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

摘要

Rotifers dominate zooplankton biomass of many aquatic environments. However, their link to food web biomass has rarely been elucidated in alkaline-saline lakes. Variations in C content, C:N ratio and stable isotope composition (delta C-13, delta N-15) of dominant rotifer species were studied from January to December 2008 in alkaline-saline Lake Nakuru to provide insights into their bioenergetics. We established that Brachionus dimidiatus dominated in terms of abundance (80,000-100,000 x 10(3) ind m(-3)) and C-biomass. Also B. dimidiatus constituted about 60-75% of the rotifer biomass in the samples. All the rotifer species exhibited significant (P0.05) seasonal differences in biomass, C and C: N ratios. Rotifers had lower mean delta C-13 than course particulate organic matter, fine particulate organic matter and fish, but higher than mean delta C-13 in the dominant phytoplankton species. In all species, delta C-13 and delta N-15 increased markedly during the rainy season reflecting the feeding onset on allochthonous food sources. The isotopic increase correlated with an increase in their C: N. Our results demonstrate that rotifers respond quickly to any increase in primary production and can cope with changes in its nature and timing.