Seasonal changes in physico-chemical status and algal biomass of lake Naivasha, Kenya

Abstract

Lake Naivasha located at latitude 0° 45' S, and longitude 36° 20' E, was designated a Ramsar site in 1995. It is an important resource to many stakeholders and has a fragile ecosystem that experiences rapid changes associated with natural and anthropogenic factors such as huge water abstraction for irrigation agriculture, fertilizer residues runoffs from the agro-based farms and sediments discharged by River Malewa among others. The purpose of the study was to evaluate the levels and seasonal changes in physico- chemical properties and algal biomass in order to provide current information on lake's water characteristics and hence propose suitable measures for sustainable management of the lake. This study was conducted in Lake Naivasha from December 2007 to April 2008, covering both dry and wet periods. Surface and bottom water samples were collected in replicates every month using Van Dom water sampler for nitrate nitrogen (N03 - N), ammonium nitrogen (NH4 - N), total nitrogen (TN), orthophosphate phosphorus (P04 -P), total phosphorus (TP) and chlorophyll a and measured using standard spectrophotometric methods. Water conductivity, pH, temperature and secchi depth were measured in situ. Dissolved Oxygen (DO) was measured by Winkler method. Trophic state index (TSI) was calculated using chlorophyll a and total phosphorus. The results show that mean f SD N03 - N values for the lake were 63 ± 31 lag L-1, NH4-N 128 f 46 Vg L-1, TN 304 196 pg L-1, P04 10 f 6 pg L-1 and TP 43 ± 26 gg L-1. There was significant seasonal and spatial variation for these nutrients p < 0.001. Mean f SD (DO) was 6.0 ± 1.3 mg L-1, temperature 21.8 ± 1.0 °C, conductivity 259 f 23 [is cm-1 and chlorophyll a 33 f 13 pg L-1. Gross primary production meant SD values ranged from 266 t 170 mg C m3 hr -1 to 473 f 230 mg C m3 hr 1. Based on the above results it can be concluded that the lake: (a) is eutrophic with respect to total phosphorus (TP) Carlson's Trophic State Index (TSI, 59.4) and chlorophyll a (TSI, 64.8) with significant seasonal difference being observed. Sewage Discharge Point (Station 2) and Malewa River Mouth (Station 3) are the two point sources for these nutrients entry into the Lake (b) Has high turbidity as indicated by low Secchi depth (c) Has a high algal biomass especially at Sewage Discharge Point and Mid Lake (Station 4). This study recommends: (1) efficient water quality monitoring through coordinated research involving research institutions, universities and nongovernmental organizations (2) Naivasha Municipal council in partnership with other stakeholders managing urban development around Lake Naivasha and the catchment should develop programs for managing waste water and soil erosion.