DOI: 10.17882/108735
title: Isotopic analyses realized on samples of Particular Organic Matter, Zooplankton, and demersal and deep-pelagic invertebrates and fishes collected during the ABRACOS expeditions in the Tropical Western Atlantic

description: This dataset compiles the results of isotopic analyses (δ¹³C and δ¹⁵N) of various ecosystem components. These analyses were conducted to investigate the trophic connections between demersal, epipelagic and mesopelagic ecosystems in the western tropical Atlantic. The results include analyses of Particular Organic Matter, zooplankton, and demersal and deep-pelagic fish, crustaceans, cephalopods, and gelatinous organisms (siphonophores and salpas).
Samples were 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), covering the continental shelf, the Fernando de Noronha archipelago, the Rocas atoll, and surrounding seamounts.
Demersal and deep-pelagic organisms were sampled day and night using three types of trawl gear: (1) a bottom trawl with a body mesh of 40 mm, a cod-end mesh of 25 mm and entrance dimensions of 28 × 10 m; (2) a mesopelagic trawl with a body mesh of 30 mm, a cod-end mesh of 4 mm and a net mouth of 16.6 × 8.4 m; and (3) a micronekton trawl with a body mesh of 40 mm, a cod-end mesh of 10 mm and a net mouth of 24 × 24 m. Bottom trawls were conducted at depths ranging from 10 to 60 metres, while pelagic trawls targeted depths between 10 and 1,100 metres.
Particulate Organic Matter (POM) was sampled by filtering 8l of seawater through pre-combusted GF/F filters (47 mm). Seawater samples were collected at different depths according to the fluorescence maximum using a CTD/rosette equipped with Niskin bottles.
Zooplankton samples were collected using four bongo nets, each with a different mesh size (64, 120, 300 and 500 μm), which were deployed simultaneously and towed from a depth of 200 metres up to the surface. After collection, the samples were pooled, sieved and divided into five size fractions for Abracos 1 (200 µm/200–500 µm/500–1000 µm/1000–2000 µm/ &gt;2000 µm) and six size fractions for Abracos 2 (100 µm/100–200 µm/200–500 µm/500–1000 µm/1000–2000 µm/ &gt;2000 µm). The captured organisms were sorted, identified and frozen at −20 °C.
White muscular tissue was extracted from each fish and crustacean and cleaned with distilled water to remove any foreign material, such as carapaces, scales and bones. For fish larvae, only the body was used; for gelatinous organisms, the whole specimen was used. Each fish, crustacean and gelatinous sample was dried in an oven at 60 °C for 48 hours and ground into a fine powder using a mortar and pestle. 
To obtain unbiased values of δ13C, the carbonates in the zooplankton and POM samples were removed by acidification. POM filters were exposed to hydrochloric acid vapour, while whole zooplankton samples were clustered into six size classes and mixed with approximately 2 ml of 0.5 mol.l−1 HCl. Both samples underwent the acidification process for four hours, after which they were dried at 40 °C for 36 hours. Untreated sub-samples of POM and zooplankton were used to measure δ15N.
Each dried sample was analysed for its carbon and nitrogen isotope ratios using a Thermo Delta V+ mass spectrometer coupled to a Thermo Flash 2000 element analyser via a Thermo ConFio IV interface at the Platform Spectrometry Ocean (PSO, IUEM, France). The stable isotope analysis results for δ13C and δ15N were derived from the relationship between the isotopic value of the sample and that of a known standard, according to the following formula: δ13C or δ15N = ((Rsample/Rstandard) – 1) x 10³, where R corresponds to the ratio of 13C:12C or 15N:14N. The accuracy of the measurements was verified through repeated analysis of internal acetanilide samples.