Natural geographic barriers and escarpments inhibited the post-glacial colonization of many lakes by fish on the eastern Canadian Boreal Shield. Hence, fishless lakes containing freshwater communities that evolved without fish predation are commonly found on the Boreal Shield next to fish-present lakes. The aim of this study was thus to assess how different top-down control from fish or invertebrate predation structure zooplankton communities in eastern Canadian Boreal Shield lakes throughout the ice-free season. We examined zooplankton community characteristics using univariate indices such as abundance, richness (S), evenness (J') diversity (H'), and multivariate species assemblages (Bray—Curtis dissimilarity) in 5 lakes with a single fish population, brook trout, Salvelinus fontinalis, and in 5 naturally fishless lakes throughout the ice-free season. The total abundance of zooplanktonic organisms was significantly higher in fishless lakes over the whole sampling period; this was related to the presence of small herbivorous species such as rotifers. Species assemblages of zooplankton were also significantly different between the 2 types of lakes throughout the season, principally due to the high abundance of rotifers and the scarcity of immature and mature daphnids in fishless lakes. Our results suggest that the low intensity of selective predation by brook trout on large herbivores and the heavy predation by Chaoborus americanus larvae on daphnids in fishless lakes appear to be the key factors for structuring the zooplankton community. Multivariate indicators identified patterns that were not revealed with the usual univariate indices, showing the importance of selecting appropriate indicators for the assessment of community structure.

Nomenclature: Edmondson, 1959; Pennak, 1978; Bernatchez & Giroux, 2000.