Case Study 3: Holocene Climate Change (Duigan & Birks 2000)
Several studies using cladoceran remains have focused on post-glacial environmental change and Quaternary palaeoecology. Often an indicator species approach was used to discern responses to changing climate.
More recently statistical techniques have been developed to correlate modern cladoceran assemblages to environmental conditions. This application has been extended by applying these relationships to palaeolimnology studies. In particular, inference models (or transfer functions) for mean summer air temperature have been developed for cladocera along an alpine mountain gradient (Lotter et al., 1997).
Duigan & Birks (2000) used these transfer functions to carry out a quantitative reconstruction of changes in air temperature for a small lake at Kråkenes inWestern Norway. The base of a cirque, which contained a glacier during the Younger Dryas is still visible on the side on a nearby mountain. Ice was absent in the Allerød period and silty organic sediment rich in plant and animal microfossils built up in the basin. Cold glacial meltwaters entered the lake during the Younger Dryas and more inorganic sediments were deposited. At the start of the Holocene the inorganic silt component decreased sharply and more organic sediments were deposited.
The responses of the cladocera during this period of climate change were assessed on the basis of the established ecology of the taxa and in the context of the quantitative temperature reconstruction. The remains in the sediment sequence were dominated by littoral chydorid taxa, with Bosmina, Daphnia and Simocephalus representing the open water zooplankton.
Chydorus piger and Daphnia were the only immediate post-glacial colonists. The start of the Allerød coincided with a rapid proliferation of Cladocera taxa which was maintained until the start of the Younger Dryas. Then the local glacier began to drain into the lake and chydorid diversity declined. Only Chydorus sphaericus and Acroperus harpae persisted throughout this period. There was a recovery in diversity in the upper Younger Dryas and it continued into the Holocene.
The reconstructed mean summer air temperature was from 8-21°C (1.8-2.5°C prediction error). The Allerød was marginally warmer than the Younger Dryas, but a progressive increase in temperature is apparent in the early Holocene period.