Holocene subfossil chironomid stratigraphy (Diptera: Chironomidae) in the sediment of Plešné Lake (the Bohemian Forest, Czech Republic): Palaeoenvironmental implications

Abstract

A faunal record of chironomid remains was analyzed in the upper 280 cm of a 543 cm long sediment core from Plesne jezero (Plesne Lake), the Bohemian Forest (Sumava, Bohmerwald), Czech Republic. The chronology of the sediment was established by means of 5 AMS-dated plant macroremains. The resolution of individual 3-cm sediment layers is ∼115 years and the analyzed upper 280 cm of the sediment core represent 10.4 cal. ka BP. As the results of DCA show, two marked changes were recorded in the otherwise relatively stable Holocene chironomid composition: (1) at the beginning of the Holocene (ca. 10.4-10.1 cal. ka BP) only oligotrophic and cold-adapted taxa (Diamesa sp., M. insignilobus-type, H. grimshawi-type) were present in the chironomid assemblages, clearly reflecting a cool climate oscillation during the Preboreal period, and (2) during an event dated in the interval 1540–1771 AD, when most taxa vanished entirely and only Zavrelimyia sp. and Procladius sp. were alternately present accompanied by Tanytarsus sp. Although, the age of this event is in agreement with the dating of the Little Ice Age, the most probable reason for the elimination of many chironomid taxa was very low sums recorded in this part of the sediment, rather than cool conditions connected with the LIA. Variations in the chironomid fauna after the Preboreal period were reflected mainly by changes in abundances of dominant taxa rather than by changes in species composition. These variations could be explained by: (1) climatic changes, namely temperature and amount of rainfall resulting in oscillations in lake level, with changes in the occurrence of macrophytes in the littoral and (2) increasingly dense afforestation which led to a considerable input of organic material into the lake and a subsequent increase in the trophic status of the lake water.