Consumer vs. resource control of ciliate protozoa in a copepod-dominated subtropical lake

Abstract

Twenty controlled experiments were performed over a one-year period, to quantify macro-zooplankton and nutrient impacts on ciliate protozoa in a shallow subtropical lake (Lake Okeechobee, Florida, USA). In triplicate treatments using small (20-L) microcosms, macro-zooplankton grazers were excluded by size-fractionation (114 μm), and/or nutrients (nitrogen and phosphorus) were added, and responses of ciliates were compared with untreated controls after 3-day incubations. Densities of dominant ciliate taxa (Strobilidium humile, Tintinnium fluviatile, Cyclidium glaucoma, and Mesodinium pulex) were not affected by zooplankton removal in most experiments. The smallest taxon (M. pulex) displayed the most frequent evidence of consumer control, indicated by significant 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. glaucoma) was more responsive to nutrients than the others, but again, significant 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 bacterioplankton (a major food resource for these ciliate taxa) also was under abiotic control in Lake Okeechobee. To fully understand ciliate dynamics, it may be necessary to supplement experimental studies with measurements of resources, consumers, and abiotic variables conducted at a finer temporal scale, along with measurements of taxon-specific growth, reproduction, and mortality rates.