STABLE ISOTOPE ANALYSIS METHODS DEVELOPMENT AND ASSESSMENT OF FOOD WEB DYNAMICS AFTER HYPOLIMNETIC OXYGENATION IN TWIN LAKES, WA
Skinner, Megan Marie
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Line-diffuser hypolimnetic oxygenation was initiated in North Twin Lake, Washington, in 2009 to mitigate reduction in Rainbow Trout (Oncorhynchus mykiss) and Brook Trout (Salvelinus fontinalis) fishery habitat due to temperature–dissolved oxygen “habitat squeeze.” Previous studies demonstrated that trout populations rapidly expanded into increased hypolimnetic habitat within the first few years of oxygenation. Previous short-term diet analyses indicated effects on fish diets; however, the long-term effects on fish ecology have yet to be established. In this study, stable isotope analysis of fish tissues suggests few differences in feeding ecology of principal coldwater fish species in North Twin Lake compared to unoxygenated South Twin Lake. When compared between lakes, Rainbow Trout and Brook Trout diets contained similar proportions of Daphnia, Chironomidae, and Chaoboridae. Littoral and epilimnetic-focused Golden Shiner (Notemigonus crysoleucas) diets were also similar between lakes. Observed similarities between Golden Shiner and trout diets suggest the effects of interspecific competition between salmonids and non-salmonids may be limiting trout growth and survival. Fisheries managers should therefore consider both habitat limitations and interspecific competition when managing for coldwater fish species in mesotrophic, dimictic lakes. To ensure proper SIA methods were utilized for the Twin Lakes food web study, I also investigated the necessity of lipid extraction of fish tissues prior to SIA and in situ Rainbow Trout isotopic turnover. I found that mathematical normalization is sufficient to account for bias in δ13C values associated with lipid content in fish tissues when C:N ratios are above 3.5. C:N ratios below 3.5 indicate tissues have insufficient levels of lipid to bias δ13C values. Additionally, I estimated stable carbon retention time of 154 (95% CI: 106 to 224) and 292 (95% CI: 257 to 763) days for Rainbow Trout liver and white muscle, respectively. For nitrogen, I estimated a retention time of 186 (95% CI: 114 to 329) days for both white muscle and liver. Generally, these findings support the use of more timely and cost-effective processing methods and improve interpretation of SIA in future aquatic food web and diet studies involving adult coldwater fish in freshwater systems.