacillariophyta (diatoms 8.4%), Dinophyta (4.20%), Chlorophyta (2.70%), Euglenophyta (1.00%), and Chrysophyta (0.41%). Phytoplankton diversity indices ranged as follows: Shannon-Wiener (H = 0.26 – 0.98), Simpson (D = 0.46 – 0.90), and Pielou's evenness (J = 0.22 – 0.44). Canonical Correspondence Analysis (CCA) demonstrated the influence of environmental variables on phytoplankton assemblage structure, while Non-metric Multidimensional Scaling (NMDS) revealed spatial and temporal differences in phytoplankton assemblages. The observed Cyanophyta dominance and low diversity index values collectively indicate that Lake Asa is heavily polluted and in poor ecological condition.