The return of diatoms: the almost complete history of phytoplankton assemblages in Lake Orta

The last comprehensive study of the phytoplankton assemblages in Lake Orta dates back to 2001, when the long-term evolution of the primary producers was analysed, since 1984 up to 1998, including the large-scale liming carried out in the lake in 1989 and 1990. The phytoplankton was analysed in its general properties (abundance, biovolume, chlorophyll-a concentration, average cell size, diversity) as well as in the species composition. The gradual decrease of the chlorophytes and the noticeable increase of the diatoms, reappearing in the pelagic phytoplankton after decades, were the most outstanding features of the post-liming assemblages. However, a first comparison of Lake Orta phytoplankton structure with that observed in other deep italian subalpine lakes, showed us that the complete recovery was not yet reached. A further study, carried out in 2001, again pointed out the differences existing between the two assemblages of lakes Orta and Maggiore. Since then, we do not have detailed information about the phytoplankton taxonomic compostion in Lake Orta: however, between 2005 and 2013 some water samples were analysed, using in vivo fluorimetry, allowing to study the composition of the phytoplanktonic biocoenosis in terms of major classes. In the same period, similar analysis were carried out in Lake Maggiore, thus allowing a new comparison of the two ecosystems.