Dansk Miljøkonference 2002 - Applications of ALMaSS

POSTER
Development of the cladoceran community in Dallund Sø, Denmark,
during the last 7000 years – based on macrofossils in the sediment
L. Johansson 1 , E. Bradshaw 2 , E. Jeppesen 1 and Peter Rasmussen 2
1 National Environmental Research Institute, Vejlsøvej 25, 8600 Silkeborg
2 Geological Survey of Denmark and Greenland, Øster Voldgade 10, 1350 København K
Resumé
The historical changes in lake ecosystems, caused by human activities, can be elucidated by
palaeoecological analyses of the sediment. A sediment core of 11 m (representing approx. 7000
years) from Dallund Sø, a lake impacted by human activities for at least the last 6000 years, was
thoroughly analysed for several physical and biological variables. We present the analysis of
cladoceran fragments and relate it to the data from terrestrial pollen, diatoms, diatom-inferred Plevel
and the minerogenic input (a proxy for soil erosion). The cladoceran fragments indicate, that
the ecosystem of the lake was rather stable until 1000 BC. Hereafter, the absolute number of
fragments and the share of macrophyte associated cladocerans increased. These results
correspond well with other parameters, showing that high erosion led to a dramatic decline in the
water level at 1000 BC, and an expansion of aquatic macrophytes. Later, from 1200 AD to the
present, the total number of fragments increased further, and the macrophyte associated
cladocerans almost disappeared. This coincides with a period of increasing P-concentration, and is
an indication of a decline in the water quality. Moreover, a shift occured from a large proportion of
Daphnia to exclusive dominance by Bosmina, indicating an increase in fish predation at this time.
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