Benthic secondary production in a mesotrophic lake and its implications for terrestrial consumer energetics
Environmental and Natural Resource Sciences
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Historically, limnological studies have focused on understanding the pelagic habitats in lakes. While pelagic habitats are important to whole lake productivity, benthic habitats are very productive and contribute to whole lake productivity as well. There are a variety of benthic invertebrates that emerge from the lake into adult form to reproduce. The flux of resources emerging from the lake may provide resources for the surrounding terrestrial habitat. Terrestrial consumers have been shown to have greater densities near water bodies, likely due to high productivity of aquatic emergent invertebrates. The transfer of organic matter across aquatic and terrestrial ecosystems allows consumers in either ecosystem to shift diets depending on seasonal invertebrate production fluxes in their respective ecosystems. Understanding the temporal aspect of invertebrate production is important when assessing resource competition between aquatic and terrestrial ecosystems. Lake ecosystems are often manipulated for management purposes through fish stocking (e.g. recreation, clarity), which may impact the availability of emerging aquatic invertebrates due to fisheries reliance on littoral-benthic habitats. The impact of stocked fish on aquatic invertebrate emergence may alter the foraging behavior of terrestrial consumers, such as bats, that forage on aquatic emergent invertebrates to some degree. The goals of this study were to understand benthic invertebrate production and aquatic and terrestrial prey availability for higher consumers (e.g. bats, fishes). Specifically, the first chapter provides an introduction to the project. The objectives of the second chapter were to 1) determine seasonal and spatial patterns of benthic invertebrate production in a mesotrophic lake and 2) estimate the emergence of invertebrates from the lake in order to determine potential availability for terrestrial consumers. Annual, seasonal, and whole-lake benthic secondary production were estimated using the size-frequency method for the dominant taxa. Emergence-to-production ratios were examined during all seasons. The results suggest that while there were abundant benthic invertebrates in Castle Lake, aquatic invertebrates did not enter the terrestrial landscape at their maximum potential possibly due to consumption by stocked fish, or to natural mortality. The objectives of the third chapter were to 1) examine the relationship between fatty acid and isotopic composition of potential aquatic and terrestrial prey sources and bat consumers, based on seasonal variations in the relative abundance of aquatic and terrestrial prey and 2) review the feeding behavior of bats and classify fatty acids as aquatic or terrestrially derived. Aquatic emergent and terrestrial invertebrate densities and biomass, carbon stable isotopes and fatty acids were analyzed to determine aquatic and terrestrial invertebrate contributions to bat energetics. Carbon stable isotope analyses indicated the relative percentages of aquatic versus terrestrial invertebrates in secondary consumer diets. Fatty acids were separated into distinct terrestrial and aquatic functional groups through the use of a principal component analysis and a synthesis of current fatty acid literature. Insectivorous bats received 20 % of their total fatty acids from both aquatic and terrestrial ecosystems. The last chapter of this thesis contains the conclusions from the second and third chapters.