Distribution patterns and biomass estimates of diatoms and autotrophic dinoflagellates in the NE Atlantic during June and July 1996

Abstract

Qualitative and quantitative analyses of microphytoplankton communities were determined from samples collected in the northeast Atlantic Ocean in the early summer of 1996 during the PRIME Cruise of the RRS Discovery. A combination of light microscopy and scanning electron microscopy techniques was used to determine the species composition of two of the main groups of phytoplankton: Bacillariophyceae and Dinophyta. Two series of samples were collected; the first set of samples was collected between 18 and 29 June 1996 during a Lagrangian time-series study in the vicinity of 59°N 20°W tracking a mesoscale cold-core eddy; the second set of samples was collected between 4 and 10 July 1996 during a transect along the 20°W meridian from 59 to 37°N. A total of 155 samples were analysed over various depths down to 150 m, and 78 phytoplankton species were identified. Samples taken during the Lagrangian time-series study were dominated by diatom species, including Ephemera planamembranacea and Pseudo-nitzschia species, whilst the main representative of the microphytoplankton dinoflagellates was Ceratium fusus. On the transect, several Ceratium species were common, including C. furca C. fusus, and C. lineatum, and three other autotrophic dinoflagellates were frequent including Prorocentrum minimum, Oxytoxum scolopax and Gonyaulax polygramma. A number of diatoms dominated the profiles along the transect including Leptocylindrus mediterraneus, Thalassiosira oestrupii, and representatives of the genera Haslea and Pseudo-nitzschia. Standing stocks of both groups were low and typical of post-bloom carbon levels. Diatom biomass exceeded that of dinoflagellate biomass in the eddy although the reverse situation was seen in the more southerly stations along the transect. Maximum abundances of the dinoflagellate communities were situated in the surface waters within the mixed layer, while depth maxima of certain diatoms were noted at around 40 m below the depth of the mixed layer both in the Langrangian time-series study and along the transect. Microstratification and nutrient stress may have contributed to losses of diatoms within the mixed layer due to sedimentation. Hierarchical classification and ordination techniques were used to identify patterns in species assemblages. Four possible clusters of phytoplankton were identified, each associated with a particular suite of environmental variables.