Do subfossil Cladocera and chydorid ephippia disentangle Holocene climate trends?

Abstract

We used sedimentary records of Cladocera and chydorid (Chydoridae) ephippia to reconstruct Holocene climate trends from Lake Arapisto in southern Finland. The quantitative temperature record ( TJulyCla) inferred from subfossil Cladocera was compared with a previously published pollen-based mean annual temperature reconstruction ( TAnn) from the same lake. Furthermore, proportions of total chydorid ephippia (TCE), indicating proportions of asexual and sexual reproduction, were examined from the sediment core to provide supplementary data. The TJulyCla record showed a consistent trend of slowly rising July temperatures (from 12–13°C to ~15°C) during the early Holocene until ~9500 cal. BP and indicated slightly higher temperatures than prior (~16°C) during the Holocene thermal maximum (HTM) at ~7000–4000 cal. BP. The TCE record was strongly similar to the TJulyCla reconstruction during the Holocene. It suggested that chydorids would have reproduced predominantly sexually (higher TCE) under environmentally unfavorable periods (early Holocene) and some other occasions, and used mainly asexual reproduction mode under periods of stable environmental conditions (HTM). The monotony of the Cladocera-based reconstruction, when compared with the previously available pollen-based TAnn record and other reference material from the adjacent region, suggests that it was partly hampered by other forcing factors more important than temperature, such as major food-web changes or adaptation ability of cladoceran species.