Lake Garda as a paradigmatic case study of ongoing ecological changes in the large lakes south of the Alps

The largest (62-368 km2) and deepest (251-410 m) lakes south of the Alps (Garda, Maggiore, Como and Iseo) are one of the most important lake districts in Europe. In the last decades, these lakes underwent important changes, which include the warming of the water column; the tendency to oligotrophication or stabilization of the concentrations of total phosphorus (TP); the introduction of new cyanobacteria, i.e. the establishment of the water-bloom forming Dolichospermum lemmermannii since the 1990s, and the very last discovery of Tychonema bourrellyi, a species producing anatoxin-a (ATX, a neurotoxin). These changes will be critically described considering in detail the results obtained in the Long-Term Ecological Research station of Lake Garda. From 1991 to 2015, the long-term increase of the mean annual water temperatures in the mixolimnion (0-50 m) showed a positive and significant trend (>0.025 °C yr-1). The warming was confirmed also considering the satellite measurements of temperature of the surface waters since the 1980s. The deep hypolimnion (< 200 m) showed periods of warming caused by a downward transport of heat by turbulent diffusion during stratification, interrupted by irregular cooling and overturn during harsh winters. Overall, the frequency of mixing episodes decreased. The last circulation was observed in 2006; since then, the waters below 200 m showed a continuous warming reaching, in 2015, unprecedent temperature values, between 8.5 and 8.7 °C. These changes were paralleled by a continuous increase of TP in the whole water column until 2002 (ca. > 20 μg P/L), followed by a decrease (17-18 μg P/L). These changes concurred to affect the ecological features of the lake. The first surface blooms of D. lemmermannii in Lake Garda were recorded in the 1990s. The introduction (in the 1960s) and expansion of this species was linked to the incipient eutrophication and to the lake warming, which is a general positive factor for the development of gas-vacuolated cyanobacteria. Nevertheless, while the impact of Dolichospermum was limited to the development of summer surface “oligotrophic” blooms, recent investigations showed that Tychonema was able to develop with biomasses as high as those of P. rubescens. These findings induce to change an important paradigm in the phytoplankton ecology of the southern perialpine lakes. In fact, until now, Planktothrix was the dominant cyanobacterium, and the only producer of microcystins (MCs, hepatotoxins). Conversely, many strains of Tychonema isolated in Lake Garda and in the other large perialpine lakes tested positive for the presence of the genes encoding ATX, and for the production of ATX. Since 2009, the increasing role of Tychonema was confirmed by the increase of ATX and the decrease of MCs. The causes will be discussed considering in particular the interactions between changes in the trophic level and lake warming.