A Survey of the Impacts of Fish Introduction and Removal on Zooplankton of Alpine Lakes in Rocky Mountain National Park, USA

Lafrancois, Toben; University of Minnesota

Studies of fish effects on zooplankton in alpine lakes show mixed or inconclusive results. When effects are detected, they are not uniform across different systems (e.g. fish increase zooplankton taxa richness in some studies but decrease richness or lead to extirpation in others). Teasing out these relationships is important because (non-native) fish removal is becoming a more predominant management directive. We surveyed 33 alpine and sub-alpine lakes in Rocky Mountain National Park in late July 2005 to capture peak copepod and cladoceran production, comparing zooplankton communities of lakes with and without fish across an elevation and productivity gradient. Both redundancy analysis and a priori t-tests (Bonferroni corrected) were used to examine the factors that explained zooplankton community composition both in terms of mean individuals and biomass of each taxa. Fish had significant impacts on both zooplankton biomass and community composition in these lakes. Non-rotifer taxa that were found in lakes with and without fish were significantly smaller in lakes with fish. Zooplankton communities in lakes with fish were almost entirely composed of rotifers, while fishless lakes supported greater numbers and kinds of larger zooplankton but fewer rotifer taxa. Fish presence, however, did not account for differences in total taxa richness or total abundance of individuals. These results show that significant changes in biomass and community composition can be masked if management decisions are based only on comparisons of diversity or raw numbers of individuals. Changes in community composition are particularly important because different zooplankton communities have very different ecological functions.

A Survey of the Impacts of Fish Introduction and Removal on Zooplankton of Alpine Lakes in Rocky Mountain National Park, USA

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