Assessing the influence of Lake Victoria flux on the Inner Murchison Bay water quality

Abstract

Lake Victoria in Eastern Africa is affected by a complex mixture of processes including increased pollution, water level fluctuations, hydrology of upstream catchments and wetland management, flora and fauna populations, etc. This study attempts to analyse effects of water level changes in Lake Victoria on the quality of water in the Inner Murchison Bay (IMB), located on the Northern part of the lake. The complex shoreline structure of bays and gulfs around Lake Victoria hinders accurate determination of the pollutant balance. Consequently, temporal variations in the pollutant concentrations in the Bay are used to monitor its water quality. Delft3D-Flow was applied to study the Bay hydrodynamics and quantify effects of dilution and mixing. This model was coupled with the Delwaq module to investigate the water quality processes related to Oxygen, Organic and Nitrogen nutrient components. Analysis of the measured concentrations over the period 2001–2011 shows that these concentration loadings generally follow an exponential distribution over time. The temporal variations are partly controlled by the lake levels through varying the water flow patterns affecting the residence time of pollutants rather than due to dilution tendencies. Lake levels accounted for only about 90% of flow in the IMB whose mean residence time was about 60 days. The Delwaq model could reasonably predict water quality variations towards the Outer Bay for NH4 + , DO and BOD as lake mixing occurs towards this Outer Bay. The high continuity errors in the Inner Bay may be attributed to high swamp inflows which were neglected in the hydrodynamic simulations.