Lake Garda: nuisance cyanobacteria and impact on the aquatic ecosystems

Cyanobacteria have a fundamental role in aquatic ecosystems. However, massive proliferations (blooms) can have negative impact on all water uses. Anthropogenic pressure and climate changes are the major stressors in lakes; they can trigger cyanobacterial blooms with the capability of producing toxic metabolites: hepatotoxins microcystins, nodularins), neurotoxins (saxitoxins, anatoxins, BMAA), cytotoxins cylindrospermopsins). Some toxins moreover can undergo bioaccumulation. Lake Garda hosts some potentially toxic cyanobacteria (Planktothrix rubescens and Dolichospermum lemmermannii). In the frame of the EULAKES project, a monitoring campaign in 2010-2012 investigated with a monthly frequency: i) the dynamics of the cyanobacterial populations, and ii) the chemical diversity and concentrations of cyanotoxins in the lake water. The survey showed that the most abundant toxins were microcystins (MCs) and anatoxin-a. Five congeners of MCs were identified and the MC RRdm was the dominant variant. MC concentrations were always low (< 1 µg L-1), while preliminary analyses did not detect MC in fish. The transfer of MCs from P. rubescens to Daphnia magna was also investigated, and we found that D. magna was very active in grazing P. rubescens, with significant MC accumulation. In conclusion, we can state that in the case of Lake Garda both cyanobacteria and cyanotoxins are present with low levels typical of oligotrophic water bodies. Nevertheless, considering that anthropogenic and natural stressors are expected to be more severe in the future, continuous research is needed to preserve the water quality.