Microbial community structure and standing stocks in the NE Atlantic in June and July of 1996

The standing stocks of nanophytoplankton and picoplankton in the northeast Atlantic Ocean in June and July 1996 were quantified using flow cytometry a nd microscopy. Diatoms and dinoflagellates were analysed by microscopy and coccolithophores, other nanophytoplankton, picoeukaryotic phytoplankton, cy anobacteria (Synechococcus spp.), prochlorophytes (Prochlorococcus spp.) an d heterotrophic bacteria by flow cytometry. The research was divided into t hree components: a lagrangian study of a nutrient replete cold-core eddy ce ntred around 59 degrees 12'N 20 degrees 12'W; a transect close to the 20 de greesW meridian from 59 degrees 18'N to 37 degreesN, which passed through c ontrasting water masses; and a lagrangian study in oligotrophic waters, cen tred around 36 degrees 42'N 19 degrees 12'W. The eddy was characterised by a bloom of the coccolithophore Coccolithus pelagicus whose standing stocks averaged 4.26 g C m(-2) over the upper 50 m. C. pelagicus and other nanophy toplankton (excluding diatoms and dinoflagellates) dominated the standing s tocks of the microbial community, averaging approx. 70% of the total microb ial standing stocks of the groups quantified. The majority of the remaining biomass was accounted for by the picoeukaryotic phytoplankton and heterotr ophic bacteria. The microbial community immediately outside the eddy was si gnificantly different in both composition and standing stocks. There were n o C. pelagicus outside the eddy and fewer nanophytoplankton, resulting in m icrobial standing stocks of approx. one-third that found in the eddy. The t ransect was characterised by a frontal region at approx. 52 degrees 30'N. T here was a general decrease in the standing stocks of all components of the microbial community from the start of the transect to the front. Just to t he south of the front, nanophytoplankton, Synechococcus spp. and heterotrop hic bacteria showed marked increases in standing stocks, especially the nan ophytoplankton, which increased from 3.43 to 7.90 g C m(-2). The nanophytop lankton dominated the microbial standing stocks throughout the transect, ev en in the oligotrophic waters where the integrated carbon biomass was 4.58 g C m(-2), representing 69% of the total microbial standing stocks, During the lagrangian study around 37 degreesN the picoplanktonic community was do minated by heterotrophic bacteria. However, heterotrophic bacteria standing stocks decreased with time, along with Synechococcus spp. and picoeukaryot ic phytoplankton. Peak biomass for these three groups shifted deeper down i n the water column with time. Prochlorococcus spp. were only present toward s the end of the transect and at the oligotrophic site. At the oligotrophic site their standing stocks increased, unlike other groups, so that they be came the dominant picophytoplanktonic group. (C) 2001 Elsevier Science Ltd. All rights reserved.