Dataset: Phytoplankton growth and grazing mortality from fluorometric chlorophyll a sampled in the Gulf of Mexico on R/V Nancy Foster cruises in May 2017 and May 2018

This dataset is from CTD hydrocasts in the Gulf of Mexico from R/V Nancy Foster cruises in May 2017 and May 2018, which were part of a NOAA RESTORE project (aka: BLOOFINZ-GoM) to investigate the epipelagic marine nitrogen cycle, plankton dynamics, and impacts on growth and survival of larval Atlantic Bluefin Tuna (ABT). These data are meant to be used in inter-species, interregional comparisons to data from the BLOOFIN-IO study of larval Southern Bluefin Tuna in the Indian Ocean spawning region.

On each cruise, we conducted multi-day quasi-Lagrangian experiments, called "cycles", during which we sampled and measured processes on a repeated daily schedule following a satellite-tracked free-drifting array (Landry et al., 2009). The drift array (Pacific Gyre, San Diego) consisted of a surface float, a 3-m drogue centered at 15 m, coated-wire with stainless-steel attachment rings for in situ bottle incubations, and a separately attached smaller float with iridium transmission (10-min position frequency) and nighttime strobe light.

For each experiment, we collected seawater daily from Niskin bottles on early-morning CTD hydrocasts (~02:00 local time) at 6 depths in the euphotic zone from 5 m to the deep chlorophyll maximum (DCM). Samples for initial concentrations of pigments, flow cytometry (FCM) and microscopy were filled directly from the Niskin bottles via silicone tubing. For each depth, we also prepared a dilution experiment that compared net population growth rates in polycarbonate bottles (2.7 L) containing unfiltered seawater (100%) and a dilution treatment consisting of ~32% whole seawater diluted with filtered seawater from that depth (Landry et al., 2008). 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 (3.7% trace-metal grade HCl; Milli-Q and seawater rinses). Dilution bottles were first given a measured volume of filtered water and then gently filled to the top with unscreened water from the Niskin bottles.

All bottles were secured into coarse net bags with top and bottom attachment clips and incubated in situ for 24 h at the depth of collection on the line below the drifter float. For the first deployment of each cycle, the entire array with bottles attached was laid out on deck before being quickly lowered by hand. For subsequent daily experiments, a new 6-depth experiment was set up in net bags on deck before recovering the drifter. The drifter was then recovered, the previous day’s experiments removed, the new experiments attached, and the drifter redeployed – a process that took ~15 min while the ship maintained position. All recovery and deployments were carried out before sunrise. Sampling for daily experiments was done in close proximity (~100 m) to the drifter position. Upon recovery, all bottles were subsampled for assessments of community composition and biomass, as described below.

Initial and final samples (250 ml) for fluorometric Chla analyses were filtered onto GF/F filters and extracted with 90% acetone in a -20°C freezer for 24 h. Extracted samples were warmed to room temperature in the dark and analyzed on a Turner Designs model 10 fluorometer calibrated against a pure Chla standard (Strickland and Parsons, 1972).

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 or pigment-associated 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).