Freshwater driven seasonal and spatial variability of chlorophyll-a and nutrients in Lagos coastal waters.

The environmental health of Lagos coastal waters has been increasingly compromised by excessive primary productivity, reducing their capacity to deliver critical ecosystem services. This study assessed the spatial and seasonal variability of key physicochemical parameters and nutrients in relation to chlorophyll-a concentrations in Lagos coastal waters. Field sampling was conducted monthly at ten locations over 6 months, encompassing the dry and wet season. Measured parameters included total phosphorus (TP), dissolved reactive phosphorus (DRP), dissolved inorganic nitrate, salinity, dissolved oxygen, turbidity, temperature, and pH, at surface and mid-water depths. The results revealed significant seasonal differences in temperature, salinity, TP, and dissolved oxygen, with chlorophyll-a concentrations generally higher in surface water than in mid-water and elevated during the wet season (mean 35.51 µg L-1 and 29.19 µg L-1 for surface and mid-water, respectively). Spatial variability among stations significantly influenced chlorophyll-a distribution, highlighting the role of local catchment activities and anthropogenic pressures in shaping eutrophic conditions. Elevated nutrient inputs associated with freshwater inflows were identified as a major driver of enhanced primary productivity, contributing to water quality degradation and impairment of ecosystem services such as navigation and fisheries. Overall, eutrophication in Lagos coastal waters is regulated by the combined effects of nutrient enrichment, hydrodynamic conditions, and spatial heterogeneity. Continuous monitoring of key water quality indicators and nutrient inflows through coastal inlets is therefore essential for identifying eutrophication hotspots and supporting effective management strategies in tropical coastal ecosystems.