A comparative study of the influence of variations in environmental factors on phytoplankton roperties of selected reservoirs in Central Kenya.
Kitur, Lesan Esther
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Limnological information on reservoirs is important as it forms a vital baseline for among others the detection of undesirable changes in water quality. The physicochemical and phytoplankton properties were investigated in uhuru, Ruiru, Ngewa, Comte and Kianjjibbe reservoirs in central and Nairobi provinces of Kenya, over a period of 12 months (between February 2002 and January 2003) The aim of the study was to determine the nature and level of variation in the physico-chemical properties and its influence on phytoplankton composition, diversity and biomass. Sampling was done during day time and samples analyzed within four hours of sample collection. Among the limnological properties measured were: Secchi depth, temperature, electrical conductivity, dissolved oxygen pH, total alkalinity, nitrite-nitrogen, nitratenitrogen, total nitrogen, ortho-phosphate, total phosphate, soIuble reactive silica, phytoplankton composition and phytoplankton biomass. The study revealed varied degrees of seasonal changes in physico-chemical and phytoplankton properties of the reservoirs. Mean Secchi depth (SD) ranged from 0.1± 0.4 (Ndewa) to 0.9 ± 0.43 m (Ruiru). In general, the highest variation was noted during the dry season. Mean EC ranged from 45.8 ± 5.7 uS cm-1 (Ruiru) to 298.9 ± 41.5 pS cm- in Kianjjibbe with the highest EC being recorded during the dry season. Mean total alkalinity (TA) at the reservoirs ranged from 18.0 ± 0.24 mg L-1(Ruiru) to 110.2 ± 15 ') mg L-1 (Kianjjibbe). Mean nitrate nitrogen (N03-N) concentration ranged from 4.2 ± 043 pg L-' in (Ngewa) to 13.6 ± 1.14 pg L_1 in (Kianjjibbe). Low mean N03-N concciltration was recorded during the dry season. Mean total nitrogen (TN) concentration a the reservoirs ranged from 32.5 ± 28.7 Itg L-1 (Kianjjibbe) to 40.5 ± 36.9 lag L- (Comte). High (TN) concentration was recorded during the wet season. Mean total phosphorous (TP) ranged from 0.70 ± 0.25 pg L-1 (Kianjjibbe) to 1.03 ± 0.82 ug L-1 (Comte). High mean TP concentration was recorded during the wet season. Mean soluble reactive silica (SRS) concentration ranged from 3.2 f 0.47 (Uhuru) to 7.3 f 0.74 pg I ,(Ngewa). High SRS concentration was recorded during the dry season. A significant difference in Secchi depth electrical conductivity, total alkalinity, nitrate nitrogen total nitrogen, total phosphorus and Soluble reactive silica was noted in all the five reservoirs (P<0.001). A total of 35 phytoplankton genera belonging to 7 divisions were 1ccntified in all the five reservoirs. However the largest number of genera (16) belongs d to the Chlorophyta while Cryptophyta had only one genus. Total biomass ranged "rom 3291.87 mg L_1 (Ngewa) to 22,338.763 mg L-1 (Kianjjibbe). Most of the biomass vas due to Dinophyta. Wet season was characterized by high biomass. Total biomass between the reservoirs were significantly different (p<0.001). Phytoplankton biodiversity was high during the dry season. Uhuru reservoir had the highest diversity of 2.4 bits The study concludes that the reservoirs investigated vary in levels of physico-chemi,- 11 and phytoplankton properties investigated. This variation was attributed to differences in rainfall, volume of outflow and use dynamics of the water of the reservoirs.