Stations were occupied on successive days, with sampling done on a consistent daily schedule, followed by late-night transit between stations. 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% of incident solar radiation (PAR, %Io).

For each depth, we prepared a two-treatment dilution experiment, 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. Each bottle was subsampled for flow cytometry (FCM) analysis (2 mL) for initial population abundances, and the bottles were placed in light-calibrated shipboard incubators corresponding to the light depths of collection and incubated for 24 h, cooled by the ship’s running seawater line. The incubators were covered 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. Listmode files were analyzed with FlowJo software for abundances of Prochlorococcus (PRO), Synechococcus (SYN), and photosynthetic (pico-)eukaryotes (EUK).

Net rates of population growth from initial and final FCM samples in diluted (k_d) and undiluted (k) treatments were used to compute rate estimates of microzooplankton grazing mortality (m, d^-1) and instantaneous growth rate (µ, d^-1) as:  m = (k_d - k)/(1 - D) and µ = k + m.