Assessment of the current trophic state and water quality of Lake Naivasha, Kenya using multivariate techniques

Abstract

Lake Naivasha is a tropical freshwater body in Kenya, providing a source of livelihoods to the surrounding communities. The lake ecosystem, however, has become degraded over time because of increasing pollutant loads from multiple sources, including human settlements and agricultural and surface runoff in its basin. Thus, it is vital to study the lake's water quality to determine its current status to inform appropriate management measures. Accordingly, the present study analysed water quality parameters and the trophic state of Lake Naivasha on the basis of water samples collected at eight sampling sites during the dry and wet seasons from December 2019 to May 2020. The physico-chemical parameters were measured in situ, while nutrients were analysed in the laboratory. The results indicated high turbidity, total nitrogen (TN), ammonium-nitrogen (NH₄⁺-N), nitrate-N (NO₃-N) and total phosphorus (TP) levels during the wet season, and at the Malewa and Karati River mouths, while pH, electrical conductivity (EC) and chlorophyll-a values were relatively high during the dry season. The water temperature and dissolved oxygen (DO) concentration, however, did not vary between sampling sites or seasons. Cluster analysis (CA) classified the sites into three clusters based on water quality spatial and seasonal variations. Cluster 1 was comprised of the isolated region (Crescent), while clusters 2 and 3 mainly comprised sites sampled during wet and dry seasons, respectively. Similarly, principal component analysis (PCA) revealed spatial and seasonal physico-chemical variations, indicating nutrient parameters (TN, TP, NO₃-N and NH₄⁺-N), pH, Secchi depth and turbidity are the major variables influencing water quality variations. Furthermore, factor analysis (FA) identified nutrients, sediment and organic sources as the main lake pollution sources. The trophic state index (TSI = 60–72) indicated the lake varied from a eutrophic to hypertrophic state during the study period. The lake was found to be more eutrophic than reported in previous studies, indicating a progressive deterioration of its water quality. The overall results demonstrated some of the sampling sites in the lake are heavily contaminated with pollutants from various sources, attributable primarily to land use patterns and anthropogenic activities. The results of the present study are useful to inform decisions regarding management of the lake water quality.