Investigation of Eutrophication Dynamics and Their Effects on Fish Mortality Events in Epe Lagoon, Lagos State

Eutrophication-driven dissolved oxygen depletion events have been repeatedly linked to mass fish mortality in Epe Lagoon, Lagos State, threatening the livelihoods of artisanal fishers and the ecological integrity of this important estuarine system. This study investigates the temporal dynamics of eutrophication indicators, specifically chlorophyll-a, dissolved oxygen, total nitrogen, total phosphorus, and phytoplankton composition, and their relationship to documented fish mortality events in Epe Lagoon over a two-year monitoring period. Biweekly sampling was conducted at six stations distributed across the lagoon, and meteorological data including wind speed, rainfall, and solar radiation were compiled for the monitoring period. Fish mortality events were documented in collaboration with fishing community informants, and concurrent water quality conditions at affected sites were characterised. Results showed that dissolved oxygen crashed below 1 mg/L at three stations on seven occasions during the monitoring period, each coinciding with phytoplankton bloom collapse events dominated by cyanobacteria genera Microcystis and Anabaena. Total phosphorus concentrations consistently exceeded 0.2 mg/L at stations near urban drainage outlets, indicating anthropogenic nutrient enrichment as the primary driver of bloom events. Salinity stratification during calm weather periods prevented reoxygenation of bottom waters, extending hypoxic episodes. The study establishes a mechanistic understanding of eutrophication-mortality linkages in Epe Lagoon and recommends nutrient load reduction targets for the Lagos State Government, with priority on redirecting urban drainage away from the lagoon system.

Keywords: eutrophication, dissolved oxygen, Epe Lagoon, cyanobacteria, fish mortality