Dataset: Size fractionated mesozooplankton biomass, elemental and stable isotope measurements derived from plankton net tows conducted on R/V Roger Revelle cruise RR2201 (BLOOFINZ-IO) in the Argo Basin region off NW Australia during January-March 2022

This dataset is derived from mesozooplankton net tows in the Indian Ocean Argo Basin off Northwestern Australia carried out from the R/V Roger Revelle cruise in January to March 2022, which were part of an NSF-OCE project (aka: BLOOFINZ-IO) led by Prof. Michael R. Landry to investigate the epipelagic marine nitrogen cycle, plankton dynamics, and impacts on growth and survival of larval Southern Bluefin Tuna (SBT). The cruise report can be found at http://hdl.handle.net/1834/43464. These data are meant to be used in inter-species, interregional comparisons to data from the BLOOFIN-GoM study of larval Atlantic Bluefin Tuna (ABT) in the Gulf of Mexico spawning region.

Oblique net tows were taken to obtain estimates of mesozooplankton standing stocks and grazing over the depth range of the upper mixed layer and the full euphotic zone. Generally, we sampled during midday (1000-1200 h) and midnight (2100-0300 h) hours repeatedly over 3-4 days following a drogued drifter (termed "Cycles"), allowing estimates of diel vertical migrant biomass by difference within the same water parcel. Additional transect stations were also sampled at variable times of the day. We used a 1-meter (m) diameter ring net with a 202-micrometer (µm) mesh size to sample the full euphotic zone, and a 60-centimeter (cm) diameter Bongo (aka: Bongo-60) net with one net having a 202-µm mesh size and the other a 50-µm mesh size to sample the upper mixed layer. All nets were Nitex mesh and fitted with General Oceanics flow meters in the center of the opening to measure volume filtered. Depth of tow was controlled by a depth sensor on the hydrowire and exact depths recorded by a depth logger fitted to the net frames. Net tow contents from the Ring net and Bongo-60 202-µm mesh sized nets were anesthetized with ice-cold carbonated water and split with a Folsom splitter, with half preserved in 4% buffered formalin and half further split into two quarters. Each quarter was size-fractionated using nested sieves into five size classes: 0.2-0.5, 0.5-1, 1-2, 2-5, and >5 millimeters (mm). In the quarter dedicated for measuring biomass, each size fraction was concentrated on a preweighed 202-µm Nitex filter, rinsed with isotonic ammonium formate to remove sea salt, dried at 60 degrees Celsius (°C), and stored dry in sealed containers with desiccant. The other quarter was dedicated to measuring gut pigments for estimating mesozooplankton grazing. Net content from the Bongo-60 50-µm mesh sized net was treated similarly except that half was preserved in formalin, and half split into 0.05-0.1, 0.1-0.2, and >0.2 mm size fractions using nested sieves and dried for measuring biomass.

In the laboratory, dry filters were weighed on an analytical balance and filter preweights subtracted to calculate zooplankton dry weight for each size fraction. Zooplankton dry weight in milligrams per square meter (mg m-2) was calculated from the measured volume, depth of tow, and fraction of sample analyzed. The dried sample was subsequently scraped off the filter, ground to a powder 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 a standard elemental analyzer, isotope ratio mass spectrometry (EA-IRMS) at the Stable Isotope Facility at University of California, Davis (https://stableisotopefacility.ucdavis.edu/). The system consists of a PDZ Europa ANCA-GSL elemental analyzer interfaced to a PDZ Europa 20-20 isotope ratio mass spectrometer (Sercon Ltd., Cheshire, UK). Acetanilide, USGS41, and facility internal standards was used for instrument stability and corrections 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.