ARMORED DINOFLAGELLATES IN THE NE ATLANTIC DURING THE BOFS CRUISES, 1988-90

The armoured dinoflagellates present in 90 plankton samples collected by the use of an Apstein closing net were enumerated. The samples were collected from various stations around the 20-degrees-W meridian and between 44 and 60-degrees-N during the 1988, 1989 and 1990 Biogeochemical Ocean Flux Study (BOFS) cruises. At most stations, samples were obtained from various depths, although in 1990 only the surface zone was sampled. A total of 126 species were identified, of which 49 have chloroplasts and are thought to be autotrophic, 47 are assumed to be heterotrophic and the nutritional type of the remainder is unknown. The samples collected during July 1988 were dominated by large numbers of the two photosynthetic species Gonyaulax polygramma and Protoceratium reticulatum (=Gonyaulax grindleyi). The much more intensive sampling of 1989 revealed several Ceratium species, C.fusus, C.furca and C.lineatum, together with Gonyaulax polygramma, as the most common dinoflagellates. In 1990 the samples, which were taken during a Lagrangian survey in May-June, were also dominated by Ceratium species. This time C.azoricum was a major component and Protoceratium reticulatum was again present in high numbers as in 1988. A number of analyses were carried out on the data collected. It was found that the majority of the more frequent species were autotrophs and most were members of the genus Ceratium. The effects of depth were shown to result in reduced numbers of cells and species, but no clear association was found between species and depth. After the application of Detrended Correspondence Analysis (DECORANA) to all surface samples, there was found to be a clear association between the species composition and both time of year and latitude. Water column stability is also probably an important factor in species composition and cell numbers. Seasonal changes in the frequency of the main species were also noted with some, such as C.lineatum, being more important early in the summer and others, such as Protoceratium reticulatum and particularly G.polygramma, becoming dominant later. The use of Two-way Indicator Analysis (TWINSPAN) revealed some potential species associations.