Diversity and seasonality of cyanotoxins in Lake Garda

The presence of toxic cyanobacteria is a matter of concern in many freshwater ecosystems because some genera can produce toxic secondary metabolites: hepatotoxins (microcystins, nodularins), neurotoxins (saxitoxins, anatoxins, BMAA), cytotoxins (cylindrospermopsins). Cyanotoxins have also the potential to accumulate in aquatic organisms at different trophic levels. In this project, we are studying the temporal dynamics of microcystins production in Lake Garda. Moreover, we have planned to perform a set of lab experiments for elucidating the fate and distribution of microcystins along the food web. Planktothrix rubescens and Anabaena lemmermannii are the most frequent potential toxic species in this water basin. The first species produces essentially microcystins while the second species produce anatoxins and microcystins. Since April 2010, Lake Garda was regularly investigated by monthly sampling and molecular diversity of toxins also analyzed by using LC/MS techniques. The desmethylated microcystin-RR was always the most abundant microcystin (more than 90% of the total). This variant is ca. 4 times less toxic than the most toxic microcystin (MC-LR). The highest concentrations of MCs were found in the summer months, coinciding with the metalimnetic development of Planktothrix populations. Results obtained from this study will help us to determine the degree of transferring of different microcystins through food webs.