Food limitation of planktonic rotifers: field experiments in two mountain ponds

1. Resource competition is thought to be important in controlling zooplankt on population dynamics and structuring zooplankton communities. Resource co mpetition requires that resources are limiting. Ten field experiments were conducted to determine the presence and intensity of resource limitation of herbivorous planktonic rotifers in two mountain ponds. The intensity of fo od limitation was measured as Deltar, the difference between population gro wth rates in food supplemented versus control enclosures. 2. Rotifers were significantly food limited (Deltar > 0) in most experiment s. The intensity of food limitation varied between species. In Grady Lake, mean Deltar was 0.26 day(-1) for Polyarthra vulgaris and 0.43 day(-1) for a n unidentified Synchaeta sp. In L1 Pond, mean Deltar was 0.07 day(-1) for K eratella cochlearis and 0.28 day(-1) for S. oblonga. The frequent and inten se food limitation suggests that edible phytoplankton in both ponds were of ten present at low densities, were of low nutritional quality, or both. The intensity of food limitation often changed rapidly over time, indicating r apid temporal variation in resource availability. 3. For P. vulgaris, Deltar was not correlated with its population growth ra te in Grady Lake. Thus, although food limitation was common, population dyn amics may have been more strongly affected by other factors. For K. cochlea ris, Deltar increased as population density increased, suggesting the exist ence of intraspecific resource competition that may regulate population dyn amics.