Coupled Hydrodynamic-Biogeochemical Modelling of Seasonal Hypoxia Formation in the Niger Delta Coastal Zone

Coastal hypoxia (dissolved oxygen below 2 milligrams per litre) has emerged as a globally expanding phenomenon with severe consequences for marine biodiversity and fisheries productivity. The formation mechanisms and seasonal dynamics of hypoxia in the Niger Delta coastal zone are theoretically unexplored despite the ecological and economic significance of the region. This research develops a coupled hydrodynamic-biogeochemical model to simulate seasonal hypoxia formation, predict spatial extent, and attribute causal mechanisms in the Niger Delta coastal zone. The three-dimensional hydrodynamic model FVCOM was coupled with a multi-element biogeochemical module (ERSEM) and calibrated against a comprehensive observational dataset collected across 24 stations over three annual cycles. Nutrient loading inputs from Niger River discharge, point sources, and atmospheric deposition were quantified and incorporated as boundary conditions. Model sensitivity analysis was used to rank the relative importance of nutrient loading, physical stratification, and organic matter decomposition in hypoxia formation. The coupled model reproduced observed seasonal dissolved oxygen dynamics with Nash-Sutcliffe efficiency of 0.81. Results indicate seasonal bottom-water hypoxia affecting approximately 820 to 1,450 square kilometres of the inner continental shelf during June to August each year. Physical stratification driven by the freshwater lens of Niger River discharge was identified as the primary prerequisite for hypoxia development, with nutrient-driven biological oxygen demand as the proximate cause. Scenario simulations demonstrate that a 30 percent reduction in agricultural nutrient loading would reduce hypoxic area extent by 22 percent under current hydrological conditions. The study provides the first three-dimensional hypoxia simulation for any Nigerian coastal system. Keywords: coastal hypoxia, hydrodynamic modelling, Niger Delta, biogeochemical modelling, dissolved oxygen.