Date of Award

Spring 1-1-2014

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Ecology & Evolutionary Biology

First Advisor

William M. Lewis

Second Advisor

Alexander Cruz

Third Advisor

Brett Melbourne

Fourth Advisor

Pieter Johnson

Fifth Advisor

Peter Blanken

Abstract

In a study of 12 Colorado mountain lakes, some containing fish and some without fish, benthic invertebrate and zooplankton community composition and production were evaluated in relation to predictions of the concept of cascading trophic interactions. The trophic cascade concept predicts that lakes with fish will show lower abundance and production of primary consumers than lakes without fish. Data collected over a period of 4 years show that fish suppress benthic invertebrate and zooplankton abundance, as predicted by the trophic cascade concept, and reduce mean and maximum invertebrate body size, as expected from the literature. Contrary to the trophic cascade concept, however, production of benthic invertebrates and zooplankton did not differ between the two lake categories. Resilience of invertebrate production occurs because smaller body size of invertebrates leads to a higher ratio of production to biomass that is sufficient to compensate for the effect of reduced biomass of invertebrates. Also, the stable isotope signatures for δ13C and δ15N show that the two categories of lakes did not differ in relative importance of energy sources for synthesis of biomass. Compensatory stability of prey production despite changes in prey biomass and composition may be a general phenomenon in food webs.

In 2013, as part of the same study, the effect of fish on lacustrine zooplankton in streams below lakes was quantified in 12 lakes with and without fish. Reductions of zooplankton abundance within lakes by fish predation caused lower transport of zooplankton to streams. Diel (24 hour) sampling showed that zooplankton export from both categories of lakes was nearly 4 times greater at night than during the day. Diel variation in export is explained by zooplankton migration from deep to shallow water at night.

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