Nutrient Enrichment and Hypoxia Threat in Urban Surface Water

Abstract

Water samples pooled from nine urban streams/rivers traversing high density residential and commercial areas in the Abeokuta metropolis, Ogun State, Nigeria, were investigated in order to determine the effect of inorganic nutrients on oxygen distribution between 2008 and 2010 by using standard methods. Results revealed that the metropolitan streams/rivers are anoxic and hypoxia-threatened due to high nutrient load (NL) except in peri-urban streams/river which served as control. The pH was 6.6–6.9 and the concentrations of dissolved organic carbon, total nitrogen and phosphorus (P-PO4 3 ) of the streams/rivers were 24.8–155.4, 3.82–15.46 and 1.35–5.71 mg/L, respectively, while dissolved oxygen (DO) ranged between 3.25 and 7.46 mg/L. The results also showed that the biochemical oxygen demand and chemical oxygen demand values ranged between 180.81 and 200.62, 250.27 and 312.83 mg/L, respectively. Analysis of variance showed significant differences between sampling sites as well as seasons (wet and dry) in terms of NL and oxygen levels in all the stream/ river water samples investigated with those from residential locations having higher NL values during the dry season. Thus, DO levels in almost all the urban streams/rivers were low, hence the need for regulatory intervention to ensure point of compliance necessary to protect inland freshwater from sewage inlets.