1465 J. Phycol. 35, 1465–1476 (1999) ECO-PHYSIOLOGY, BIO-OPTICS AND TOXICITY OF THE ICHTHYOTOXIC CHRYSOCHROMULINA LEADBEATERI (PRYMNESIOPHYCEAE) 1 Geir Johnsen, 2 Runar Dalløkken Trondhiem Biological Station, Institute of Natural History, VM, Norwegian University of Science and Technology, N-7034 Trondheim, Norway Wenche Eikrem Department of Marine Botany, Biological Institute, University of Oslo, P.O. Box 1069, Blindern, N-0316 Oslo, Norway Catherine Legrand Department of Natural Sciences, University of Kalmar, P.O. Box 905, S-39129 Kalmar, Sweden and Jan Aure, Hein Rune Skjoldal Institute of Marine Research, P.O. Box 1870, N-5024 Bergen-Nordnes, Norway 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. Chryso- chromulina leadbeateri dominated at total cell densi- ties of 2  10 6 cells·L -1 ; at lower total cell densi- ties, C. leadbeateri was accompanied by other Chry- sochromulina species, peridinin-containing dinofla- gellates, and diatoms. Bio-optical characteristics and pigmentation in laboratory and field strains of C. leadbeateri allowed for the interpretation of the op- tical 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. leadbea- teri bloom. A laboratory study was conducted to as- sess the potential impact of finfish on C. leadbeateri 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 en- hanced during the winter of 1990–1991 due to the overwintering of 1.5  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 ichthyo- toxins making ‘‘hypertoxic complexes’’ with the polyamines enhancing growth in the mixotrophic C. leadbeateri. 1 Received 9 November 1998. Accepted 28 August 1999. 2 Author for reprint requests; e-mail geir.johnsen@vm.ntnu.no. Key index words: bio-optics; caged salmon; chl c 3 - group; Chrysochromulina; harmful algal bloom; her- ring; ichthyotoxins; mixotrophy; polyamines; putres- cine The relationship between harmful algal blooms in Nordic fjords and nutrients, temperature, salinity, and light has been described by many authors (Braa- rud et al. 1958, Sakshaug and Myklestad 1973, Gra- ne ´li et al. 1989, Bjergskov et al. 1990, Johnsen et al. 1997b). Since 1985, several toxic or harmful blooms of chlorophyll (chl) c 3 –containing prymnesiophytes (Prymnesium parvum (Carter), Green et al.; Prymne- sium patelliferum, Green et al., Chrysochromulina poly- lepis Manton et Parke; Chrysochromulina leadbeateri, Estep et al. 1984) and the dinoflagellates Gyrodinium mikimotoi Miyake et Komiami ex Oda (this strain was earlier called G. aureolum), and Gymnodinium gala- theanum Braarud have been reported in the Skag- errak, the North Sea, and the Norwegian coast and sea (Johnsen and Sakshaug 1993 and references therein). Common to all these blooms are stratified surface waters (low salinity, high temperature), sun- ny conditions and calm weather. The blooms end as a consequence of nutrient depletion, grazing, and bacterial and viral activity (Bratbak et al. 1995). Much effort has focused on characterizing the vari- ability in the optical properties of these chl c 3 –con- taining prymnesiophytes and dinoflagellates both in the laboratory and in the field (Schofield et al. 1990, Johnsen et al. 1994, 1998). This work has focused on developing techniques that can characterize pop- ulations in the field in a nonintrusive manner. The chl a–specific absorption coefficient (a (), *  400–700 nm, m 2 ·[mg chl a] -1 ) reflects the photoac- climational status of the cells and can thus be po- tentially related to the overall cellular ‘‘physiological state’’ of the algae. The corresponding scaled fluo-