Water quality in aquatic systems is important because it maintains the ecological processes that support biodiversity. However, declining water quality due to environmental perturbations threatens the stability of the biotic integrity and therefore hinders the ecosystem services and functions of aquatic ecosystems. Lake Naivasha is one of the aquatic ecosystems that have faced a myriad of environmental perturbations that have transformed the lake from a clear to muddy eutrophic turbid state, which has resulted in a decline in ecological quality, impacting heavily on fish population and tourism. Though there has been regular data collection on water levels and fish catches, little has been done in monitoring the water quality dynamics in Lake Naivasha. This research aimed at studying the water quality in Lake Naivasha, Kenya. The specific objectives were to assess the overall water quality status; establish the trophic status; assess retrospectively the water quality condition in the last decade; study effect of succession of fish community; and investigate the mechanisms that influence the water quality dynamics in Lake Naivasha. These objectives were achieved through coupling field measurements, geo-information and earth observation, and system modelling. The field measurements were collected weekly from January to June and bi-weekly from July to November 2011 at seven locations in the lake. Water temperature, pH, conductivity, Secchi depth, and turbidity were measured in-situ while others were analysed from water samples in the laboratory. Geo-information and earth observation was used in the retrieval of chlorophyll-a concentration from June 2002 to June 2012 from Moderate Resolution Imaging Spectroradiometer (MODIS-Aqua) satellite images. The modelling objective was achieved using Delft3D Flow module to simulate the hydrodynamics and simple stirred-tank reactor model to simulate the water quality in Lake Naivasha. Principal Component Analysis (PCA) and Cluster Analysis (CA) revealed spatial variability in physiochemical parameters, nutrients and main ions. Northern region, main lake, and Crescent Lake sectors of the lake were distinct. Water quality parameters association indicated that the quality of water is influenced by agricultural activities, and domestic effluent around Lake Naivasha. The Northern sector (close to rivers input) seemed to be influenced by agricultural activities. The North East sector of the Lake was dominated by domestic effluent and close association with the crescent lake which is influenced by natural mineral composition associated with its volcanic origin. Discriminant analysis (DA) of the trophic state indices (TSI) revealed that the trophic state was indeed heterogeneous with three distinct sectors which include: the northern part of the lake, the mid and southern sector, and the Crescent Lake. Water quality modelling revealed that response of the lake ecosystem to reduction of pollutants is gradual and it would take 40 years to reach the equilibrium state if the loading remained constant. Ground water seepage could be the main reason behind the freshness of Lake Naivasha despite non-existence of a visible outlet. Investigation of the driving forces behind the spatial variability in water quality revealed that currents which might have been responsible for the transport of sediment and other constituents from the input rivers, were mainly wind-driven in Lake Naivasha. There exists mixing which could be responsible of substance (suspended particles, sollutes, and pollutants) redistribution in the Lake. The nutrient mixing can enhance proliferation of algal biomass through nutrient enrichment in the water column leading to high turbidity levels.
|Award date||27 Jun 2014|
|Place of Publication||Enschede|
|Publication status||Published - 27 Jun 2014|