A toxic phytoplankton bloom, dominated by the prymnesiophyte Chrysochromulina leadbeateri Estep, developed in the Ofotfjord-Tysfjord area (North Norway) in mid-May and ended in late June 1991 in Vestfjorden and the adjacent fjord areas, Chrysochromulina leadbeateri dominated at total cell densities of >2 x 10(6) cells . L-1; at lower total cell densities, C. leadbeateri was accompanied by other Chrysochromulina species, peridinin-containing dinoflagellates, and diatoms, Bio-optical characteristics and pigmentation in laboratory and field strains of C. leadbeateri allowed for the interpretation of the optical signatures within the bloom. The bio-optical data suggested healthy and actively growing cells during the bloom. About 600 metric tons of pen-raised Atlantic salmon were killed by the C. leadbeateri bloom. A laboratory study was conducted to assess the potential impact of finfish on C. leadbeaferi growth. It was found that the polyamine putrescine enhanced cell biomass and hemolytic activity. Given this, a possible scenario for the development of this bloom and the level of toxicity is hypothesized: (1) The nutrient loading in the Ofotfjord area was enhanced during the winter of 1990-1991 due to the overwintering of 1.5 x 10(6) metric tons of herring from a depth of 0-250 m, This may have sustained a large stock of the mixotrophic C. leadbeateri in early spring before light regime (irradiance, spectral irradiance, and day length) made net photosynthesis possible, (2) The release of polyamines during; the decay of dead fish (e.g. putrescine, cadaverine, and histamine) may have acted as cofactors with ichthyotoxins making "hypertoxic complexes" with the polyamines enhancing growth in the mixotrophic C, leadbeateri.