Diatoms as sensitive indicators for climatic and anthropogenic impacts on the palaeoecosystem of Lake Holzmaar (West Eifel, Germany)
by
Janina Baier
GeoForschungsZentrum Potsdam, Germany
Coauthors: Andreas Lücke (Forschungszentrum Jülich, Germany), Jörg F.W. Negendank (GeoForschungsZentrum Potsdam, Germany), Bernd Zolitschka (Geomorphologie und Polarforschung GEOPOLAR, Institut für Geographie, Universität Bremen, Germany)

A high-resolution quantitative and qualitative diatom study in combination with geochemical analyses was carried out in order to reconstruct the palaeolimnological development of Lake Holzmaar (West-Eifel, Germany) with regard to climatic and/or anthropogenic impacts during the late Holocene focusing on the time window from 6300 to 1500 varve years BP. The fossil diatom taxa were used for the reconstruction of total phosphorus and pH. Furthermore, diatom fluxrates were calculated. These results are discussed in parallel with accumulation rates of geochemical data including total organic carbon (TOC), biogenic silica (BSi), total sulphur and the clastic contribution. . Noticeable are time lags between shifts in fossil diatom assemblages and changes in sediment composition and bulk accumulation rates. Stephanodiscus minutulus, Nitzschia paleacea and Asterionella formosa were the dominant planktonic diatoms between 6300 and 5100 varve years BP, suggesting that Lake Holzmaar was eu- to polytrophic. High accumulation rates for BSi, TOC and for minerogenic material as well as high diatom flux rates were characteristic for this time period. Reconstructed pH remained high throughout (pHmax = 9, 4). Possibly enhanced catchment erosion and intense upwelling increased nutrient supply to the lake. Diatom communities changed after 5100 varve years BP due to increases in Cyclotella species, like C. comenis and C. ocellata as well as in Fragilaria species (e. g. F. tenera, F. nanana). These changes are consistent with a period of low phosphorus concentration, low diatom productivity, low pH and low accumulation rates of particulate matter. Maybe strong and long lasting summer stagnation prevented intense diatom growth. The above trend started to change around 3700 varve years BP when Stephanodiscus minutulus reappeared. Taxonomic shifts were associated with limnological changes, however appearing approximately 200 years later. Coincidentally only reconstructed pH increased, whereas diatom-inferred TP did not start to increase sharply until around 3500 varve years BP. Diatom flux rate was subject to higher fluctuations after 3700 varve years BP, but only increased slightly. Diatom flux rate tripled around 2850 varve years BP, whereas SARbsi even decreased at that point. The results reflect a change in the fossil diatom community that was obviously not triggered by external nutrient supply. Changes in e.g. internal circulation processes (coupled with climatic changes) could have been responsible for this gradual change in the lacustrine system. The most striking shift to higher accumulation rates appeared around 2600 varve years BP – the beginning of the Iron Age. SARmin showed a three-fold rise around 2600 varve years BP relative to the values around 2700 varve years BP. Interestingly, there was a 17-years lasting time-lag before SARbsi tripled, SARtoc doubled and before diatom fluxrates showed a 9-fold increase. The diatom community reacted as well to this change in sedimentation character: Nitzschia paleacea disappeared around 2600 varve years BP while Cyclotella radiosa reappeared in the fossil diatom community in low abundances. Diatom-inferrred trophic status and pH successively dropped after 2600 varve years BP. Again, diatom composition was subject to a major change around 2200 varve years BP, when Stephanodiscus minutulus populations reduced to low numbers and Cyclotella species (C. radiosa, C. comensis and C. ocellata) became dominant, accompanied by higher abundances of Asterionella formosa. Diatom-inferred TP suggests that Lake Holzmaar turned oligo-/mesotrophic to mesotrophic and reconstructed pH was low (pH 6, 6 to 7, 16). SARmin and SARbsi increased slightly, whereas diatom flux rates decreased compared to the beginning of the Iron Age.The dominance of-species suggest that the Si:P-ratio steadily increased after 2200 varve years BP. A change in external nutrient supply and/or lake internal processes were responsible for this change. The lacustrine condition changed once more around 1650 varve years BP. Cyclotella radiosa and Asterionella formosa dominated the fossil diatom community and Stephanodiscus minutulus reoccured in small numbers. This change is also reflected by a rise in diatom-inferred TP and pH. Accumulation rates for minerogenic matter, BSi and TOC decreased simultaneously. This detailed and high-resolution lacustrine record provides a multi-proxy approach to improve the understanding of a complex ecosystem. The ultimate goal, a discrimination of climatic and anthropogenic forcing factors of palaeoenvironmental changes, has not yet been conclusively obtained. This poster belongs to a poster cluster of the DFG priority programme "Changes of the Geo-Biosphere during the last 15.000 years. Continental sediments as evidence for changing environmental conditions".

http://www.uni-frankfurt.de/fb11/ipg/spp/Postergallery/Postergallery.htm

Date received: March 29, 2001

Copyright © 2001 by the author(s). The author(s) of this document and the organizers of the conference have granted their consent to include this abstract in Atlas Conferences Inc. Document # cagc-50.