Estimation of Trophic State Indices And Phytoplankton Quotients in Kisumu Bay, Nyanza Gulf, Lake Victoria

Abstract

Kisumu Bay is greatly impacted by pollution from anthropogenic activities around Nyanza Gulf and from increasing levels of industrial and municipal waste discharges \ from Kisumu City. This has resulted in significant changes; in the level of eutrophication and the general ecology of the bay, impacting negatively on water quality, biodiversity, fisheries and livelihoods. These changes need to be monitored constantly for the effective environmental management of the gulf. The level of eutrophication of the Kisumu Bay has, however, not been determined. This study aimed at estimating the Trophic State Indices and Phytoplankton Quotients, 'as key indicators of eutropbication, in Kisumu Bay, and to determine the level of eutrophication of Kisumu Bay. Water quality measurements were conducted from April 2009 to March 2010. Physico-chemical parameters including Secchi depth was determined with a Secchi disc, turbidity,' temperature, conductivity, alkalinity and dissolved oxygen concentration were measured in situ using a seabird multi-parameter water quality probe, whereas nutrients levels (ammonia, nitrates, nitrite, total nitrogen, soluble reactive phosphorus, total phosphorus and chlorophyll a) and phytoplankton analyses were done by spectrophotometric and microscopic techniques, respectively. There were significant spatial differences in the dissolved oxygen concentrations (ANOVA, p< 0.01) within the bay. These differences were highly pronounced at the Kisat, Maboko and Yacht Club stations which are associated with sewage discharges from Kisumu City. Significant differences (ANOVA, p < 0.05) associated with discharges from Kisumu City and seasonal nutrient runoffs from storm water were also observed in the spatial and temporal distribution of phosphorous, ammonia, nitrates, nitrites and silicates within the bay. Significantly higher (ANOVA, p<O.05) chlorophyll a concentrations were recorded during the dry v season when compared to the rainy season, probably as a result of higher turbidity during the rainy season, which reduces light penetration into the water, and thereby reducing the rates of phytoplankton production. Cyanophyceae was the most abundant phytoplankton group contributing 5']010 of the total phytoplankton ;~unt, followed by Chlorophyceae (28%), Desmidaceae (11%), Bacillariophyceae (4%) and Euglenophyceae (1%). Among the Cyanophyceae, the most dominant species were Microcystis spp,Chroococcusspp, Anabaena spp and Cylindrospermopsis spp, Different phytoplankton distribution patterns were observed between the offshore Maboko station and the inshore stations. The mean phytoplankton quotient for Kisumu Bay was 4.1, whereas the Trophic State Index mean value was 145.3, indicating that the bay is highly eutrophic. The study attributes this observation to high nutrient loads from anthropogenic activities in the catchment area and industrial and municipal waste discharges from Kisumu City, and recommends stricter enforcement of the established policies on waste discharges from municipal and industrial establishments. The study also recommends the institution of environmental education and awareness creation targeting the catchment area and lake side communities as a policy to abate pollution in the bay and Lake Victoria as a whole.