Abrupt and consistent responses of aquatic and terrestrial ecosystems to the 8200 cal. yr cold event: a lacustrine record from Lake Arapisto, Finland

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A 8400—7700 yr cal. BP section of the sediments of a small lake in southern Finland was studied by high-resolution pollen and cladoceran analyses to examine the response of terrestrial and aquatic ecosystems to the 8200 cal. yr BP cold event. Furthermore, a pollen-based Tann reconstruction and proportions of chydorid ephippia were used to determine changes in temperature and in the length of open-water season during the event. The pollen data, with abrupt decreases of the early-flowering Alnus, Corylus and Ulmus at c. 8350 cal. yr BP, may partly reflect vegetation dynamics but possibly also decreased pollen productivity and reproduction caused by cold winters and springs. The reconstructed Tann indicated a rapid temperature fall and a subsequent recovery. Proportions of chydorid ephippia suggested a considerable reduction in the length of open-water season at the beginning of the event, followed by periodically shorter open-water seasons. In the aquatic ecosystem only one species, the dominant planktonic cladoceran Bosmina longirostris, responded by decreasing dramatically and almost became extinct during the coldest period. However, we were unable to reconstruct the causative link between the inferred climate change and its decrease. The response of both aquatic and terrestrial proxies at c. 8350 cal. yr BP was consistent, indicating an abrupt start of the event and also an abrupt end at c. 8100 cal. yr BP.