Consistent patterns in δ¹³C and δ¹⁵N of multiple trophic levels across the Laurentian Great Lakes.

Stable isotope ratios of nitrogen (δ¹⁵N) and carbon (δ¹³C) are valuable tools for studying food web structure, but broad-scale variations in baseline δ¹⁵N and δ¹³C are not well-understood for freshwaters. We use the Great Lakes as a case study to test for variation in isotopic ratios in lakes that exhibit gradients in residence time, watershed characteristics, and trophic state. We used a nested ANOVA to investigate differences in δ¹³C and δ¹⁵N of organisms of multiple trophic levels (zooplankton, benthos, rainbow smelt, and lake trout) across lakes and seasons, accounting for within-lake variation due to region and year. There were significant effects of lake on baseline values of δ¹³C and δ¹⁵N; patterns were generally consistent with expectations, with more oligotrophic lakes (Superior) being relatively depleted and more mesotrophic (Ontario) to eutrophic (Erie) lakes being enriched in both δ¹³C and δ¹⁵N. Season was a significant factor for δ¹³C in all taxa except benthos and for δ¹⁵N in zooplankton and rainbow smelt. Zooplankton and benthos δ¹⁵N values were highly variable, but this variation was dampened in fish, as expected due to integration of prey items over time. Post-hoc analysis of δ¹³C and δ¹⁵N versus lakewide average chlorophyll, residence time, and watershed characteristics indicate that these properties are correlated with stable isotopes values. This study suggests that baseline isotopes ratios in large lakes may be predictable based on lake and watershed characteristics, as has been demonstrated for some marine and smaller freshwater systems. [ABSTRACT FROM AUTHOR]

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