Secular ecological evolution of Lake Ledro Trentino) as outlined by paleolimnological studies

Lake Ledro (zmax=48 m) is an hardwater, mid-altitude (650 m a.s.l.) lake located in SW Trentino (Eastern Italian Alps). It has been modified for hydroelectric exploitation since the late 1920s and at present represents a relevant socio-economic resource for the region in relation to recreation and summer tourism. The lake underwent anthropogenic eutrophication from the 1970s to the 1980s, which has been followed by water quality improvement over the past two decades, after the sewage treatment of municipal wastewater. Despite the present lake mesotrophic conditions, cyanobacteria contribution recently increased, culminating in a very intense bloom of Planktothrix rubescens in autumn-spring 2009/2010. The Autonomous Province of Trento promoted a multidisciplinary research project aimed at understanding the environmental and biotic factors driving the development of P. rubescens populations in Lake Ledro. Within the project, a complementary paleoecological investigation was conducted in order to reconstruct the lake secular trophic evolution (thus identifying its reference conditions) and the variations of P. rubescens in relation to climatic variability, human impact and lake basin management. Several short sediment cores were retrieved from the deepest point of the lake in winter 2011. The master core (83 cm long) was dated based on Pb and Cs radioisotopes, and subsamples were analysed for basic geo-chemical proxies (i.e. wet density, and water/organic content). Biological proxies included sub-fossil algal and cyanobacteria pigments and diatom remains. A parallel sediment core (78 cm long) was opened longitudinally in order to analyse the varved structure of the sediment and to identify the records of major hydrological related events occurred during the last few centuries. The present contribution reports the preliminary results obtained during the study of the sediment cores. The results are discussed comparing the secular trends with the data recently recorded during the limnological surveys