A recently proposed model for the investigation of diffusivity in planktonic systemscontaining toxin-producing phytoplanktons is here reconsidered. We show the existence ofplanktonic travelling waves. Numerical simulations validate the analytical findings, toelucidate the sensitivity of the results in dependence of the diffusion coefficients.

The occurrence of new phytoplankton species in a coastal area may be explained by the import of shellfish containing whole live algal cells in their digestive tracts. Indeed, shellfish containing toxic algal cells can induce both primary contaminations in safe areas (initially free from toxic microalgae), and secondary contaminations of other shellfish growing in the same area via the faeces of the imported animals. To mitigate this problem, shellfish need to be placed in a separate holding tank and their intestinal content purged. For a deeper understanding of the risks associated with transferring contaminated shellfish, oysters (Crassostrea gigas) and mussels (Mytilus edulis) were purposely fed either Alexandrium minutum or A. catenella (Dinophyceae) or Pseudo-nitzschia calliantha (Bacillariophyceae) toxic algae for 2 h. They were then transferred into individual tanks where they were continuously fed with a non-toxic alga, Tetraselmis suecica. Biodeposit production, faeces composition, and filtration rates were monitored for the shellfish over a 6-h period. The effect of temperature differences and different initial toxic algae concentrations were compared. This study revealed a relationship between temperature and cell lysis in the oyster digestive tract. It also indicated that toxic algae concentration did not seem to influence gut passage time in oysters, while a significant effect was observed in mussels, and confirmed the existence of a difference between oyster and mussel feeding patterns.

Chrysochromulina and Prymnesium are important bloom-forming organisms in marine and brackish waters, respectively. Both genera include toxic species, which are primarily implicated in fish kills. Previous analyses of small subunit (SSU) rDNA sequences from Chrysochromulina and Prymnesium spp. indicate that Chrysochromulina is paraphyletic. C. polylepis, which produced a spectacular, harmful bloom in 1988, is more closely related to toxic Prymnesium species than to most other Chrysochromulina species based on rDNA sequence comparisons. Signatures were identified in the SSU rRNA gene specific for a clade that comprised primarily toxic taxa (C. polylepis, P. parvum, P. patelliferum and P. calathiferum) and that recognized C. polylepis alone. Oligonucleotide probes complementary to these regions were designed, and their specificity tested using dot-blot hybridization on PCR products of the SSU rRNA gene from 28 strains of Chrysochromulina and Prymnesium. Whole-cell hybridizations were performed with FLUOS- as well as Cy3-labelled probes on cultured species from both genera, and were detected with both epifluorescence microscopy and flow cytometry. The probes afforded easy identification of clonal isolates of C. polylepis and a cluster of closely related species including C. polylepis and Prymnesium spp. The feasibility of using these probes for species identification and studies of population dynamics in the field is discussed.