The ecology of planktonic ciliates in tropical lakes of East Africa.

摘要

Planktonic ciliates were studied in 17 East African lakes of different salinity and trophic status from September to December in 1998 and between August and December in 1999 to evaluate their role in the food webs and to compare their abundance, biomass and species composition with those of temperate and subtropical lakes. Oligotrichs (mainly Strombidium and Strobilidium) and scuticociliates (e.g., Cyclidium, Cristigera and Pleuronema) were the most abundant ciliates in the lakes during the period of study. The abundance of oligotrichs decreased while that of scuticociliates increased with increasing chlorophyll a and conductivity. Other important orders both numerically and in terms of biomass were Cyrtophorida, Peniculida, Heterotrichida, Prostomatida, Hypotrichida and Haptorida.; Conductivity and trophic status were the most important environmental variables that influenced the distribution of ciliate species in East African lakes. Strobilidium, Strombidium, Stichotricha, and Spiretella were more abundant in freshwater and oligotrophic-mesotrophic lakes while the abundances of Dysteria, Cyclidium, Aspidisca, Cristigera, Holophrya, Chlamydodon, Monodinium, and Frontonia were higher in saline and eutrophic lakes. Halteria, Vorticella, Urotricha, Lagynophrya, and Linostoma were found in most East African lakes.; Experiments with 1-μm fluorescently labeled beads showed that Cyclidium, Halteria, Dysteria, Aspidisca, and Vorticella ingested fluorescently labeled beads and are bacterivorous, while other ciliates were herbivorous, carnivorous, omnivorous or mixotrophic.; Mean ciliate abundance ranged from 2 to 1,220 ciliates·mL −1 and the biomass range was between 1.9 and 1,900 μg C·L −1. Both ciliate abundance and biomass had positive relationships with chlorophyll (mg·m−3), bacteria (·mL−1) and conductivity.; Seasonal ciliate biomass maxima coincided with bacterial abundance maxima in saline, eutrophic lakes but with phytoplankton biomass in mesotrophic ones. In stratified lakes, most ciliate species were found in the epilimnion but ciliate biomass peaks occurred at oxyclines or thermoclines.; Regression models predicting abundance and biomass of ciliates from chlorophyll showed that East African, subtropical (Florida), and temperate (Quebec) lakes have similar ciliate abundance and biomass per unit chlorophyll. The eutrophic lakes of East Africa with higher ciliate abundance and biomass relative to chlorophyll are saline, and these type of lakes were not included in the other two data sets.; Ciliate growth rate estimates ranged from 0.18·d−1 to 6.56·d−1 and the potential production range was from 8.22 μg C·L−1·d −1 to 2,334.7 μg C·L−1·d −1 (mean 255.5 μg C·L−1·d −1). Bacterivorous and herbivorous ciliates accounted for 36% and 39% respectively of the total ciliate production in these lakes. However, the ciliates were consumed by metazoan zooplankton, whose predation rates were between 0 and 0.958·d−1. My data suggest that much ciliate production is consumed within the ciliate community. Nevertheless, ciliates are an important link between the microbial components and larger organisms (e.g., zooplankton and some fish) in the food webs of East African lakes, as they are for lakes at higher latitudes.