Wet nitrogen and phosphorus deposition in the eutrophication of the Lagos Lagoon, Nigeria.

Air pollution is influenced by wind-aided particulate suspension, open-air waste burning, and fossil fuel combustion. The pollutants from these sources eventually deposit on ambient surfaces. Atmospheric wet deposition into Lagos Lagoon may be significant additions to the nutrient levels of the eutrophic lagoon. Precipitation was monitored at three stations in the Lagos Lagoon basin from May to November, 2012, in order to estimate the contribution of wet deposition to the nutrient cycles of the lagoon. Water samples were digested with potassium persulfate, and the species of phosphorus (P) and nitrogen (N) were analyzed by colorimetric methods. The mean [NO3-+NO2-]-N level was 0.39 ± 0.51 kg ha-1 month-1. The average total N was 3.16 ± 6.39 kg ha-1 month-1. The mean soluble reactive P was lower than the [NO3-+NO2-]-N averaging 0.06 ± 0.09 (at control site S2) to 0.24 ± 0.10 kg ha-1 month-1 (at site S1). Average total P was 1.25 ± 0.82 kg ha-1 month-1. The annual total N (May-September) was 4.55 (at S2) to 32.4 kg ha-1 year-1 (at S3). The annual total P (May-November) over Lagos Lagoon basin was 5.06 kg ha-1 year-1 (at S2). This study demonstrated that wet deposition of anthropogenically derived nutrients to the Lagos Lagoon is ongoing and may represent a considerable proportion of the total nutrient loading to it. The increased P availability in the wet deposition is likely responsible for the water hyacinths, which usually blossom on Lagos Lagoon during the late rainy season, and the reported harmattan-season bottom water hypoxia.