Seasonal variability of secondary production of cladocerans and rotifers, and their trophic role in Lake Tana, Ethiopia, a large, turbid, tropical highland lake

Abstract

Daily and annual production rates of eight cladoceran and two rotifer species, and their seasonal variation and trophic role in the large, turbid, tropical Lake Tana, Ethiopia, were assessed in 2003–2005. Laboratory cultures were used to infer cladoceran development times, and secondary production was estimated using the growth increment summation and recruitment methods. Production for both taxa was highest in October–November, after the rainy season, and lowest in January–April during the dry season. Cladocerans and rotifers comprised 24% of the metazoan zooplankton biomass of 45.1 mg DW m⁻³, but comprised 53% of its production. Daily production for cladocerans and rotifers, respectively, was 1.23 and 0.94 mg DW m⁻³ d⁻¹, and annual production was 447.9 and 353.5 mg DW m⁻³ y⁻¹. Energy transfer efficiency from producers to zooplankton was 1.3% and 4.4% from zooplankton to planktivores. Herbivores consumed 3.4% of primary production and planktivores 36% of zooplankton production. High biomass turnover rates of cladocerans and rotifers sustain planktivores and, after a month's delay, decomposed Microcystis provides their main food source during the pre- and post-rainy months in Lake Tana.