5 Conclusion

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The objective of this thesis was to assess whether and to which extent phytoplankton pigments in Lake Baikal can indicate changes of the recent and past phytoplankton community structure as well as climatic and other environmental changes. Moreover, the information gathered by the phytoplankton pigments should be used to complement our knowledge on the current and historical productivity variations in Lake Baikal in conjunction with the paleoclimate project CONTINENT and the long-term monitoring programme of the State University Irkutsk.

Here, I show that the analysis of recent phytoplankton pigments in the water column of Lake Baikal was a useful approach to characterise the recent phytoplankton assemblage and its variations induced by environmental changes. Pigment-based studies were demonstrated to greatly aid our understanding of the ecology of large lakes as well as to be a considerably improvement on traditional monitoring by microscopic counts. Even planktonic groups, e.g. the picoplanktonic Eustigmatophyceae, which cannot be distinguished by light or epifluorescence microscopy, could be traced by this method. Chla concentrations, as a measure of potential productivity, strongly differed between the regions, years and seasons. However, the community structure varied more than the total productivity with changing environmental conditions, with temperature and stratification being the main driving forces. Therefore, we can expect that predicted future warming in the lake would lead to significant changes in the phytoplankton composition. Moreover, this change would be towards small phytoplankton species at the expense of the characteristic large endemic diatoms.

The use of fossil phytoplankton pigments to determine past community structural changes and environmental influences was more complex than the use of recent phytoplankton pigments to determine the recent community composition. During sedimentation out of the euphotic zone and even at the surface sediment, Chla or phytoplankton pigments in general were considerably affected by degradation processes. The degradation was mainly caused by oxidation and grazing. The manner and extent of the degradation processes varied regionally even among the open basins, indicating the limits of extrapolations within the lake. Transfer functions were provided in this thesis for different regions in order to further improve reconstructions based on fossil phytoplankton pigments.

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However, although the degradation of the phytoplankton pigments before permanent burial in the sediment was strong, the ratio of fossil Chla plus its degradation products vs. organic carbon accurately tracked published past climate changes, even those that occurred more than 100,000 years ago. Hence, sedimentary Chla in Lake Baikal was shown to be a reliable indicator of phytoplanktonic response to published climate changes and may serve for validation of future climate models in continental regions. Chlb plus its degradation products provided important additional information on the past development of Chlorophyta. Most other sedimentary phytoplankton pigments were found to be unsuitable to determine past phytoplankton community structures in Lake Baikal, because their degradation products could not be definitely related to the parent pigments.

The ratio of fossil Chla plus its degradation products vs. organic carbon (as a measure of the productivity) indicated that strong oscillations of the productivity occurred during the Holocene (from c. 10,000 years BP to present day) and the last interglacial (from c. 129,000 to 117,000 BP). These results highlighted the great natural variability at times before any human impact on a pristine lake. Productivity optima were found that even surpassed the present productivity.

Taken together, pigment-based analyses were shown to accurately reflect phytoplankton variation caused by environmental changes of natural or human origin in Lake Baikal. In conjunction with the EU project CONTINENT and long-term monitoring by the State University Irkutsk, the phytoplankton development determined from the last interglacial up to the early 21st century presented in this thesis will be a useful basis for future research and modelling of climate changes as well as for the protection of Lake Baikal.


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