A multi-proxy sediment study to assess long-term effects of nutrients and climate variability on the ecological dynamics of the largest Italian lake (Lake Garda)

The study of lake sediments allows to place limnological investigations within a secular temporal perspective, providing a longer time-span compared to monitoring data. A multi-site and multi-proxy paleoecological approach was applied to Lake Garda, the largest Italian lake, in order to disentangle the effects of local anthropogenic forcings, such as nutrients, and climate variability on the lake ecosystem during the last few centuries.Short sediment cores were collected from the deepest point of the two lake basins: Brenzone (350 m depth) and Bardolino (81 m depth). Biological indicators(diatoms and Cladocera) were used to reconstruct changes in the aquatic food web and to define the lake reference conditions, while sediment geochemistry, analyzed by wavelength-dispersive X-ray fluorescence spectroscopy (WD-XRF), was investigated to obtain information on different physical or chemical processes affecting the lake and its catchment. The selected biological proxies suggested stable oligotrophic conditions of Lake Garda until the 1960s, while the following lake nutrient enrichmentled to a drastic change in the phytoplankton community. The major climatic anomalies,i.e. the Medieval Climatic Anomaly and the Little Ice Age, did not apparently affect planktonic diatom taxonomic composition, while Cladocera showed changes in total abundance and speciescompositions. On the other hand, diatoms showed an indirect response to climate variabilitysince the beginning of the nutrient enrichment phase in the 1960s, while Cladocera revealed a weaker climate-response during this nutrient-driven period. This different response to nutrients and climate was put in relation with the thermal dynamics of large and deep lakes. In fact, climate variability regulates magnitude and frequency of thermal circulationin large and deep lakes, which in its turn controlthe degree of nutrient fertilization of the entire water column and the related phytoplankton growth. Geochemical data showed a pronounced change in elemental composition since the middle of the 20th century, when major elements and lithogenic tracers started to decrease, while elements related to redox conditions and contamination (trace elements) increased. The general trends agreed with the biological records. However, some differences recorded in the two different basins of Lake Garda reflected the effects of local hydrological and sedimentation patterns.