Dataset: Phytoplankton growth and grazing from flow cytometry in the eastern Indian Ocean

We sampled 21 stations, most on a south-to-north transect from 39.5 to 11.5°S along longitude 110°E, west of Australia. Stations were occupied on successive days, with sampling done on a consistent daily schedule, followed by late-night transit between stations. Mean light extinction coefficients determined from morning CTD hydrocasts were used to compute sampling depths corresponding to the transmission characteristics of light-calibrated shipboard incubators. Experimental water was collected on the evening (~21:00 local) hydrocasts at depths corresponding to 75.6, 31.7, 18.0, 7.6, 2.6 and 1.3% Io.

For each depth, we prepared a two-treatment dilution experiment (Landry et al. 2008, 2011), with one polycarbonate bottle (2.7 L) containing unfiltered seawater (100%) and the second (diluted) bottle consisting of ~33% whole seawater with filtered water from the same depth. Seawater was filtered directly from the Niskin bottles using a peristaltic pump, silicone tubing and an in-line 0.2-µm Suporcap filter capsule that had previously been acid washed. Each bottle was subsampled for flow cytometry (FCM) analysis (2 mL) for initial microbial concentrations, and the bottles were placed in their respective light boxes for 24 h, cooled with constant high flow from the ship’s running seawater line. The incubators were covered to protect from deck lighting during nighttime operations and received full solar lighting during the daytime.

Picoplankton FCM samples were preserved with 1% paraformaldehyde and frozen at –80°C. Thawed samples were stained with Hoechst 34580 (1 µg mL-1) and analyzed at a flow rate of 30 µL min-1 with a Beckman-Coulter CytoFLEX-S instrument with 4 lasers (Selph, in review). Side scatter, forward angle light scatter (FALS) and fluorescence signals were collected using laser excitation (EX)/emission (EM) filters of EX375/EM450±45 for Hoechst-stained DNA, EX488/EM690±50 for chlorophyll, and EX561/EM585±42 for phycoerythrin. Listmode files (FCS 3.0) were analyzed with FlowJo software (v.10.6.1) for abundances of Prochlorococcus (PRO), Synechococcus (SYN), photosynthetic (pico-)eukaryotes (EUK) and heterotrophic bacteria (HBact).

We determined rate profiles for phytoplankton growth (µ, d-1) and microzooplankton grazing (m, d-1) from each pair of dilution experiment bottles and for each FCM population according to the following equations:

m = (kd - k)/(1 - D)
and
µ = k + m,

where kd and k are the measured net rates of change between initial and final concentrations in the diluted and undiluted treatments, respectively, and D is the portion of unfiltered water in the dilution treatment (Landry et al., 2008; Selph et al., 2011).