Plankton Communities and Water Quality in Nigerian Rivers: A Review of Physico-chemical Drivers, Community Composition and Bioindicator Potential

Rivers supply drinking, domestic and fishery water for much of Nigeria, yet most receive untreated domestic, agricultural, abattoir and industrial waste. This review synthesises published studies on the physico-chemical conditions and plankton communities of Nigerian rivers, from the Sudan savanna in the north to the Niger Delta in the south, and evaluates how far plankton can serve as indicators of river condition. Across the studies reviewed, surface water temperature lies between roughly 25 and 32 °C, pH is near-neutral to mildly alkaline, and dissolved oxygen falls below 5 mg/L at sites receiving organic effluent. Phosphate, nitrate and several metals (iron, lead, cadmium) frequently exceed WHO and Nigerian drinking-water limits, particularly in urban and oil-producing catchments. The phytoplankton of free-flowing rivers is dominated by diatoms (Bacillariophyceae), with green algae (Chlorophyceae) second; cyanobacteria and euglenoids become dominant at organically enriched sites. Zooplankton communities are typically led by rotifers, followed by cladocerans and copepods, with protozoa and cyclopoid copepods increasing where organic or hydrocarbon pollution is heavy. Taxa such as Oscillatoria, Euglena, Brachionus and Keratella recur as pollution-tolerant indicators, while diverse diatom and calanoid assemblages characterise cleaner reaches. Seasonality (rainy-season dilution and flushing versus dry-season concentration) and nutrient enrichment are the most consistent drivers of community change. Persistent gaps include short sampling periods, inconsistent taxonomy, limited multivariate analysis and the absence of a Nigerian plankton-based biotic index. Standardised, longer-term, basin-scale monitoring that pairs plankton with water chemistry is recommended.