Seasonal Dynamics of Dissolved Oxygen Depletion in the Hadejia River and Its Implications for Aquatic Ecosystems, North West Nigeria

This study analyses the seasonal dynamics of dissolved oxygen depletion in the Hadejia River in North West Nigeria, empirically characterising the oxygen regime across contrasting hydrological seasons and identifying the biogeochemical processes responsible for hypoxic conditions that cause fish kills and aquatic ecosystem degradation. Seasonal fish kills in the Hadejia River have been reported by fishing communities in Jigawa State, but the environmental engineering processes driving these events have not been systematically studied. A twelve-month monitoring programme was established at seven stations along a 120-kilometre reach of the Hadejia River covering wet season high flow, receding flow, and dry season low flow conditions. Dissolved oxygen, temperature, pH, biochemical oxygen demand, chlorophyll-a, turbidity, and nutrient concentrations were measured fortnightly at each station. Oxygen consumption rate experiments were conducted on river water and sediment samples collected across seasons. During the driest months, dissolved oxygen concentrations in the lower reach decline to below 2 milligrams per litre on multiple occasions. Oxygen sag curve analysis attributes the decline to the combined effects of high biochemical oxygen demand from organic waste inputs, elevated water temperatures reducing oxygen solubility, and high sediment oxygen demand from organic-rich bed sediments. Reaeration is reduced by low flow velocities in the dry season. Agricultural fertiliser-driven algal growth provides diurnal oxygen supersaturation in the morning but contributes to nocturnal oxygen depressions through overnight respiration. The study identifies dry-season wastewater dilution capacity as the critical limiting factor for river oxygen balance. Keywords: dissolved oxygen, river quality, seasonal dynamics, Hadejia River, aquatic ecosystem.