Changes in whitefish scales (delta)13C 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 (delta)13C 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 (delta)13C in the absence of any significant changes in the trophic status of the lake. In Lakes Constance and Geneva, changes in whitefish scales (delta)13C 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 (delta)13C exhibited only minor changes during the survey. Changes in whitefish scale (delta)13C 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 (delta)13C 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.