Blooms of nitrogen-fixing toxic cyanobacteria are of concern in ecosystem management worldwide. The aim of our project is to understand role of the bloom-forming cyanobacteria in the productivity of the Baltic Sea in the light of climate-related changes. To do that, we are developing a new approach combining molecular and isotopic analyses and use it to quantify grazing on bloom-forming cyanobacteria in the pelagic food webs. Our results quantifying the contribution of cyanobacterial blooms to zooplankton feeding are critical for food web and nutrient cycling models that current management decisions are based on, including models accessing fish production and nutrient reductions in the Baltic Sea.