Cyanobacterial abundance and microcystin occurrence, in Mediterranean water reservoirs in Central Spain:: microcystins in the Madrid area

The occurrence and concentration of microcystins were analysed in sestonic samples from seven water reservoirs in the Madrid region (Spain) between July and November in two consecutive years (2002 and 2003). The data collected indicate that microcystins were present on several occasions in both years in all the reservoirs studied. The months of maximum risk for cyanotoxin occurrence were July, September and October, when Microcystis typically dominated the phytoplankton community. Four of the seven reservoirs exhibited conspicuous blooms, three of which were toxic. Of the samples in 2002 and 2003, 45% and 70%, respectively, contained microcystins (Mc). In four of the reservoirs, microcystin concentrations were higher than the WHO recommended limit for drinking water (1 mu g Mc1(-1)) and, in one of the reservoirs (Santillana), its concentration, was extremely high (up to 70 mu g Mc1(-1)). Concentrations above 1 mu g Mc1(-1) were found during several months in this reservoir. Mc/chl a ratios in net-concentrated samples ranged from 0.0002 to 2.24 mu g Mc (mu g chl a)(-1) with a mean value of 0.31 mu g Mc (mu g chl a)(-1). From the data obtained we can conclude that M. aeruginosa is the main producer of microcystins in freshwaters from the Madrid region.