Diatoms as indicators of environmental change in ancient Lake Ohrid during the last glacial-interglacial cycle (ca. 140 ka)

Lake Ohrid (Macedonia/Albania) is probably the oldest lake in Europe, with an estimated age of 2 - 10 Ma. It is widely recognized that the palaeolimnological records preserved in the sediments of ancient lakes such as Ohrid are important archives of Quaternary climate change, providing long-term, high-resolution, continuous sequences which are terrestrial equivalents of the marine and ice-core records. These lakes are also hotspots of biodiversity, harbouring exceptional diversity of endemic taxa. As part of multi-proxy palaeolimnological research in advance of an International Continental Deep Drilling Programme coring campaign in 2012, this study focuses on taxonomy in presenting the results of a diatom-based study of a 14.5 m long sequence (core Co1202), which spans the last ca. 135 ka of lacustrine history. The authors aimed at testing diatom response to major climate events and the impact of tephra from Italian volcanic eruptions across two glacial-interglacial transitions. Pursuing the taxonomic theme, this extended to analysis of sub-species level morphological variability. The results indicate that in Ohrid the diatoms are extremely sensitive palaeoclimate indicators, tracking glacial-interglacial shifts and more subtle global events including MIS3. We have also identified a sub-species level response in planktonic diatom taxa which is likely to contribute additional proxy data in future palaeoclimate studies. They are the only biotic organisms to be preserved throughout glacial- interglacial cycles, suggesting their value extends also to testing theories of evolution and biodiversity over the Quaternary as a whole. A total of 179 taxa was identified in the fossil record; descriptions of the most frequent taxa are presented here. Formal description is provided of two new species which are present in the Holocene and modern environment: Encyonema cespitosiforme Cvetkovska & Levkov spec. nov. and Mastogloia ohridana Cvetkoska & Levkov spec. nov. Two planktonic taxa are dominant in varying proportions throughout. Cyclotella fottii showed variation in valve size classes between glacial and interglacial periods (although possibly linked to preservation potential rather than ecological response). C. ocellata exhibited sufficient variation in core Co1202 to recognize 22 different morphotypes, with a clear shift in types between successive interglacials. One of the key hypotheses to be raised by this study is that the higher diversity of the C. ocellata complex during MIS 5 compared to the Holocene represents evolution at a subspecies level.