Climate-induced changes in the trophic status of a Central European lake

We present a case study of the development of Sacrower See, a stratified, eutrophic lake in northeastern Germany, over the past 13,000 years. Total epilimnetic phosphorus (TP) concentrations were reconstructed quantitatively using a diatom-TP transfer function. Fossil chironomid assemblages were used to support the trophic reconstruction and helped assessing past hypolimnetic oxygen availability. The results indicate eutrophic and anoxic conditions during the Younger Dryas cold period (~12,700-11,600 cal. BP) preceding the present interglacial. Throughout the early and mid-Holocene (~11,600-4000 cal. BP) stable oligo- to mesotrophic conditions with diatom-inferred TP values of ~20 μg L-1 prevailed. First evidence of increasing Holocene TP is recorded at ~3500 cal. BP associated with Bronze Age human impact and for the past 900 years diatoms indicate increasing TP values of 30-60 μg L-1. During the early Holocene and the past two millennia chironomids indicated anoxic hypolimnetic conditions. The chironomid fauna is considered typical of oligo- to mesotrophic lakes. As a consequence of strongly increased primary production the hypolimnion of Sacrower See became anoxic again during the past 140 years. Our results indicate that highly productive eutrophic conditions can exist prior to cultural eutrophication. At Sacrower See the shift from eutrophic conditions in the Lateglacial to oligo-mesotrophic conditions in the early and mid-Holocene was associated with the climatic warming at the Younger Dryas/Holocene transition. The high productive state during the cold Younger Dryas is associated with changes in seasonality: the prolonged winters caused longer ice cover, stronger stratification, anoxia in the hypolimnion, and consequent internal phosphorus loading. During the warm Holocene, however, hypolimnetic anoxia and internal phosphorus loading decreased significantly, resulting in a substantially lower productivity.