Expansion of new cyanobacteria to the large Italian lakes: ecological and management implications

Since the 1990s, the large lakes south of the Alps (Garda, Iseo, Como and Maggiore) were colonised by Dolichospermum lemmermannii. The development of this cyanobacterium was followed, in 2014, by the discovery of Tychonema bourrellyi. While the impact of Dolichospermum was limited to the development of surface "oligotrophic" water blooms during the summer months, a very recent investigation carried out in Lake Garda showed that Tychonema developed with biomasses comparable or higher than those of Planktothrix rubescens. Until now, Planktothrix was considered the dominant cyanobacterium in the southern perialpine lake district, and the principal producer of microcystins (MCs). Conversely, many isolates of Tychonema tested positive for the presence of the genes encoding anatoxin-a (ATX), and for the production of ATX. The increasing importance of Tychonema in Lake Garda could be confirmed by the increasing trend of ATX observed since 2009, which was followed by a concurrent decreasing trend of MCs. These changes are strongly altering the cyanobacterial community and the associated cyanotoxin profile of the large Italian lakes. The causes will be discussed considering in detail the analysis of the long-term data recorded in Lake Garda.