As part of the second mission of the GOCART (Gauging Ocean organic Carbon fluxes using Autonomous Robotic Technologies) project, funded by the European Research Council, a Slocum glider (unit-405, Doombar) spent about 4 months surveying the top 1000m of a low oxygen region of the Northern Benguela Upwelling region, off the coast of Namibia. The goal of the mission was to characterize the temporal variability in organic carbon flux and remineralisation depth during the spring bloom in a highly productive but low oxygen region in the Benguela Current, off the Namibian coast.

The glider was a Teledyne Webb Research Slocum G2, equipped with:

• Seabird Glider Payload CTD (pumped), measuring temperature, conductivity and pressure
• Sea-Bird WETLabs ECO Puck Triplet BB2FL-SLC scattering fluorescence sensor, measuring chlorophyll fluorescence and optical backscattering at 700 and 532nm
• Aanderaa 4831 oxygen optode, measuring dissolved oxygen concentration.

The glider was deployed at 11.225°E, 19.331°S on 14th February 2018 from the RV Mirabilis, the vessel of the Namibian Ministry of Fisheries, during the second leg of the 2018 Hake Survey off the northern Namibian shelf and recovered on June 19th, 2018 during the DY090 cruise on board RSS Discovery, during the COMICS cruise (Controls over Oceanic Mesopelagic Interior Carbon Storage), funded by the Natural Environmental Research Council. The latter cruise was in the vicinity of the glider from 1st-19th June, 2018. To validate/calibrate the glider sensors, we conducted several targeted casts (where gliders and ships started profiles simultaneously) and non-targeted casts (unplanned matchups where gliders and ships CTD-profiles were within an acceptable range of each other). Bottle samples were collected on all ship CTD profiles. For calibration purposes, we evaluated and determined that casts within 5 km and 12 hours of each other were considered glider ship matchups. These matchups exhibited strong correlation (linear regression r^2 ≥ 0.95).Variables calibrated were salinity, chlorophyll concentration and oxygen concentration.

Doombar’s mission was slightly modified 3 times during the 4-month deployment:

1 – Sampling around a 12 km triangle at BN0 (centred at 10.80°E, 18.25°S) from 19/02/2018-27/03/2018: Once the glider reached the site (BN0), it was tasked to survey a triangle with 12 km side, centred at that location. The location of the triangle was chosen based on low currents to help constrain advective processes and the predominantly westward surface currents in the region . The 12km sided triangle was chosen based on the time it took the glider to do a complete circuit, aiming for the glider to take around 1.5 days around the triangle.

2 – Sampling around a 12 km triangle at BN (centred at 10.95°E, 18.05°S) from 27/03/2018-08/06/2018: Given that the mission was to survey a low oxygen region, and oxygen concentrations monitored by the glider weren’t consistently low, on March 26th the glider was re-tasked to establish a new triangle (BN) centred at 10.95°E, 18.05°S, Northeast of the initial sampling site, where lower oxygen concentrations were expected.

3 – Station-keeping (“virtual-mooring”) 1.5km North of BN from 08/06/2018-19/06/2018: Throughout its deployment, Doombar gradually reduced its forward speed from about 4km per 1000m dive to about 1.5km. This meant that the glider could no longer cover the triangle in less than 2 days. So, while the ship was in the vicinity, Doombar was assigned a station keeping mission 1.5 km from the cruise main station, to not only avoid the risk of hitting the glider, but also so that any ship data could contribute to validating glider sensor data.

Further information on sensor validation can be found in the netcdf file as well as documented in Lovecchio et al, 2022.