ZOOPLANKTON SPATIAL PATTERNS IN 2 LAKES WITH CONTRASTING FISH COMMUNITY STRUCTURE

Horizontal and temporal patterns in crustacean zooplankton communities were analyzed in two small, oligotrophic lakes which were morphologic ally and chemically similar, but had contrasting fish communities. Ran ger Lake was dominated by two bass species and the planktivores number ed < 25 ind. ha-1. Mouse Lake had no large piscivores and planktivores numbered > 1200 ind. ha-1. There were significant differences in the distribution of zooplankton taxa and size classes between sampling sta tions. In Ranger Lake, the smallest size classes were more abundant at the deeper stations and the larger individuals were more abundant at the shallower stations. In Mouse Lake, the smaller individuals were mo re common at the shallow stations and the larger individuals were more common at the deeper stations. These differences suggest medium scale patterns induced by vectorial forces, but modified by species specifi c migration patterns. We tested the hypothesis that horizontal heterog eneity should be influenced by planktivore density and found that none of the taxa showed significant between-lake differences in the varian ce-mean regressions. We also tested the hypothesis that larger taxa sh ould be more heterogeneous and we found that cladocerans were more het erogeneous than copepods and nauplii. In terms of sampling methodology our data suggest that the between-station variability was so high tha t a single mid-lake sample would certainly lead to completely unaccept able errors in the estimation of population densities and biomasses.