Abstract

The role of Mara River basin wetland in reduction of nitrogen load to Lake Victoria was investigated. Reconnaissance survey was carried out to identify the suitable sampling points in the wetland. Transects were developed in the inlet and outlet zones of the wetland through which three sampling points were established at each transect. Data for physical-chemical parameters such as pH, temperature and dissolved oxygen were determined in situ and nitrite-nitrogen, nitrate-nitrogen, ammonia-nitrogen and total Kjedahl nitrogen were determined in the laboratory at Mwanza. Sediment samples were collected from the field by using soil auger and samples were taken to the laboratory for examination of nitrogen content in the sediments. The determination of nitrogen mass in plants involved determination of plant dominance, plant density and plant biomass before samples were taken to determine nitrogen in biomass. The results indicate that Mara River wetland receives about 0.70 ~ 1.56 mg/L of nitrogen largely in the form of organic nitrogen (63.6%) and nitrates (29.1%). The wetland removes about 28.8% of this nitrogen largely through net-loss to sediments and uptake by wetland plants. It is estimated that about 75 tons of nitrogen is removed annually, which is equivalent to 3.67 kg/ha/year. About 0.38 gN/kg of dry sediments were trapped in the benthic layer and 67.9 gN/m² of wetland is trapped in plant biomass. It was concluded that Mara River wetland was effectively protecting Lake Victoria by reducing nitrogen load entering the lake.

Key words: Lake Victoria, Mara River wetland, nitrogen removal, nitrogen load.