Changes in whitefish scales δ13C 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 C-13 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 C-13 in the absence of any significant changes in the trophic status of the lake. In Lakes Constance and Geneva, changes in whitefish scales delta C-13 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 313 C exhibited only minor changes during the survey. Changes in whitefish scale delta C-13 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 C-13 during the last decades mirrored the lakes' trophic history. The responses of the Lakes Geneva and Constance to changes in P-tot 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.