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 and microscopy. Diatoms and dinoflagellates were analysed by microscopy and coccolithophores, other nanophytoplankton, picoeukaryotic phytoplankton, cyanobacteria (Synechococcus spp.), prochlorophytes (Prochlorococcus spp.) and heterotrophic bacteria by flow cytometry. The research was divided into three components: a lagrangian study of a nutrient replete cold-core eddy centred around 59° 12′N 20° 12′W; a transect close to the 20°W meridian from 59° 18′N to 37°N, which passed through contrasting water masses; and a lagrangian study in oligotrophic waters, centred around 36° 42′N 19° 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 nanophytoplankton (excluding diatoms and dinoflagellates) dominated the standing stocks of the microbial community, averaging approx. 70% of the total microbial standing stocks of the groups quantified. The majority of the remaining biomass was accounted for by the picoeukaryotic phytoplankton and heterotrophic bacteria. The microbial community immediately outside the eddy was significantly different in both composition and standing stocks. There were no C. pelagicus outside the eddy and fewer nanophytoplankton, resulting in microbial standing stocks of approx. one-third that found in the eddy. The transect was characterised by a frontal region at approx. 52° 30′N. There 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 the south of the front, nanophytoplankton, Synechococcus spp. and heterotrophic bacteria showed marked increases in standing stocks, especially the nanophytoplankton, which increased from 3.43 to 7.90 g C m−2. The nanophytoplankton dominated the microbial standing stocks throughout the transect, even 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°N the picoplanktonic community was dominated by heterotrophic bacteria. However, heterotrophic bacteria standing stocks decreased with time, along with Synechococcus spp. and picoeukaryotic phytoplankton. Peak biomass for these three groups shifted deeper down in the water column with time. Prochlorococcus spp. were only present towards the end of the transect and at the oligotrophic site. At the oligotrophic site their standing stocks increased, unlike other groups, so that they became the dominant picophytoplanktonic group.