The cyanobacterial community of Lake Trasimeno

Lake Trasimeno is a large, shallow lake in central Italy. Its water regime mainly depends on rainfalls, with periodical floods alternating to low water discharge. The presence of potentially toxic cyanobacteria posed the need of an accurate characterisation of its cyanobacterial community. The study was conducted on a complete annual cycle, with periodical morphotype counting, strain isolation, integrated morphological, morphometric and molecular identification and characterisation, and cyanotoxin determination. Dense populations of potentially toxic cyanobacteria were observed from early summer to late autumn but cyanotoxin concentrations never reached alarming levels. The tropical species Cylindrospermopsis raciborskii, fast spreading in temperate zones, was significantly present in the lake, but did not produce significant amounts of toxins. The phylogenetic analysis confirmed the diversity of the lake cyanobacteria and showed the existence of previously unknown clusters of heterocytous (Nostocales) cyanobacteria. The fluctuating environment of Lake Trasimeno, although presently healthy, is at risk due to the periodical increase of the density of potentially toxic cyanobacterial populations. The modification of the structure of its cyanobacterial community should therefore be thoroughly monitored.