Changes in whitefish scales d13C during eutrophication and reoligotrophication of subalpine lakes

We measured the isotope composition of whitefish (Coregonus lavaretus) scales from 50-yr collections to determine the pattern of change of d13C during the eutrophication and reoligotrophication of two subalpine lakes: Lakes Constance and Geneva. The isotope composition of scales from Lake Annecy, a subalpine lake that has been protected from eutrophication, was also determined, to provide a control value for modifications in the values of whitefish scale d13C in the absence of any significant changes in the trophic status of the lake. In Lakes Constance and Geneva, changes in whitefish scales d13C were closely correlated with those in the mean annual phosphorus concentrations and, to a lesser extent, also reflected the efficiency of winter mixing in Lake Geneva. In contrast, in Lake Annecy, whitefish d13C exhibited only minor changes during the survey. Changes in whitefish scale d13C with the trophic status of Lakes Geneva and Constance were not linked to either a shift in whitefish feeding behavior or to alterations in the length of the pelagic trophic chain. Hence, the pattern of variation of whitefish d13C during the last decades mirrored the lakes’ trophic history. The responses of the Lakes Geneva and Constance to changes in Ptot were strictly synchronous and parallel, with a response during restoration that may have been the result of the antagonism between the effects of the falling phosphorus concentration and increasing temperature. Stable isotopic analysis (SIA) has been extensively used to study trophic relationships in lakes worldwide (Kling et al. 1992; Cabana and Rasmussen 1994; Post 2002). As the carbon isotopic composition (d13C) of the consumer reflects that of its prey with nearly no enrichment (10.39‰ [Post 2002]), d13C can be used to track the primary producer at the base of the trophic chain. In addition, any isotope modification at the bottom of the lake food web can be transmitted up to the top predators, such as fish, which can therefore potentially be used as integrators (Vander Zanden and Vadeboncoeur 2002). It has been shown that the d13C of pelagic components depends on the primary production of the lake (Schelske and Hodell 1991; Gu et al. 1996; Schindler et al. 1997). When the lake’s primary production increases, the pelagic components get enriched in 13C. Many processes and mechanisms combine to produce this pattern, but the end result is that the d13C of the pelagic components could provide a reliable indicator of the primary production of the lake (Lehmann et al. 2004). Eutrophication and reoligotrophication have affected many lakes worldwide, and may have thus altered the d13C of pelagic components. So far, d13C changes in response to eutrophication and reoligotrophication of lakes have been studied from the organic and 1 Corresponding author. Present address: Department of Biology, University of Victoria, P.O. Box 3020, Stn CSC, Victoria, BC V8W 3N5, Canada ([email protected]).