Phytoplankton productivity in Lake Naivasha

Abstract

Lake Naivasha being an endorheic basin, within an intensive agricultural region requires continuous assessment of basic factors controlling phytoplankton productivity, because agricultural activities can contribute to basic ecological changes. Such changes include those attributed to eutrophication, which could be detrimental to the well developed fishery in Lake Naivasha. Phytopiankton productivity was determined in thres different ecological zones in Lake Naivasha, i.e. littoral, open-water and a semi-enc1osed basin. Nearisothermal conditions were observed in all the stations, with a seasonal change of 3°C. The water was well oxygenated with orthograde curves, however dissolved oxygen stratification was well developed at Crescent Island Lake. Secchi disc transperency, conductivity, alkalinity, and nutrient concentrations were greatest at Crescent Island Lake. Phy topi ank ton biornass as chlorophyll a, showed significant differences in concentration between stations and over time, with ranges of 26.5-40.5, 20.6- 59.8 and 11.2-41.9 pg/1 at Safariland, Hippopoint and Crescent Island Lake respectively. This could be attributed to differences in species composition: Closterium. Micrpcystis. Ceratium. and Melosira spp.were abundant at Main Lake stations, while Melosira spp. dominated Crescent Island Lake. Photosynthetic rates were maximal in sub-surface waters (0.5-1.0 m depth) in all the stations, coinciding with more than the 15 '/. level o"f incident irradience measured as Secchi depth and PAR light attenuation. Maximum gross photosynthesis (Gpmax) was highest at Safari land between January and March 1990 and Crescent Island Lake took there after. Crescent Island Lake had the highest values of specific photosynthetic rate (Pmax) than the other stations with higher chlorophyll a concentration in January to March.