Primary productivity dynamics in the Winam Gulf of Lake Victoria

Abstract

Primary productivi ty in the Winam Gulf arid the various f actors that are likely to influence it were studied from February, 1986 to May,1987. Surface water temperature varied little among the file sampling stations established in the Gulf. However, slightly lower temperatures were encountered at Nyakach Bay, which was attributed to the influence of colder water from Rivers Nyando and Sondu. Surface water temperature ranged between 25.7 ± 0.4c to 27.6 ±. 1. 0 Temperature varied by 3.5c during a 24th period at Nyakach Bay on 29 and 30 December, 1986. Solar radiation influenced water temperature to a great extent with values rising during periods of high insolation and vice-versa. The water column was generally thermally uniform with differences between surface and bottom temperatures not exceeding o0.8 C. Concentration of dissolved oxygen ranged between 8.0 + 0.6 and 8.5 + 1.0 mg-1I The highest value measured was 11.1-11 at Nyakach Bay in July, 1986. Concentrations were usually higher near the surface than at greater depths. On a diel basis, concentrations reached maximum levels at 1500 h which coincided with highest productivity values. Secchi disc visibility varied seasonally ranging from 0.2 to 1.4 m and havinq an inverse relationship to ch1orophylly 11 a, (t'= - 0.695). The penetration of the photosynthetically active radiation (PAR) declined with increasing depth and recorded the lowest value at Nyakach Bay where highest chlorophyll productivity and turbidity values were reached. Water pH ranged between 7.5 to 9.0 during the period of this study. Maximum conductivity of 145 pS cm -1 was recorded at Nyakach Bay. Alkalinity of the Gulf ranged between 52 and 77 mg CaCO 3 + HCO 3-1I Both conductivity and alkalinity rose during the dry season and dropped during the wet season. Phytoplankton species and biomass remained generally uniformly distributed in the water column. The blue-green alga, Microcytis aeruginosa Kutzing. was the most dominant phytoplankton species in the Gulf, its highest biomass being 150 mg m -3 Its biomass together with that of Anabaena sp., the next most important blue-green alga, reached maximum during the rainy season. Nitzschia sp. was the most important diatom in the Gulf, followed by Me1osira sp. Chlorophyll a concentration reached a maximum level of 50 pg 1-1 at Nyakach Bay in may, 1986 when the highest phytoplankton volumes were recorded. Two peaks of chlorophyll a concentrations were encountered, a major one during May to June and a minor one during October to January. Concentrations of chlorophyll a were lower at the surface than just below the surface. Concentrations of nutrients (P04- P and N03 - N) were higher the shallow enclosed bays, notably Nyakach Bay and Kisumu Bay and inrose during the rainy season. A high concentration (34 ur- g PO 4-P-1 1 was recorded at Nyakach Bay in May,1987 while the lowest was recorded at Kisumu Bay in February , 1986…………………………