Zooplankton partitioning in a tropical alkaline–saline endorheic Lake Nakuru, Kenya: Spatial and temporal trends in relation to the environment

Abstract

Spatial and temporal zooplankton variations were studied for 1 year in tropical alkaline-saline Lake Nakuru to determine how they partition in the habitat, relative to environmental variables. Monthly samples were collected at 10 sampling sites, with subsurface tows, using 33.5-lm mesh plankton nets. Physicochemical parameters displayed clear seasonal variations associated with precipitation patterns. Nine species, belonging to two main zooplankton taxonomic groups (ciliates; rotifers), were identified in the samples. Brachionus dimidiatus dominated the samples, accounting for 80% of the total zooplankton abundance. Kruskall-Wallis tests indicated significant (P < 0.05) temporal and spatial variations among all taxonomic groups. Different zooplankton species displayed a clear succession throughout the year. The total abundance of the rotifers and ciliates peaked at sampling sites near inlets during the long rainy seasons, while those in the inshore sites displayed variable succession patterns. Spatiotemporal structure of the zooplankton assemblages, and its correlation with environmental variables, indicated each species displayed distinct niche-based partitioning. The ciliates niche was associated with increasing soluble reactive phosphorus, total phosphorus and nitrite-nitrogen (NO 2 -N) concentrations. Niche partitioning in rotifers was associated with nitrate-nitrogen (NO 3 -N), conductivity and pH. These results indicate physical niche separation, even in a small, relatively homogenous lake among species of rotifers and ciliates, providing information from which future changes in their abundance and spatial distributions can be predicted, given continuous water quality changes.

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

“…In the alkaline-lakes, rotifers attain high population densities quickly, especially when large, competitively superior zooplankton species, such as Daphnia, are rare or absent (Gilbert, 1988;Echaniz et al, 2006). However, during a short time period of the year, the size of their population changes rapidly (Echaniz et al, 2006;Oyoo-Okoth et al, 2011). The predominance of rotifers in the eutrophic alkalinesaline lakes concords with their rapid reproductive capacity during optimal conditions, their short generation time and their ability to develop large populations under eutrophic environments (Ghadouani et al, 1998).…”

“…Previous studies in Lake Nakuru, pointed to the dominant rotifer species of genus Brachionus (Harper et al, 2003;Oyoo-Okoth et al, 2011;Burian et al, 2013). Particularly, OyooOkoth et al 2011 established that Brachionus dimidiatus, B. plicatilis, Keratella tropica and Filinia longisepa were the dominant rotifers in the lake at all sampling sites, albeit with some rapid changes in a short period of times.…”

“…Vareschi, 1982;Vareschi & Vareschi, 1984;Vareschi & Jacobs, 1985;Finlay et al, 1987;Gilabert, 2001;Yasindi et al 2007;Oyoo-Okoth et al, 2011;Ong'ondo et al, 2013). Generally rotifers thrive well under eutrophic conditions, for example, they may reach population densities as high as 20,000 ind l -1 (Gulati et al, 1992;Ooms-Wilms et al, 1999;Bonecker & Aoyagui, 2005).…”

“…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 alkaline-saline lakes in East Africa, have a restricted rotifer composition (Hecky & Kilham, 1973;Nogrady, 1983;Finlay et al, 1987;Gilabert, 2001;Oyoo-Okoth et al, 2011). Previous studies in Lake Nakuru, pointed to the dominant rotifer species of genus Brachionus (Harper et al, 2003;Oyoo-Okoth et al, 2011;Burian et al, 2013).…”