Abundance and biovolume of Zooplankton collected during the ABRACOS expeditions in the Tropical Western Atlantic

This dataset presents taxonomical identification and quantification of samples of zooplankton collected during the ABRACOS 1 (29 September–21 October 2015) and ABRACOS 2 (8 April–9 May 2017) expeditions along the northeastern Brazilian margin (approximately 3°S to 9°S and 35°W to 31°W), encompassing the continental shelf, the Fernando de Noronha Archipelago, Rocas Atoll, and adjacent seamounts. Zooplankton was sampled using two gears : (i) a bongo frame with four mesh size (64, 120, 300 and 500 µm) and (ii) a closing net (WP2, 200 µm).  (i) Bongo sampling was conducted by towing a regular bongo frame with four nets fitted with mesh sizes of:  - 64 µm (30 cm mouth diameter),  - 120 µm (30 cm mouth diameter),  - 300 µm (60 cm mouth diameter),  - 500 µm (60 cm mouth diameter).  At each station, one oblique haul was conducted from 200 m depth to the surface or from approximately 10 m above the seabed to the surface at stations shallower than 200 m. A Hydro-Bios flowmeter was mounted inside each net to estimate the volume of filtered seawater. . Samples were preserved in 4 % buffered formaldehyde (sodium tetraborate, 0.5 g L⁻¹; (Newell and Newell, 1977)).  During ABRACOS 1, the entire sample from each mesh size was used for taxonomic identification and quantification.  During ABRACOS 2, samples from each net were split into two equal fractions, one dedicated to taxonomy and the other to stable isotope analyses. - 64µm (large microzooplancton) samples were split until a minimum of 200 individuals was reached. Organisms were identified under a stereomicroscope to the finest possible taxonomic level primarily according to Boltovskoy (1999) and counted. Abundances were standardised to numbers of individuals per cubic metre (ind m⁻³) (Tosetto et al., 2024) and reported as “Density” in the datafiles. - 120µm samples were analysed under a stereomicroscope (Zeiss Stemi 2000-C). For ABRACOS 1 survey, the organisms were identified by large taxonomic group. The samples were divided into subsamples by a factor ranging from 2 to 512 (field Subpart) to obtain at least 300 organisms per sample. For each individual zooplanktonic organism, the major and minor axis (fields Major and Minor) were measured in mm.  Individual elliptical biovolume (field EBV) in mm³ was calculated as (4/3) × π × (Major/2) × (Minor/2)². Standardized empirical biovolume per m3 (field EBV_STD) in mm³ m⁻³ for the considered individual was computed as EBV × Subpart)/Filtered_Volume, where  Filtered_Volume is the total volume filtered for the considered net (Figueiredo et al., 2020). For ABRACOS 2 survey, organisms were identified to the finest taxonomical level possible and counted; measurement of organisms and individual biovolume estimation were not performed. For each organism, abundance was standardized to number of individuals per m3 (ind m⁻³) and reported as Density in the datafile. Users should note that taxonomic resolution differs between surveys for 120 µm samples, which may affect comparability.  - 300µm and 500µm were analyzed under a ZooScan system. According to the concentration of organisms, the samples were split by a Motoda splitter into subsamples by a factor ranging from 2 to 512 (field Subpart) to obtain up to 2000 objects in each scan. Subsamples were digitised using the ZooScan (Hydroptic model ZSCAN03) with a 2400 dpi resolution, following the protocol established by Grosjean et al. (2004; http://www.zooscan.obs-vlfr.fr/). Images were then processed using the ZooProcess software (Version 7.19) to isolate each object into a single vignette and generate a range of quantitative descriptors (size, grey level distribution and shape parameters) for each. To classify the vignettes into pre-established taxonomic groups, a semi-automatic approach was initially used for ABRACOS 1 samples, using the Plankton Identifier software according to Gorsky et al. (2010) (Figueiredo et al., 2020), while EcoTaxa software was used for ABRACOS2. The samples from ABRACOS 1 were subsequently re-classified using EcoTaxa to ensure methodological consistency with ABRACOS 2. After classification, all results were manually validated and identified to the smallest taxonomic level possible by multiple specialists. For each individual zooplanktonic organism identified under the Zooscan, the following measurements are available:  - The major and minor axes (fields Major and Minor) of each organism were measured in mm. - The area of the organisms (field Area) in mm2 was estimated as π × (Major/2) × (Minor/2) - Their equivalent spherical diameter (field ESD) in mm was estimated as √(Minor * Major) - Individual elliptical biovolume (field EBV) in mm³ was estimated as EBV = 4/3 × π × (Major/2) × (Minor/2)2  - Spherical biovolume (field SBV) was also inferred as 4/3 × π × (Area/π)3/2. - Standardized biovolumes per organism per m3 (fields EBV_STD and SBV_STD) in mm³.m⁻³ were calculated for both EBV and SBV, using the formulas:  EBV_STD = ((EBV x Subpart)/Filtered_Volume) / Fraction SBV_STD = ((SBV x Subpart)/Filtered_Volume) / Fraction,  where Filtered_Volume is the filtered volume in m3 and Fraction is the ratio of this filtered volume used for taxonomy (1 for ABRACOS 1, 0.5 for ABRACOS 2).   (ii) Vertical zooplankton sampling was conducted using a WP2 closing net (200 µm mesh size).  At each station, two vertical tows were performed:  - One vertical tow 200 m to thermocline (identified as “inf” in the Layer field of the data files) - One vertical tow upper thermocline to surface (idendified as “sup”) Samples were fixed using formaldehyde 4%.  The organisms were identified at the more precise taxonomic level possible and counted under stereomicroscope.  Abundances were standardized in number of individuals per 1 m3   Data volume: During ABRACOS 1, 35 and 14 successful hauls were performed for taxonomy using bongo net and WP2 net, respectively. During ABRACOS 2, 49 and 30 successful hauls were performed for taxonomy using bongo net and WP2 net, respectively.   Data organization: This dataset has been organised in several CSV files (one for each type for both survey): - ReadMe_Zooplankton_Abracos.txt : ReadMe file describing the structure of all data files. - ZooPlankton_Sampling_Abracos1.csv & ZooPlankton_Sampling_Abracos2.csv: description of the zooplankton sampling operations - Taxonomic_Reference_Zooplankton_Abracos.csv : Common taxonomic list for both surveys and sampling modes - Zooplankon_WP2_Abracos1.csv and Zooplankon_WP2_Abracos2.csv: data tables from WP2 sampling (density per taxa) - Zooplankton_64_Abracos1.csv and Zooplankton_64_Abracos2.csv: data tables from bongo 64 µm sampling (density per taxa) - Zooplankton_120_Abracos1.csv and Zooplankton_120_Abracos2.csv: data tables from bongo 120 µm sampling (biovolume per individual) - Zooplankton_300_500_Abracos1.csv and Zooplankton_300_500_Abracos2.csv : data tables from bongo 300 & 500 µm sampling (biovolume per individual)