Water Column Sampling:
Water column sampling was performed on four cruises during the spring and the summer of 2011 and 2012 at the Bermuda Atlantic Time-series Study station (31’40°N 64’10°W, BATS) and in the mesoscale eddies found in the surrounding area of the Sargasso Sea. For each cruise, two stations were sampled, usually in the center of a mesoscale eddy and at BATS. The edge of the eddy was sampled two times, as well. To be able to get a better reproducibility of data, each experiment was replicated.

For each experiment, seawater samples were collected pre-dawn (on deck 2:30-4:00, local time) at four different depths within the euphotic zone (20m, 50m, 80m and the Deep Chlorophyll Maximum, DCM). Twenty-one 10L Niskin bottles were attached to a rosette with conductivity, temperature, depth sensors (CTD), and an in vivo fluorometer. This sensor allowed for recording in real time of chlorophyll fluorescence and the DCM for each station. The water that was collected from the 10L Niskin bottles was sampled for abundance and biomass of the plankton community.

Bulk measurements:
Chlorophyll-a was extracted from seawater (250 ml and 400 ml depending on the dilution), with 90% acetone and measured after 24hrs at 4 degrees C in the dark onboard the ship using a TD 700 Laboratory Fluorometer using the non-acidification technique (Welschmeyer 1994). These data were used as a proxy for the phytoplankton biomass in the water column and to calculate the bulk growth and grazing rates of the phytoplankton community.

Microscopy Analyses:
To determine cell abundance and the biomass of the protist community (other than ciliates), epifluorescence miscoscopy was used. Ciliate abundance and biomass was determined using bright-field inverted microscopy (Amacher et al. 2009; Neuer and Cowles 1994). Epiflourescence microscopy: 25-50ml of seawater from each depth was filtered onto black membrane filters with 0.2 um pore size. Each sample was fixed first with 0.1 ml of 50% of cold glutaraldehyde, stored for 24 hours at 4 degrees C, and then filtered after addition of 0.2 ml of 1% 4', 6-diamino-2-phenylindole (DAPI). Slides were stored frozen at -20 degrees C onboard ship until transport back to the laboratory at ASU, and stored at -40 degrees C until analysis. The organisms were counted using a ZEISS Axioplan Epifluorescence Microscope equipped with a 100x Plan-NEOFLUAR 100x/1.30 oil, objective lens. Pico, nano and micro plankton were identified and separated in categories based on their approximate geometric shape, size, and on their fluorescence under blue and UV light excitation as described in Table 1 (Amacher et al. 2009, Hansen et al. in prep). Organisms were counted in one to several stripes across the slide. Abundance was then calculated based on number of counted cells, fraction of slide area counted and sample volume. The 95% confidence interval of each organismal count was determined as a function of total cells counted in a given category, according to Lund et al. (1958). The following equations were applied, where x stands for the number of cells counted on each slide:
LL = x + 1.42 - 1.960 (sqrt(x + 0.5))  [Lower limit]
UL = x + 2.42 - 1.960 (sqrt(x + 1.5))  [Upper limit]

Biomass calculations were done for each category of organism counted. Biovolume for each group was determined based on size and shape of the organism by approximating the closest geometric shape (Hillebrand et al. 1999) and then converted into units of carbon based on the carbon to volume ratio (Menden-Deuer and Lessard 2000).

Flow cytometry analyses:
Collection and fixation of flow cytometry samples was carried out according to established methods of the BATS program (http://www.bios.edu/research/projects/bats/) and analyzed by the group of Co-PI Dr. Mike Lomas.

Refer to the original dataset legend (PDF) for more information.