How fast can it go? Rapid lake-ecosystem responses to catchment processes during the Copper Age at Lago Grande di Avigliana (Northern Italy)
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Over the past decades, increasing effort has been spent on understanding the effects of recent cultural eutrophication of European waters. However, simply reverting to conditions ~150 years ago is generally insufficient since both human activities and natural climate changes have influenced the landscape and nutrient levels of rivers and lakes for several millennia. Limnological, geochemical and terrestrial ecological records (μXRF, LOI, pollen, macrocharcoal and diatom analyses) were collected from an annually laminated lake sediment (sampled at a contiguous 15 yr resolution) to investigate the resilience of lake ecosystems under natural and pre-industrial conditions. We focus on a sediment section that contains ~1600 years and which includes an abrupt transition to darker, organic-richer sediments at 4800 cal. yrs BP (Copper Age). Pollen and diatom records suggest minimal changes in the terrestrial and aquatic ecosystems prior to the transition. Across the transition, no clear human impact was detected in the vegetation which appeared to be stable. However the fire regime indicates more frequent fires across the transition. Furthermore, a major change in lake water chemistry occurred as indicated by diatom assemblages and silica and diatom inferred total phosphorous (DI-TP). The ecosystem switched to a silica limited lake (low Si:P ratio). A climatic cause for this might be increased erosion, potentially driven by wetter conditions. However another cause might be extended ice cover during a colder period, leading to internal phosphorus loading due to extensive anoxia. Evidence for this cold period can be found in other Central European lake level studies. In conclusion, this study indicates a short term climatic interval, which has a major effect on the lake ecosystem. Furthermore the transition of the lake-ecosystem was rapid (~30 years) and showed a response similar to recent cultural eutrophication.