Sedimentation of Nodularia spumigena and distribution of nodularin in the food web during transport of a cyanobacterial bloom from the Baltic Sea to the Kattegat

•Transport by surface currents of a toxic cyanobacteria (Nodularia spumigena) bloom, from the Baltic Sea, through the Öresund strait, into the Kattegat.•Satellite images confirms a dense bloom of Nodularia spumigena in the central and southern Baltic Sea during the study period, and surface current measurements in the Öresund strait supports the assumption of a transport of the bloom in the more saline waters.•Measurements of plankton at different depths in the water column in the northern part of the Öresund strait (on the border to the Kattegat area) shows that the concentration of Nodularia spumigena cells and the toxin nodularin first increase and then decrease during a one month period in connection to the bloom in the Baltic Sea.•Both Nodularia spumigena and nodularin is tracked well below the strong halocline, both in water samples and in sedimentation traps.•Nodularin is found both in pelagic (copepods, herring) and benthic (blue mussels, flounder) parts of the ecosystem.•A laboratory study shows a decreased buoyancy, and even sinking of Nodularia filaments when they are exposed to a gradually increasing salinity, simulating the transport of Nodularia, from the Baltic Sea into the Kattegat.•The manuscript conclude that Nodularia spumigena blooms and the toxin nodularin is sinking to the bottom, reaching the benthic food web, to a higher extent in more saline waters, than being the case within the brackish Baltic Sea, where the major part of the blooms are believed to decay in the surface waters.