| Summary: | Phytoplankton community structure in relation to some physico-chemical parameters in Kalgwai Dam, north-western Nigeria was undertaken monthly in three stations for a period of six months spanning from January to June 2018. Physico-chemical parameters and phytoplankton were collected once in a month on each sampling occasion. Physico-chemical parameters such as Dissolved Oxygen (DO) concentration was lowest in Station 1 (3.79 ± 0.53 mgl−1), and phosphate, pH and turbidity were highest in Station 3. Tukey’s Honestly Significant Difference (HSD) test showed that the mean values of DO, TDS, EC, and turbidity were significantly different (p < .05) among the three sampled stations. A total of 379 phytoplakton individuals belonging to 6 divisions and 41 taxa were identified in the study area, and Baccillariophyta was the most predominant division of phytoplankton in the dam. Slightly higher abundance of phytoplankton was recorded in the dry season than in wet season. Analysis of variance (ANOVA) calculated for biological indices showed no significant difference among the the months sampled (p> .05). Canonical Correspondence Analysis (CCA) model revealed a weak relationship between phytoplankton community structure and measured physico-chemical parameters. Species such as Microcystis sp., Melosira sp., Closterium sp., and Ulothrix sp. indicated a strong positive correlation with increasing physico-chemical parameters such as nutrients and BOD5 at Station 1. These species were considered indicators of prevailing environmental conditions in the study area. Cluster analysis (Jaccard similarity) showed species of phytoplankton to be clustered by season rather than by stations. The study revealed anthropogenic pressures to contribute to the current deteriorating state of the dam.
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