A REVIEW OF PSEUDO-NITZSCHIA, WITH SPECIAL REFERENCE TO THE SKAGERRAK, NORTH-ATLANTIC, AND ADJACENT WATERS

The Pseudo-nitzschia flora of the Skagerrak, North Atlantic, and adjac ent waters, comprising P. pungens, P. multseries, P. seriata, P. fraud ulenta, P. heimii, P. delicatissima, and P. pseudodelicatissima, has b een examined. Except for P. australis, all Pseudo-nitzschia species sh own to produce the toxin domoic acid are present in the area although an outbreak of amnesic shellfish poisoning has never been reported. Fo r comparison of morphological and taxonomic characters, Pseudo-nitzsch ia seriata f. obtusa, P. australis, P. subfraudulenta, P. subpacifica, P. lineola, P. inflatula, and P. cuspidata have been included in this investigation. Fine details of band structure and poroid occlusions, previously ignored or unresolved, have proven to add to the morphologi cal distinction between P. pungens and P. multiseries, P. seriata and P. fraudulenta, P. seriata and P. australis, and P. delicatissima and P. pseudodelicatissima. Additional information on the structure of the proximal mantle compared to that of the valve face has revealed simil arities in most of the species but differences between P. pungens and P. multiseries. The species' seasonal and long-term distributional pat terns during the sampling period (October 1978 through September 1993) in the Skagerrak area are outlined. The greatest abundances of P. ser iata, a cold-water species most likely restricted to the northern hemi sphere, occurred in the spring, and those of the presumably cosmopolit an diatoms P. pungens, P. multiseries and P. pseudodelicatissima, in t he autumn. Whereas P. multiseries seems to have decreased in abundance in the 1990s, P. pseudodelicatissima has apparently increased.