CONSUMER VS. RESOURCE CONTROL OF CILIATE PROTOZOA IN A COPEPOD-DOMINATED SUBTROPICAL LAKE

Twenty controlled experiments were performed over a one-year period, t o quantify macro-zooplankton and nutrient impacts on ciliate protozoa in a shallow subtropical lake (Lake Okeechobee, Florida, USA). In trip licate treatments using small (20-L) microcosms, macro-zooplankton gra zers were excluded by size-fractionation (114 mu m), and/or nutrients (nitrogen and phosphorus) were added, and responses of ciliates were c ompared with untreated controls after 3-day incubations. Densities of dominant ciliate taxa (Strobilidium humile, Tintinnium fluviatile, Cyc lidium glaucoma, and Mesodinium pulex) were not affected by zooplankto n removal in most experiments. The smallest taxon (M. pulex) displayed the most frequent evidence of consumer control, indicated by signific ant increases in density in zooplankton-free treatments. However, this response occurred only in 3 experiments. On most occasions, nutrient additions also did not affect ciliate densities. One taxon (C. glaucom a) was more responsive to nutrients than the others, but again, signif icant density increases occurred only in 3 experiments. These findings indicate that other factors, perhaps abiotic ones, primarily control ciliate population densities in this subtropical lake. This finding is consistent with previous research. which indicated that bacterioplank ton (a major food resource for these ciliate taxa) also was under abio tic control in Lake Okeechobee. To fully understand ciliate dynamics, it may be necessary to supplement experimental studies with measuremen ts of resources, consumers, and abiotic variables conducted at a finer temporal scale, along with measurements of taxon-specific growth, rep roduction, and mortality rates.