Lake ecosystem response to climate change 8200 years ago. A multi-proxy study at Lake Højby Sø, Denmark

The assessment of present-day climate effects on lake ecosystems in lowland Europe is difficult for two<br/>reasons: i. the absence of long-term monitoring data (usually less than 30 yrs) which makes it difficult to<br/>determine natural variation, and ii. the problem of distinguishing between the effects of climate and the<br/>effects of human activities. These problems also complicate the prediction of possible future climate<br/>influence on lake ecology. A way of circumventing these problems is the use of lake sediment records which<br/>contain a wealth of information about past lake history over long time scales and under different climate and<br/>land-use regimes. Using a sediment record from Lake Højby Sø, Denmark, the aim of the present study was<br/>to explore ecological response to the most pronounced climate change known from the Holocene: an abrupt<br/>cooling event around 8200 cal. yr BP, also known as the ‘8.2 kyr cooling event’. The event has been<br/>recorded as a negative &amp;#948;18O excursion in the central Greenland ice cores, lasting c. 160 years from<br/>8247–8086 cal. yr BP (Thomas et al. 2007, Quat. Sci. Rev. 26, 70–81). In Greenland the maximum cooling<br/>was estimated to be 6 ± 2oC while in southern Fennoscandia and the Baltic countries pollen-based<br/>quantitative temperature reconstructions indicate a maximum annual mean temperature decrease of around<br/>1.5 oC (Alley et al. 1997, Geology 25, 483–486; Seppä et al. 2007, Clim. of the past 3, 225–236). Today<br/>there is a general consensus that the primary cause of the cooling event was the final collapse of the<br/>Laurentide ice sheet near Hudson Bay and the associated sudden drainage of the proglacial Lake Agassiz<br/>into the North Atlantic Ocean (e.g. Kleiven et al. 2008, Science 319, 60–64). This freshwater outflow<br/>reduced the strength of the North Atlantic thermohaline circulation and thereby the heat transported to the<br/>North Atlantic region, resulting in an atmospheric cooling (e.g. Barber et al. 1999, Nature 400, 344–348).<br/>The climatic consequences of this meltwater flood is assumed to be a good geological analogue for future<br/>climate-change scenarios, as a freshening of the North Atlantic is projected by almost all global-warming<br/>models (e.g. IPCC 2007). In Denmark the 8.2 kyr event pre-dates the introduction of agriculture (by more<br/>than two millennia) and any other major human impact on the environment, so allowing the anthropogenic<br/>factor to be disregarded. At Lake Højby Sø the 8.2 kyr event was identified using AMS 14C dating and<br/>high-resolution multi-proxy studies of the sediments involving geochemistry, magnetic susceptibility,<br/>pollen, macrofossil, diatom and algal pigment analysis. The 8.2 kyr climatic anomaly involved a distinct<br/>increase in regional precipitation as inferred from an abrupt increase in catchment soil erosion. This lasted<br/>for at least 250 years and gave rise to a drastic increase in sedimentation rates of inorganic and organic<br/>material. The enhanced soil erosion also increased the input of nutrients leading to an increase in lake<br/>productivity as reflected by high algal pigment accumulation rates in the period c. 8400–7950 cal yr BP.<br/>After c. 7950 cal yr BP algal productivity declined somewhat but the lake did not return to its pre-8400 cal<br/>yr BP conditions remaining a more productive and nutrient rich lake than before the climate-induced change.<br/>Thus, at this point the lake seems to have experienced a climate-driven regime shift. The pollen data from<br/>Lake Højby Sø reveal a drop in temperature during the 8.2 kyr event as inferred from a reduced pollen<br/>production from thermophilous deciduous trees. However, a temperature effect was not demonstrable in the<br/>lake ecosystem, at least based on the proxies analysed. If present, a temperature forcing on the aquatic<br/>ecosystem appears to have been fully overridden by climate-induced catchment processes. In conclusion,<br/>our study suggests that during the 8.2 kyr cooling event hydrological change was of more importance for<br/>lake ecosystem process than the change in air temperature.

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