Dataset: Mesozooplankton grazing rates from samples collected in the oceanic Gulf of Mexico on R/V Nancy Foster cruises NF1704 and NF1802 in May 2017 and May 2018

This dataset is from zooplankton net tows in the Gulf of Mexico on R/V Nancy Foster cruises in May 2017 and May 2018, which were part of a NOAA RESTORE project (aka: BLOOFINZ-GoM) led by Dr. John Lamkin 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.

Oblique net tows were taken to obtain estimates of mesozooplankton standing stocks and grazing over the depth range of the euphotic zone. Generally, we sampled during midday (1100-1400 h) and midnight (2200-0100 h) hours following a drogued drifter, allowing estimates of diel vertical migrant biomass by difference. We used a 1-m ring net with 202-µm Nitex mesh and a General Oceanics flow meter to measure volume filtered. Depth of tow was controlled by a depth sensor on the hydrowire. Net tow contents were anesthetized with ice-cold carbonated water and split with a Folsom splitter, with half preserved in 4% buffered formalin and half size-fractionated using nested sieves into five size classes: 0.2-0.5, 0.5-1, 1-2, 2-5 and >5 mm. Each size fraction was concentrated on a preweighed 202-m Nitex filter, rinsed with isotonic ammonium formate to remove sea salt, and frozen at -85°C for lab analysis.

In the laboratory, frozen size-fractioned zooplankton on the Nitex filters were thawed, set briefly on blotting paper to remove excess water, and weighed moist for total sample wet weight (WW). Wet samples were subsampled for gut pigment analyses by removing replicate portions of the biomass and recording weights before and after each subsampling (fraction of total WW removed). The remaining wet biomass on the filters was oven dried at 60°C for 24 h before weighing dry (DW:WW ratio). For each size fraction, zooplankton dry weight (mg m-2) was calculated from the measured WW (less initial filter weight), DW:WW ratio, measured volume and depth of tow, and fraction of sample analyzed. The remaining dried sample was subsequently scraped off the filter, ground to a power with a mortar and pestle, and subsampled by weight for carbon (C), nitrogen (N).

Wet weight subsamples were placed in borosilicate glass tubes with 7 mL of 90% acetone and homogenized (multiple 20-sec bursts) in an ice bath with a Vibracell sonicator probe. They were then extracted overnight (18-24 h) in a -20°C freezer and warmed to room temperature in a dark container prior to analysis. The homogenate was shaken and centrifuged (5 min at 3000 rpm) to remove particulates. Concentrations of chlorophyll a (Chla) and phaeopigments (Phaeo) were then measured by the acidification method using a 10AU fluorometer. Water-column estimates of depth-integrated Chla for the euphotic zone were made similarly from analyses of duplicate 0.25 L samples collected from CTD hydrocasts, extracted for 24 h in 90% acetone, and measured on the same fluorometer.

For each size-fraction analyzed, we computed the depth-integrated concentration of gut pigment as GPC = [Phaeo] * D / (vol * f), where GPC is gut pigment content (mg m-2), [Phaeo] is the measured Phaeo value (mg), f is fraction of sample analyzed, D is depth of tow (m) and vol is the volume of water filtered (m^3).

We estimated grazing rates (G, mg pigment m-2 h-1) for each size fraction and for the total zooplankton assemblage as G = GPC * 60 * K, where K (min-1) is the gut evacuation rate constant. For K, we used a gut passage rate of 2.1 h-1 measured under similar surface water temperatures in the equatorial Pacific. To compute dry-weight or carbon-specific rates of phytoplankton grazing by the zooplankton assemblage and individual size classes, we divided G by DW or carbon biomass (mg m-2).