Empirical Assessment of Ecosystem-Level Impacts of Aquaculture Effluent on Receiving Rivers in South West Nigeria

The rapid expansion of intensive aquaculture in South West Nigeria has increased nutrient loading to receiving rivers, yet integrated ecosystem-level assessments of these cumulative impacts are absent from the scientific literature. This study conducted an empirical ecosystem-level assessment of aquaculture effluent impacts on three receiving rivers in Ogun, Lagos, and Oyo States. A Before-After-Control-Impact (BACI) sampling design was employed. Biological assessments at each river included phytoplankton community structure, benthic macroinvertebrate assemblages, and fish species composition. Water quality measurements included dissolved inorganic nitrogen, soluble reactive phosphorus, biochemical oxygen demand, dissolved oxygen, and coliform counts. Continuous water quality logging using multi-parameter sondes was deployed at control and impact stations for 30-day periods. Structural equation modelling was used to partition direct and indirect pathways through which nutrient enrichment affected biological endpoints. Total nitrogen loads from aquaculture farms ranged from 4.2 to 11.8 kilograms per day per farm, exceeding background river loads. Phytoplankton biomass increased threefold at impact stations relative to control, with cyanobacteria proportion increasing from 8 to 41 percent. Benthic macroinvertebrate ASPT scores declined by 2.3 units at impact stations, equivalent to a shift from good to moderate ecological status. Fish species richness showed no significant change attributable to the aquaculture effluents per se, though dissolved oxygen depressions below 4 milligrams per litre were documented at low-flow periods. Structural equation modelling confirmed that cyanobacteria expansion mediated dissolved oxygen depression as the dominant impact pathway. The study provides the first BACI-designed aquaculture impact assessment in Nigeria. Keywords: aquaculture impact, BACI design, nutrient enrichment, receiving rivers, South West Nigeria.