Grazing rate of microzooplankton measured experimentally in the North Atlantic during the Maria S. Merian cruise MSM26, spring 2013

The microzooplankton grazing dilution experiments were conducted at stations 126, 127, 131 and 133-137, following Landry & Hassett (1982). Seawater samples (whole seawater - WSW) were taken via Niskin bottles mounted on to a CTD Rosette out of the chlorophyll maximum at each station. Four different dilution levels were prepared with WSW and GF/F filtered seawater - 100% WSW, 75% WSW, 50% WSW and 25% WSW. The diluted WSW was filled in 2.4 L polycarbonate bottles (two replicates for every dilution level). Three subsamples (250 - 500 mL depending on in situ chlorophyll) of the 100% WSW were filtered on to GF/F filters (25 mm diameter) and chlorophyll was extracted in 5 mL 96% ethanol for 12-24 hours. Afterwards it was measured fluorometrically before and after the addition of HCl with a Turner fluorometer according to Jespersen and Christoffersen (1987) on board of the ship. In addition, one 250 mL subsample of the 100% WSW was fixed in 2% Lugol (final concentration), to determine the microzooplankton community when back at the Institute for Hydrobiology and Fisheries Science in Hamburg. Also, one 50 mL subsample of the 100% WSW was fixed in 1 mL glutaraldehyde, to quantify bacteria abundance. The 2.4 L bottles were put in black mesh-bags, which reduced incoming radiation to approximately 50% (to minimize chlorophyll bleaching). The bottles were incubated for 24 hours in a tank on deck with flow-through water, to maintain in situ temperature. An additional experiment was carried out to test the effect of temperature on microzooplankton grazing in darkness. Therefore, 100% WSW was incubated in the deck tank and in two temperature control rooms of 5 and 15°C in darkness (two bottles each). The same was done with bottles where copepods were added (five copepods of Calanus finmarchicus in each bottle; males and females were randomly picked and divided onto the bottles). In addition, two 100% WSW bottles with five copepods each were incubated at in situ temperature at 100% light level (without mesh-bags). All experiments were incubated for 24 hours and afterwards two subsamples of each bottle were filtered on to GF/F filters (25 mm diameter); 500 - 1000 mL depending on in situ chlorophyll. One 250 mL subsample of one of the two replicates of each dilution level and each additional experiment (temperature and temperature/copepods) was fixed in 5 mL lugol for microzooplankton determination. One 50 mL subsample of one of the two 100% WSW bottles as well as of one of the additional experiments without copepods was fixed in 1 mL glutaraldehyde for bacteria determination later on. Copepods were fixed in 4% formaldehyde for length measurements and sex determination.

Abundance of bacteria and virus during the METEOR cruise M87/1

In temperate, subpolar and polar marine systems, the classical perception that bacteria are carbon limited by end of winter and respond in activity and abundance to the production of new carbon during the diatom spring bloom and post bloom. Contrary to this view, we here document an strong increase in bacterial abundance and activity (latter measured by increasing high nuclei acid (HNA) to low nuclei acid (LNA) bacteria ratio) during the winter-spring transition, where phytoplankton smaller than 10 µm dominate. Further DNA-virus were enumerated and revealed the virus to bacteria ratio (VBR) to be decreasing during winter-spring transition, indicating that the virus did not increase in number accordingly to bacteria. During repeated visits to stations in the deep Icelandic and the Norwegian Basins and the shallow Shetland Shelf (26 March to 29 April 2012), we investigated the abundance of bacteria and the succession of HNA:LNA bacteria and VBR. Water samples were collected from CTD rosette .10 L Niskin bottles and fixed in glutaraldehyde (final conc. 5%), flash frozen in liquid Nitrogen and stored at -80°C until analysis.

Abundance of pico- and nanophytoplankton during the Meteor cruise M87/1

In temperate, subpolar and polar marine systems, the classical perception is that diatoms initiate the spring bloom and thereby mark the beginning of the productive season. Contrary to this view, here we document an pre-bloom of pico- and nanophytoplankton prior to the diatom bloom; a period with excess nutrients and deep convection of the water column. During repeated visits to stations in the deep Icelandic and the Norwegian Basins and the shallow Shetland Shelf (26 March to 29 April 2012), we investigated the succession and dynamics of <10 µm phytoplankton. Water samples were collected from CTD rosette 10 L Niskin bottles and fixed in glutaraldehyde (final conc. 5%), flash frozen in liquid Nitrogen and stored at -80°C until analysis.