Seasonal Abundance of Two Species of Rotifers (Brachionus calyciflorus and Keratella tropica) in Kandy Lake, a Tropical Urban Water Body in Sri Lanka

Monsoon-bound seasonal abundance of two species of rotifers (Brachionus calyciflorus and Keratella tropica) was examined in relation to the population densities of Cladoceran zooplankton and phytoplankton and some limnlogical characteristics in Kandy Lake, a tropical urban water body in Sri Lanka, using standard techniques. Kandy Lake did not show marked seasonal variation influenced by the monsoons with respect to limnological parameters except for bicarbonate alkalinity and chlorophyll-a content. The population densities of Brachionus calyciflorus declined and Keratella tropica became dominant during the first inter-monsoon (March-April) when the abundance of total cladocerans and Microcystis aeruginosa, a cyanobacterial species with toxigenic strains, were high under relatively turbid conditions. Brachionus calyciflorus was re-established progressively during the subsequent season with decreasing populations of Microcystis aeruginosa and increasing densities of Aulacoseira granulata, a filamentous centric diatom, and Pediastrum simplex, a colony forming green algae. The likely explanation for the marked suppression of Brachionus calyciflorus during the first inter-monsoon could be the abundance of toxin forming cyanobacteria strains of Microcystis aeruginosa or else a combined effect of Microcystis aeruginosa and interference competition resulting from high densities of total cladocerans under unfavourable environmental factors such as high turbidity.