The assessment of present-day climate effects on lake ecosystems in lowland Europe is difficult for two
reasons: i. the absence of long-term monitoring data (usually less than 30 yrs) which makes it difficult to
determine natural variation, and ii. the problem of distinguishing between the effects of climate and the
effects of human activities. These problems also complicate the prediction of possible future climate
influence on lake ecology. A way of circumventing these problems is the use of lake sediment records which
contain a wealth of information about past lake history over long time scales and under different climate and
land-use regimes. Using a sediment record from Lake Højby Sø, Denmark, the aim of the present study was
to explore ecological response to the most pronounced climate change known from the Holocene: an abrupt
cooling event around 8200 cal. yr BP, also known as the ‘8.2 kyr cooling event’. The event has been
recorded as a negative δ18O excursion in the central Greenland ice cores, lasting c. 160 years from
8247–8086 cal. yr BP (Thomas et al. 2007, Quat. Sci. Rev. 26, 70–81). In Greenland the maximum cooling
was estimated to be 6 ± 2oC while in southern Fennoscandia and the Baltic countries pollen-based
quantitative temperature reconstructions indicate a maximum annual mean temperature decrease of around
1.5 oC (Alley et al. 1997, Geology 25, 483–486; Seppä et al. 2007, Clim. of the past 3, 225–236). Today
there is a general consensus that the primary cause of the cooling event was the final collapse of the
Laurentide ice sheet near Hudson Bay and the associated sudden drainage of the proglacial Lake Agassiz
into the North Atlantic Ocean (e.g. Kleiven et al. 2008, Science 319, 60–64). This freshwater outflow
reduced the strength of the North Atlantic thermohaline circulation and thereby the heat transported to the
North Atlantic region, resulting in an atmospheric cooling (e.g. Barber et al. 1999, Nature 400, 344–348).
The climatic consequences of this meltwater flood is assumed to be a good geological analogue for future
climate-change scenarios, as a freshening of the North Atlantic is projected by almost all global-warming
models (e.g. IPCC 2007). In Denmark the 8.2 kyr event pre-dates the introduction of agriculture (by more
than two millennia) and any other major human impact on the environment, so allowing the anthropogenic
factor to be disregarded. At Lake Højby Sø the 8.2 kyr event was identified using AMS 14C dating and
high-resolution multi-proxy studies of the sediments involving geochemistry, magnetic susceptibility,
pollen, macrofossil, diatom and algal pigment analysis. The 8.2 kyr climatic anomaly involved a distinct
increase in regional precipitation as inferred from an abrupt increase in catchment soil erosion. This lasted
for at least 250 years and gave rise to a drastic increase in sedimentation rates of inorganic and organic
material. The enhanced soil erosion also increased the input of nutrients leading to an increase in lake
productivity as reflected by high algal pigment accumulation rates in the period c. 8400–7950 cal yr BP.
After c. 7950 cal yr BP algal productivity declined somewhat but the lake did not return to its pre-8400 cal
yr BP conditions remaining a more productive and nutrient rich lake than before the climate-induced change.
Thus, at this point the lake seems to have experienced a climate-driven regime shift. The pollen data from
Lake Højby Sø reveal a drop in temperature during the 8.2 kyr event as inferred from a reduced pollen
production from thermophilous deciduous trees. However, a temperature effect was not demonstrable in the
lake ecosystem, at least based on the proxies analysed. If present, a temperature forcing on the aquatic
ecosystem appears to have been fully overridden by climate-induced catchment processes. In conclusion,
our study suggests that during the 8.2 kyr cooling event hydrological change was of more importance for
lake ecosystem process than the change in air temperature.