Large variation in periphyton δ13C and δ15N values in the upper Great Lakes: Correlates and implications

Abstract

Stable isotope ratios of carbon (δ13C) and nitrogen (δ15N) are frequently used to examine food web structure. Despite periphyton's importance to lake food webs, little is known about spatial variation of periphyton δ13C and δ15N values in the Great Lakes. We present periphyton δ13C and δ15N values from 28 sites the upper Great Lakes, including Lake Superior, the north shore of Lake Michigan, and Green Bay. We also examined variation in periphyton isotope values relative to several water quality parameters (TP, TN, TKN, NO3−, Kd) as well as periphyton C:N. There was a large range in both periphyton δ13C (range = 13.5‰) and δ15N (range = 10.2‰) among sites. Periphyton in more eutrophic sites had more depleted δ13C and more enriched δ15N compared to more oligotrophic sites. Our finding of high variability in periphyton isotope values in the Upper Great Lakes has implications for stable isotope-based reconstructions of food web structure.