The occurrence of toxic blue-green algae in Lake Ringsjon, southern Sweden, despite nutrient reduction and fish biomanipulation

Algal nuisance blooms have occurred during summers for more than 40 years in Lake Ringsjon, southern Sweden. Cattle deaths were recorded in connection with a high biomass of blue-green algae as early as 1968. A mouse bioassay in the 1980s confirmed the occurrence of hepatotoxic algae. In 1995, phytoplankton biomass and microcystin concentrations were measured. A mouse bioassay was carried out simultaneously showing that from the end of June, when the blue-green algae appeared, and until their disappearance at the end of October, the blooms were hepatotoxic. The blue-green alga Microcystis wesenbergii dominated over most of this period, although, Microcystis botrys, M. flos-aquae, M. viridis, Aphanizomenon klebahnii and Woronichinia naegeliana also occurred frequently. However, the algal toxicity was not related with any specific alga or with any of the measured parameters. The highest toxin production was measured in July, although the blue-green algal biomass was low at that time.