Indication of climatically induced natural eutrophication during the early Holocene period, based on annually laminated sediment from Lake Holzmaar, Germany

Abstract

A multidisciplinary study is presented for the period from 10,128–9102±10 cal BP from Lake Holzmaar in western Germany. Concurrent signals in sediment chemistry and diatom assemblages are recorded in the varved sediment sequence, and several distinct periods in the lake's response to climatic amelioration during the early Holocene period are revealed. Diatom-based transfer functions were used to estimate the lake's trophic status. In conjunction with high-resolution geochemical analysis, these data were applied to quantify the magnitude of natural nutrient enrichment. The inferred changes in the lake nutrient status are discussed with reference to the changing environmental conditions and development of the aquatic ecosystem. The major perturbation of the lake occurred with the rapid eutrophication at 9820 cal BP. Following a period of decreasing productivity due to a meromictic phase of Lake Holzmaar, an accelerated release of phosphorus from the sediment occurred and diatom-inferred total phosphorus concentrations increased rapidly. Longer hydraulic residence times in Lake Holzmaar are assumedly related to elevated evaporative transpiration rates and increasing temperatures. Indication of milder winter and increasing summer temperatures at this time is in agreement with interpretation of the pollen data and changes in the seasonal characteristics of the diatom assemblages.