High concentrations of nitrate and phosphate which was above the normal natural background levels which ranged between 1.4 mg l-1 ±0.1 - 6.4 mg l-1 ±0.3 and 0.7 mg l-1 ±0.0 - 2.2 mg l-1 ±0.2 were recorded in Oyun reservoir, Offa, Nigeria, a shallow tropical African reservoir. The relatively high nitrate and phosphate load was observed to have come from a non-point source of run-off of nitro-phosphate fertilizers from nearby farmlands into the reservoir. This resulted in slow cultural eutrophication. This was observed to have impacted on the catch and fish assemblages of the reservoir with high abundance and dominance of pelagic phytoplanktivorous fishes such as cichlids, while benthic and non-phytoplanktivorous species were found in low abundance. Other possible impacts of the eutrophication when it becomes accelerated were discussed. Measures to reduce the eutrophication in order to enhance adequate fish assemblages and management techniques for sustainable exploitation of the fisheries in the reservoir were suggested.
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