Description: The overall objective of the NORCOHAB cruise was to study the coastal oceanographic processes and mechanisms underlying the dynamics of key toxic bloom species and the biogeographical distribution of their toxins in the water column of different North Sea coastal waters. The cruise was endorsed by SCOR-IOC Programme GEOHAB (Global Ecology and Oceanography of Harmful Algal Blooms), under the auspices of the Core Research Project on HABS in Fjords and Coastal Embayments. Following the prescribed format of GEOHAB research the studies were international, multidisciplinary and comparative with the ultimate aim of modeling dynamics and behavior.We assessed and compared key genotypic and phenotypic properties of HAB species in relation to grazing and toxic and allelochemical interactions in Scottish and Norwegian/Danish (Skagerrak) coastal areas. For the first time, on board measurements of known phycotoxins collected directly from the plankton were subjected to analysis by high resolution tandem mass-spectrometry coupled with liquid chromatography (LC-MS/MS). These measurements included domoic acid and a large variety of lipophilic toxins associated with marine plankton. We also performed grazing experiments with protists and copepods on selected key toxin algal species in on-board incubations. The following main goals of the cruise programme were successfully addressed: 1) the phenotypic and genotypic characteristics of populations of selected HAB species were compared, including that of a new dinoflagellate identified as the proximal source of azaspiracid poisoning (AZP) in the North Sea; 2) the toxin profile and content of HAB species and toxin transfer and metabolism to the next trophic level were assessed and compared; and 3) insights into the relative importance of grazing by both metazoa and protists as a potential “top down” regulatory mechanism for population dynamics of HAB species was achieved.