Fluttuazioni delle popolazioni di Planktothrix rubescens in relazione alle variabili ambientali nel Lago di Ledro

Lake Ledro is a mesotrophic lake located in the Eastern Italian Alps (Trentino). The basin is exploited for hydroelectric power production. After a period characterized by eutrophic conditions (1970s-1980s), the water quality of the lake improved as a consequence of the installation of a wastewater treatment plant. Despite this, a strong algal bloom of the cyanobacterium Planktothrix rubescens occurred in 2009, during the autumn and winter months. Besides aesthetic impact on the water, P. rubescens is well known to produce toxic compounds (hepatotoxic microcystins). Therefore this episode raised concern about water quality conditions, with possible negative impact on recreational activities (such as fishing and water sports) and, in general, on the touristic attractiveness of the lake. In order to sort out this problem, the Autonomous Province of Trento funded a research project aimed at identifying the factors that regulate the development of P. rubescens. The environmental variables and phytoplankton were studied with a monthly frequency between June and December 2012. The study revealed high transparency values (max = 10.4 m) and moderate nutrients concentrations (average TP = 23 ìg/L and NO3-N = 743 ìg/L). Phytoplankton was characterized by the dominance of P. rubescens (up to >90% of total biomass). During the summer thermal stratification, this cyanobacterium mainly developed in a 2-3 m thick layer just below the limit of the euphotic zone, reaching a maximum biovolume of 5000 mm3/m3. The populations were successively entrained to the surface during winter mixing. These results are fully consistent with the autoecological characteristics of the species, which requires a well illuminated epilimnion and metalimnetic light intensity values around 2-20 ìmol m2s-1. Overall, these findings suggest that the decrease of the trophic status of the lake and improved light regime set the stage for the development of metalimnetic populations of P. rubescens. From a management perspective, a reduction of the species necessarily will require a further reduction of nutrients