1Department of Biology, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada
2Current address: Department of Zoology, Egerton University, P.O Box 536, Njoro, Kenya

Abundance, biomass and estimated production of planktonic ciliates in Lakes Victoria and Malawi

The abundance, biomass and composition of planktonic ciliates were studied in Lakes Malawi and Victoria between June 1997 and June 1999, to investigate their role in the food webs of these lakes. Oligotrichs (Strombidium, Strobilidium and Halteria) were the most abundant ciliates in both lakes. Other important ciliates, in terms of abundance and/or biomass, included heterotrichs, peritrichs, haptorids, and peniculids. Mean ciliate abundance was 2.2 ± 0.7 ciliates ml-1 and 12 ± 4.3 ciliates ml-1 in Lakes Malawi and Victoria, respectively. Ciliate biomass ranged from 0.03 to 7.82 µgC l-1 (mean 2.3 ± 0.7 µgC l-1) in Lake Malawi and ranged from 24.2 to 61.82 µgC l-1 (mean 43 ± 13.2 µgC l-1) in Lake Victoria. Both ciliate abundance and biomass were higher in offshore than inshore waters of both lakes despite that chlorophyll exhibited an opposite trend. Ciliate growth rate estimates ranged from 0.74 to 4.20.d-1 in Lake Malawi and 0.66 to 5.71.d-1 in Lake Victoria while estimated ciliate production was 9 and 175 µgC l-1 d-1 in Lakes Malawi and Victoria, respectively. The majority of ciliates in both lakes were herbivores accounting for 63% and 70% of estimated ciliate production in Lakes Malawi and Victoria. In Lake Victoria, grazer manipulation experiments demonstrated that most of the ciliate production was consumed by metazoan zooplankton whose estimated predation rate on ciliates was 1.68 d-1.