Spatial and seasonal variations in phytoplankton community structure in alkaline–saline Lake Nakuru, Kenya

Abstract

Spatial and seasonal variations of phytoplankton, expressed in terms of species composition, cell density, biovolumes and biomass, collected at 10 sampling sites in alkaline-saline Lake Nakuru, Kenya, were investigated monthly from March 2004 to February 2005, in relation to selected physical and chemical water quality parameters. A total of eight species, belonging to five genera and three classes, were recorded. The Cyanophyceae comprised the bulk of the phytoplankton, followed by Euglenophyceae, while Bacillariophyceae were the least abundant. Arthrospira fusiformis was the most dominant species, in terms of density, among the Cyanophyceae, while Euglena spp. and Navicula elkabs dominated the Euglenophyceae and Bacillariophyceae density, respectively. Seasonal variations in phytoplankton species composition, density and biomass were significant (P < 0.05) with minimum cell density and biomass occurring during the dry season, and being maximum following the end of the two rainy seasons from May to November 2004, suggesting the possible influence of various environmental factors on the lake. Sampling sites located close to the inlets of inflowing rivers exhibited significantly (P < 0.05) higher phytoplankton cell density and biomass than those by the inshore sites. Soluble reactive phosphorus, ammonia-nitrogen, conductivity and total alkalinity accounted for most of the variations in the cell densities of phytoplankton species.

Elemental ratios (C:N) and stable isotopic composition of dominant rotifer species in a tropical eutrophic alkaline–saline Lake Nakuru (Kenya)

“…A bimodal rainfall pattern is normally experienced in the region. Long rains often fall during the months of March-May, while short rains occur during September-October (refer to Okoth et al, 2009 andOyoo-Okoth et al, 2011 for rainfall patterns). High solar radiation and adequate nutrient supply support the growth of primary producers resulting in a super-saturation of dissolved oxygen (17.2 ± 7.9 mg l -1 ) in the uppermost 50-cm layer during the day (Oduor & Schagerl, 2007).…”

“…The high alkalinity and salinity coupled with low environmental stability in these lakes render them to characteristically sustain low floral and faunal diversity (Hecky & Kilham, 1973;Finlay et al, 1987;Gilabert, 2001;Harper et al, 2003;Grant, 2006;Oyoo-Okoth et al, 2011;Ong'ondo et al, 2013). Nevertheless, alkaline-saline environments are rich in terms of species biomass and productivity (Talling et al, 1973;Vareschi, 1982;Agasild & Nõges, 2007;Oduor & Schagerl, 2007;Okoth et al, 2009). The higher productivity, presumably reflects higher ambient temperatures, higher incident solar radiation throughout the year and unlimited access to inorganic carbon (Melack & Kilham, 1974;Grant, 2006).…”

“…High solar radiation and adequate nutrient supply support the growth of primary producers resulting in a super-saturation of dissolved oxygen (17.2 ± 7.9 mg l -1 ) in the uppermost 50-cm layer during the day (Oduor & Schagerl, 2007). The phytoplankton is dominated by Arthrospira fusiformis (Okoth et al, 2009).…”

“…The mean temperature varies from 8 to 28°C. The variation in water volume results in large fluctuations of physico-chemical parameters such as water temperature and salinity (Okoth et al, 2009). For example, salinity ranges between 10 and 120% (Williams, 1996) and daily temperature fluctuations result in strong diurnal cycles of stratification and mixing (Melack & Kilham, 1974).…”

“…The geo-referenced sites had been established during a previous Kenya Wildlife Service monitoring programme and also sampled in earlier expeditions (Talling & Talling, 1965;Vareschi, 1982) and recently (see Okoth et al, 2009;Oyoo-Okoth et al, 2011;Ong'ondo et al, 2013). They included inlet sites (Hippopoint, Metal pole, Njoro Shore, Makalia Shore and Nderit Shore), which were closer to the discharge point of the incoming rivers and offshore areas (Njoro East, Jetty West, Jetty Mid, Jetty East and Makalia/ Nderit), which were further away from the point of discharge by the river inlets (Table 1; Fig.…”