Assessing water quality in Lake Naivasha

Abstract

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. This research aimed at studying the water quality in Lake Naivasha, Kenya. A myriad of environmental perturbations in Lake Naivasha’s ecosystem 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. 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, geoinformation 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 insitu while others were analysed from water samples in the laboratory. Geoinformation 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 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. This study also indicated that Lake Naivasha lies between eutrophic and hypereutrophic state. In comparison to previous water quality studies, this study showed that a further deterioration of the trophic status. 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. Graphical representation of the deviations of chlorophyll-a (TSI-CHL) and total phosphorus (TSI-TP) trophic state indices indicated that the lake was predominantly phosphorus limited (TSI-CHL > TSI-TP). Further scrutiny revealed that close to Mouth of Malewa (river input), North East (near the waste water treatment plant effluent discharge point and Kihoto informal settlement) and Kamere Beach (near Kamere informal settlement), the turbidity constituents were mainly dominated by sediments or other organic matter rather than chlorophyll-a (TSI-CHL < TSI-SD (SD=Secchi depth )). In Crescent Lake, the TSI-CHL exceeded the TSI-SD (TSI-CHL > TSI-SD) which was an indication of the presence of algae species with a more filamentous or colonial structure than in the rest of the lake. This study affirms the possibility of retrospective analysis of spatial variations and temporal trends in chlorophyll-a concentration’s using MODIS-Aqua satellite data, and therefore provide data at times when routine ground measurements do not exist. The existence of a large inter-annual spatial variation in chlorophyll-a concentration over the lake was evident particularly in the monthly composite maps. The results portray a large temporal variability which was partly caused by seasonal influences such as climate (rainfall) and seasonal agricultural practices. This was also evident in the long-term trend variations that correlate to the lake level, which could explain dilution and concentration effects.