The quick death of a lake: human impact on Lake Tresssee (N Germany) during the last 6000 years – an approach using pollen, Cladocera and sedimentology

Human-induced fluctuation of lake levels has been a common phenomenon in Europe since Neolithic times. At present, Lake Tresssee is a eutrophic lake covering less than 5 ha in northern Germany, but its sudden shrinking from ~125 ha before 1800 is considered a consequence of anthropogenic lowering of the lake level. We investigated the history of anthropogenic vegetation changes and water level fluctuations by multiproxy studies of a 4 m core from the former lake area. Our analyses of pollen and Cladocera subfossil, chemistry and sedimentological features yielded important conclusions about interactions between land-use history and climate impacts on the lake and its surroundings. The results indicate that the highest lake level persisted until the Late Atlantic. Since the Subboreal there have been several fluctuations, mostly in consequence of climate impacts. Later, different phases of sediment input to the lake from tributary streams and probably also from aeolian processes from an adjacent dune field were observed. At ~2800 BC the sedimentation rate decreased in consequence of fluvial impacts, as the lake basin was nearly filled up with deposits. As a result of greater human impacts, from the Early Bronze Age the macrophyte zone expanded in the lake, the oxygen content of the water continuously decreased, and heathlands developed in the surroundings. From the Late Iron Age and in the Early Medieval, pollutants probably from ironworks are detectable by geochemical analyses in the corresponding segments. In the pollen diagram the Migration Period is clearly visible, but the suggested radiocarbon date is younger than in Lake Belau in the neighboring region of Schleswig-Holstein. Most probably our pollen diagram did not register the absolute maximum values of Fagus related to the Migration Period. From the Early Medieval a clear phase of resettlement occurs. Since the Early Modern period, the lake level has shrunk rapidly in consequence of historically documented human activity.