The influence of hydrographic, biological and meteorological variables on the abundance of Dinophysis spp. and the concentration of diarrhetic shellfish toxin (DST) in the plankton population were investigated in a mussel (Mytilus edulis) farm area on the Swedish west coast. This location provided an opportunity to simultaneously compare Dinophysis spp. cell numbers, concentration of DST in natural phytoplankton assemblages and toxicity of mussel tissues. Sampling was performed every other day from October 10 to November 5, 2000, and on each occasion, 5 randomly selected sites were sampled. During this period, 3 distinct water masses passed through the vicinity of the mussel farm. The second water mass, characterized by low salinity and nitrogen concentration, was probably advected into the area from surface waters in the nearby Skagerrak. This low salinity water also contained a high abundance of Dinophysis spp., and high concentrations of DST were recorded in the phytoplankton population. Multivariate analysis (projection to latent structures by means of partial least squares, PLS) determined that the principal variables influencing the concentration of DST in the plankton assemblage were the causative species (D. acuminata, D. acuta and D. norvegica) and salinity. The abundance of the 3 Dinophysis spp. was inversely correlated to salinity. A rapid increase in the toxicity of mussels in response to the high levels of DST was observed. The concentration of DST had doubled within 2 d of the appearance of Dinophysis spp. After 8 d, the water mass containing Dinophysis spp. was replaced and cell numbers again returned to low levels. The concentration of DST in the phytoplankton samples remained high for another 2 d after the number of Dinophysis spp. had declined and the toxicity of mussels continued to be high for the remainder of the study. Causes of the rapid intoxication versus slow detoxification of mussels are discussed. These results suggest that present monitoring programs are insufficient to provide early warning of toxic blooms to aquaculturists on the Swedish west coast.

Many diatom species have the ability to form benthic resting stages, but the importance of these stages as a supply for planktonic blooms is uncertain. A mesocosm study was carried out in December 2005 to January 2006 in Mangalore, India. Mesocosms were inoculated with various combinations of benthic and/or planktonic cells, sampled from the coastal SE Arabian Sea, and the development of the planktonic community was followed. Diatoms dominated the phytoplankton community in all mesocosms, irrespective of inoculum. The most significant differences among inoculum types were altered species composition, and the timings of the maximum cell abundances, which lagged behind in the sediment mesocosms. Populations of Thalassiosira were initiated by both plankton and benthic propagules. Taxa known from temperate coastal areas to seed bloom by benthic propagules, such as Chaetoceros and Skeletonema, were predominantly seeded by planktonic cells in this experiment; this implies differential seeding strategy within the same species at different latitudes. The species assemblage encountered in the plankton and sediment was similar, which indicates that the benthic resting stages seed an autochthonous phytoplankton flora in the area. High species diversity in all inoculated mesocosms was maintained throughout the experimental period, although the actual number of species was fewer at the end. The hydrographic conditions and timing of formation, survival, and germination of diatom resting stages in SE Arabian Sea are discussed.