Primary productivity dynamics in the winam gulf of Lake Victoria

Abstract

Primary productivity in the Winam Sulf and the various factors that are likely to influence it were studied from February, 1986 to May, 1987. Surface water temperature varied little among the fi/e 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 o o o + 0.4 C to 27.6 +1.0 C. Temperature varied by 3.5 C during a 24-h 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 o 0.8 C. Concentration of dissolved oxygen ranged between 8.0 + 0.6 and 8.5 + 1.0 mg 1 ^ . The highest value measured was 11.1 mg 1 at Nyakach Bay in July, 1986. Concentrat ions were usually higher near the surface than at greater depths. Dn 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 having an inverse relationship to chlorophyll a - xvii - (rs - 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 a, productivity and turbidity values were reached, between 7.5 to 9.0 during the period of this conductivity of 145 pS cm -1 was recorded Alkalinity of the Gulf ranged between 52 and 77 1 1 . Both conductivity and alkalinity rose season and dropped during the wet season. Water pH ranged study. Maximum at Nyakach Bay. mg CaCO„ + 3 HCO during the dry Phytoplankton species and biomass remained generally uniformly distributed in the water column. The blue-green alga, Microcytis aeruoinosa 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 so.. the next most important blue-green alga, reached maximum during the rainy season. Nitzsch ia sp. was the most important diatom in the Gulf, followed by Melosira sp. Chlorophyll a concentration reached a maximum level of 50 pg 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 Concentrations of nutrients (F'O^ - P and NO^ - N) were higher in the shallow enclosed bays, notably Nyakach Bay and Kisumu Bay and rose during the rainy season. A high concentration (34 ug FO - P

1 ) was recorded at Nyakach Bay in May, 1987 while the lowest (2 jag PO - P 1 * > was recorded at Kisumu Bay in February, 1986. Similarly a maximum NO^ - N concentration of 85 ug 1 -* was recorded at Nyakach Bay in February, 1986 and the minimum (2.5 ^.ig 1 * ) at Kisumu Bay in February, 1986. The water column exhibited a more marked stratification of NO - N than F'0 . - P at the relatively deeper Ndere Island Station. _ Q _ 1 A high photosynthetic rate of 1.2 g 0^ m J h was recorded at Nyakach Bay in January, 1987. This value was nearly double the -1 highest (0.65 g 0^ m h ) reported by Tailing (1965). During the same period the highest mean productivity in the water column was 0.74 g 0 ^ nf^ h-* . Two peaks of productivity were encountered, one during June to August and another one during November to February. In the water column productivity declined with increasing depth as did water transparency. Productivity was positively correlated with nutrients, particularly P0^ - P (r = 0.913) and chlorophyll a <r = 0.873).