Palaeolimnology of Lake Orta: what we can learn from the past for the future

Lake Orta is amongst the world best case study of freshwater environments. Following the establishment of a rayon factory in 1926, this lake has experienced high levels of metal pollution, ammonium sulphate and subsequent acidification. Long-term limnological data dating back to the beginning of 1900 showed in great detail the history of its heavy metals and ammonium sulphate pollution, a dramatic event that sadly determined among limnologists the well world-wide knowledge of this large, deep and important Italian water resource. A liming intervention was planned in 1989 aimed at a complete oxidation of ammonium and restoration of total alkalinity. The first sediment core analyses were carried out in 1958, but it is only from 1978 onward that a modern palaeolimnological approach was applied to the study of Lake Orta. The main objectives were: (1) to detail and infer past primary production using fossil pigments; (2) to reconstruct changes in algal assemblages using species or group specific carotenoids to estimate the natural variability and the pollution impact on diatoms, Cladocera and Oligochaete community. It was proved that there was a transition towards smaller sized organisms from different kingdoms and trophic levels (diatom, thecamoebian, and cladoceran remains). Changes in size reflected not only shifts in the taxonomic composition of the assemblages, but also the reduction of body size within a single taxon (i.e. the diatom Achnanthes minutissima). Recently, Cladocera and rotifer resting eggs were also studied to address ecological, micro-evolutionary during chronic copper pollution and recovery. Responses of Rotifera resting eggs to environmental changes were clear and consistent with those previously observed on Cladocera.