CLIMATE-LAKE INTERACTIONS RECORDED IN VARVED SEDIMENTS FROM A SWEDISHBOREAL FOREST LAKE

A 1100-year long record of lake ecosystem response to climate and catc hment change with precise chronological control is reported. Diatom an d pollen assemblages of an annually laminated (varved) sediment from a northern Swedish lake (Kassjon, Vasterbotten) were used as records of lake diatom communities and catchment vegetation. These data were com pared with summer temperature estimates based on tree-ring records of the same geographical area to identify the effects of climate change a nd catchment disturbance on diatom assemblages in the lake. In a canon ical ordination, 23% of the variability in the total diatom assemblage s for the period AD1040-1804 was accounted for by changes in pollen da ta which reflect agricultural development in the catchment. Diatom spe cies richness, however, exhibited a stronger relationship with summer temperature and, significantly, declined with the lower temperatures a ssociated with the Little Ice Age minimum (early 17th century). Summer temperature accounted for 23% of the variability in diatom species ri chness 20 years later. The mechanism behind this time-lag is unclear, but may be related to catchment-mediated effects, given recent evidenc e for lags in the response of boreal-forest vegetation regeneration cy cles to climatic variability. These results suggest that climate-relat ed effects on lakes occurring over medium timescales can be resolved i n lake sediments. Moreover, it is possible to identify these effects d espite cultural-related signals, but as the latter become more extreme in the late 20th century the climate signal is obscured.