The limnology and management of thika dam Ndakaini, Kenya

Abstract

The sources of water for cities include Rivers and streams, lakes and reservoirs. The capacities of these sources to meet these demands are however challenged by sedimentation and enrichment of lakes and reservoirs by plant nutrients. There are various environmental impacts of dam construction on River channels. This has necessitated studies on the limnology and management of lakes and reservoirs especially Thika dam. During this study, various samples were collected to measure the vanous attributes of physico - chemical parameters, aquatic plant nutrients, chlorophyll a, zooplankton and fish. The samples were collected from the influent Rivers, the transitional zone and open water zone, which are the major zones of a reservoir. They then were analysed using various standard methods to describe the limnology of Thika reservoir. The results revealed that Thika dam is a cold-water reservoir that exhibits thermal, gaseous and chemical stratification, with longitudinal and vertical patterns of distribution of physicochemical and biological properties. The water is well oxygenated (surface value of 8.71 ± 0.29 mg/I.) with even higher values during the wet cold periods. The hypolimnion is cold and anoxic. The influent nutrient levels are low but the transitional zones have magnified level of both nitrogen and phosphorus, which are attenuated with depth and towards the dam wall. Chlorophyll a levels are high near the dam. wall (10.24 ± 2.02) and were positively correlated with nutrients. Thermocyclops spp. dominated zooplankton group with its distribution skewed towards the influent zone. Individuals of O-;'"iochromis niloticus were most abundant individuals collected during the fishing exercise and their growth condition factor was greater than one. It was shown that Thika reservoir is a mesotrophic reservoir and integrated management interventions are required to maintain its capacity to supply water to Nairobi