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Stable isotope differences among the Lake Michigan 2015 CSMI transects
Citation:
Hoffman, J., A. Trebitz, T. Hollenhorst, A. Cotter, M. Sierszen, J. Scharold, Dave Bolgrien, S. Miller, A. Opseth, W. Bartsch, J. Lietz, AND L. Mohn. Stable isotope differences among the Lake Michigan 2015 CSMI transects. International Association for Great Lakes Research, Guelph, ON, CANADA, June 06 - 10, 2016.
Impact/Purpose:
not applicable
Description:
During the Lake Michigan 2015 Cooperative Science and Monitoring Initiative (CSMI), eight transects situated near tributaries that present a gradient of phosphorus loads were sampled from nearshore to offshore during May, July, and September. Our objective was to evaluate associated patterns in carbon and nitrogen stable isotope ratios of particulates and bulk zooplankton samples. Each transect was comprised of three stations arranged nearshore to offshore (18-110 m depth). At each station, samples for stable isotope analysis were obtained from both the epilimnion and the metalimnion or deep chlorophyll layer. We found a substantial isotopic gradient in zooplankton. Zooplankton were relatively 13C-enriched and 15N-depleted in epilimnetic waters sampled in July and September, though a tributary-associated difference was not detected. In contrast, zooplankton sampled in May and in the metalimnion were relatively 13C-depleted and 15N-enriched. These isotopic patterns are opposite that associated with differences in trophic level among zooplankton, indicating that systematic differences in biogeochemistry or food web inputs are likely the cause. Our results demonstrate that both seasonal and depth-specific differences in zooplankton must be recognized for correct stable isotope-based pelagic food web interpretation.