The impact of nonnative species and cultural eutrophication on the Lake Tahoe food web over time

Aquatic ecosystems are impacted from anthropogenic stressors resulting primarily from increased nutrient loading and the introduction of non native species. In either case, these stressors may alter food web energetic pathways. This thesis evaluates the energetic consequences of cultural eutrophication and invasive species introductions on Lake Tahoe's food web. A novel approach comparing biotic stable isotope levels from preserved and contemporary specimens is utilized to measure impacts on food webs over time. In Chapter 1, the impact of 40 years of increased pelagic primary production on zoobenthos is measured. Increases in pelagic production shift the depth of 1% light penetration resulting in a decrease of periphyton production. As a result zoobenthic primary consumers receive a greater amount of their energy from newly sedimented pelagic primary production rather than from periphyton primary production. In Chapter 2, a retrospective analysis of over 120 years of species introductions and their impacts to the food web structure is quantified focusing on the differing impacts of vertebrate (lake trout) versus invertebrate (Mysis) introductions. Lake trout introductions did not result in significant food web structure changes however the elimination of the lake's only native predator (cutthroat trout) occurs. Mysis introduction result in a "middle out" effect shifting the trophic niche of chubs and lake trout by lowering their trophic position and shifting their energetic reliance on pelagic primary production. In Chapter 3, the impact Mysis introduction is evaluated on multiple levels of ecological organization (community and ecosystem). At the community level, Mysis eliminate their preferred cladoceran prey source. As a result they also reduce their reliance on herbivorous zooplankton over time and feed on algal-detrital material. On an ecosystem level Mysis may be playing an important role in controlling carbon export to the lake bottom by directly feeding on sedimented carbon.