Effects of climate change and nutrients on the secular evolution of the planktonic community in Lake Garda (Northern Italy): a multi-proxy approach

Lake Garda, the largest Italian lake (368 km2), is a key resource for drinking water supply and irrigation, for tourism, and for biodiversity conservation. The evaluation of the lake’s vulnerability to human stressors within the current climate change emerges as a stringent necessity. Paleoecological methods were used to understand long-term environmental and ecological changes. Two short sediment cores were collected from the deepest NW basin (Brenzone, 350 m depth) and in the shallower SW basin (Bardolino, 81 m). According to radiometric dating, the cores cover ~700 years. Subfossil diatoms were analyzed to reconstruct the lake total phosphorus and to identify the lake’s reference conditions. Until the 1960s, the two basins were ultraoligotrophic and inert toward climatic variability. Since the nutrient enrichment in the 1960s, meso- to eutraphentic planktonic pennate and filamentous centrics increased and partially displaced the formerly dominant oligotraphentic taxa. Analyses of cladoceran remains supported and supplemented the diatom results. Since the 1960s, the drastic change in plankton species composition, from oligo- to mesotrophic taxa, was interpreted as a result of the combined effects of nutrient enrichment and climate change. A peak in sedimentation rates in the mid-1940s, followed by a clear decrease in mineral content, reflects the beginning of the hydroelectrical exploitation of the lake catchment. The study confirms the strength of the multi-proxi paleoecological approach to complement limnological investigations and to understand ecosystem changes at secular scale.