Dataset: Mesozooplankton wet, dry, carbon and nitrogen biomass and isotope data 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) and stable isotope (13C and 15N) analyses.

CN subsamples were weighed in small tin boats, packed into pellets, and analyzed by standard elemental analyzer, isotope ratio mass spectrometry (EA-IRMS) at the Isotope Biogeochemistry lab at Scripps Institution of Oceanography. The continuous flow system consisted of a Perkin Elmer CHN analyzer coupled to a Thermo/Finnigan Delta Plus IRMS. Acetanilide was used as the standard for instrument stability for both elemental and isotopic measurements on every run. C and N biomass estimates (mg m-2) were computed for each size fraction from C:DW and N:DW ratios. Stable isotope values are reported in standard delta (‰) notation relative to atmospheric N2 and Vienna Pee Dee Belemnite for carbon.