Atmospheric deposition of phosphorus and nitrogen in the Northern Lake Victoria Watersheds

Abstract

Atmospheric deposition occurs when pollutants as air or particles are transferred from the air to the earth's surface. This study was undertaken to determine the contribution of atmospheric deposition to the nitrogen (N) and phosphorus (P) loading of Lake Victoria, and hence provide some insight into the role of atmospheric deposition to eutrophication of the Lake Victoria ecosystem. Quantities of P and N were estimated based on their rates of dry and wet deposition forms. Dry deposition samples were collected after every 14days by placing a litre of de-ionized distilled water into a high density polythene bucket placed on a flat platform and exposed for 24hrs. Wet deposition samples were collected on event basis, by exposing an empty bucket on a platform to collect a sample of a rainfall event. To estimate the quantity of P and N, deposition rates were determined at each site and later extrapolated over the whole lake area (68,800Km2). Dry deposition rates of N ranged from 127.8 to 1075.93μMm-2day -1. Kampala had the highest rates of N while Entebbe had the least. N in wet depositions ranged from 71.8 to 213.5 μM m-2 Event -1. Potential estimates of N deposited annually over Lake Victoria were at 143.3Ktons (20.8 Kg ha-1 Yr-1), with 87% and 13 % as contributions due to dry and wet depositions respectively. Dry deposition of N thus appears to be the dominant pathway through which N enters the lake. P loading rates in dry depositions were between 8.3 μM m-2 day -1 and 43.4 μM m-2 day -1. The average rate of P in dry deposition among the sampled sites was at 17.33 μM m-2 day -1. Wet deposition rates of P varied from 6.8-24.85 μM m-2 Event -1 and averaged 16.54μM m-2 Event -1 among the sites. Wet deposition samples had a greater portion of P as SRP (56.5%) compared to 31.2% in dry depositions. Wet deposition thus appears a significant route by which bio-available phosphorus enters the lake. Extrapolated over the whole lake area, annual P loading in this study was 14.11Ktons or 2.04±0.12Kg ha-1 Yr-1. Of total P determined, SRP contributed 41.6% while the rest was in particulate form. Like N deposition, dry deposition rates of P (68%) dominated over wet P (32%). Likely sources of P and N are industrialization, urbanization, and inappropriate land-use practices particularly deforestation and biomass burning. Results suggest that atmospheric deposition is a potential source of N and P to the nutrient budget of Lake Victoria.