The Cape São Tomé Eddy Transect during the ILHAS 3 Research Cruise

Description

During the ILHAS 3 expedition, a strong coherent cyclonic mesoscale meander was detected between 22ºS and 24ºS, about 100 km offshore of the Rio de Janeiro State, Brazil. The feature was surveyed with a hydrographic transect between 09 and 11 December 2019 (end of spring) onboard the University of São Paulo's R/V Alpha Crucis.

The transect was oriented in the SW-NE direction, roughly following the 2000 m isobath. The ship moved NE, occupying 19 stations. The oceanographic sampling instruments included: a shipboard Acoustic Doppler Current Profiler (ADCP) RDI-75 kHz; a Profiling Natural Fluorometer System (PNF-300); and a Niskin rosette with a payload consisting of Niskin bottles for water sampling, and a Sea-Bird SBE 9 Conductivity-Temperature-Depth profiler (CTD) carrying sensors for temperature (SBE 3plus), conductivity (SBE4), dissolved oxygen (SBE43), and fluorescence (Turner Cyclops) for Chlorophyll-a. Water samples were collected at standard depths for the determination of nitrate, nitrite, phosphate, and silicate using an AutoAnalyzer-3 SEAL Analytical System. Aliquots of 1 L were retrieved, filtered onto Whatmann GF/F filters, and for later Chlorophyll-a measurements with a Tuner Design AU-10 Fluorometer. Flow cytometry was performed with an Attune Nxt Flow Cytometer for pico- and nano-phytoplankton community analyses. Primary production experiments were performed in the euphotic zone at two stations (in the eddy center and the outermost station).

Acknowledgments

The Brazilian Navy, Petrobras (Petróleo Brasileiro S.A.), and ANP (Brazilian National Agency of Petroleum, Natural Gas and Biofuels) for facilitating the fieldwork under the Cooperation Terms SIGITEC 2018/00451-6 and 2018/00452-2.

Captain José Rezende, commanding officer of the R/V Alpha Crucis, and crew, as well as others of the technical staff, undergraduate and graduate students on board, whose assistance was indispensable for data collection.