Assessment of Nitrogen Enrichment Effects on Aquatic Biodiversity in Rivers Feeding Lekki Lagoon, Lagos State

Nitrogen enrichment from agricultural runoff, urban wastewater discharge, and aquaculture operations is a leading driver of aquatic biodiversity loss in coastal lagoon systems globally. Lekki Lagoon in Lagos State, within Nigeria's South West geopolitical zone, receives inputs from multiple rivers and drainage channels in one of Nigeria's most densely populated peri-urban corridors, creating conditions conducive to eutrophication and biodiversity impoverishment. This study empirically assesses nitrogen enrichment levels and their effects on aquatic biodiversity in four rivers feeding the Lekki Lagoon system. Water quality parameters, including total nitrogen, phosphorus, dissolved oxygen, and chlorophyll-a, were measured monthly over 12 months at 24 sampling stations. Aquatic biodiversity assessments encompassed macroinvertebrate benthic surveys, fish diversity sampling, and submerged aquatic vegetation cover estimation. Results indicate that total nitrogen concentrations in three of the four study rivers exceed NESREA water quality standards. Macroinvertebrate diversity indices are significantly lower at high-nitrogen sites, with tolerance-indicator taxa dominating. Fish diversity at impacted sites is reduced by 55 percent relative to reference conditions at the least impacted river. The study contributes the first integrated assessment of nitrogen enrichment and aquatic biodiversity in the Lekki Lagoon catchment and recommends a coordinated catchment-level nutrient management strategy, involving the Lagos State Environmental Protection Agency, agricultural extension services, and the urban drainage authority. Keywords: nitrogen enrichment, aquatic biodiversity, Lekki Lagoon, South West Nigeria, eutrophication