Toxic algae and fish mortality in a brackish-water lake in angstrom land, SW Finland

The brackish-water Lake Vargsundet (SW Finland) was affected by multiple al gal toxins and extensive fish mortality in July 1997. Dense populations of the ichthyotoxic haptophyte Prymnesium sp. and the hepatotoxic cyanobacteri um Planktothrix agardhii were observed simultaneously, but usually in disti nct layers. Fish died in the relatively clear epilimnion where 10-40 millio n Prymnesium cells per litre were recorded (chlorophyll a range 6-18 mu g l (-1)). Fish survived in the metalimnion dominated by the hepatotoxic P. aga rdhii (chlorophyll a 20-40 mu g l(-1)). The deeper layers were anoxic. The Prymnesium-rich water remained ichthyotoxic until the end of August but the toxicity was less marked in tests with Artemia salina. As even inconspicuo us haptophyte blooms with low chlorophyll a concentrations may be harmful, safety and monitoring issues need reconsideration.