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Gough relief cruise report
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The South African polar research vessel, Agulhas II, embarked on its voyage to Gough Island not only to conduct oceanographic research in the Southern oceans, but also to relieve the researchers that were at Gough throughout the year and send off the team to continue with the research.
To foster and enhance collaboration, the Oceans and Coasts Branch of the Department of Environmental Affairs (DEA) offered SAEON an opportunity to participate in the Gough relief voyage aboard the SA Agulhas II.
The voyage was aimed at conducting ship-based as well as island-based research. Research institutions such as the Council for Scientific and Industrial Research (CSIR), University of Cape Town (UCT), University of Pretoria (UP), Rhodes University (RU), South African Weather Service (SAWS) and other institutions were on board to do work in their respective fields.
Ship-based scientists were divided into two groups - one for the day shift and one for the night shift. The night shift group leader was Gavin Tutt (DEA scientific technician) and the day shift leader was Mthuthuzeli Gulekana (DEA oceanographer and the chief scientist for the trip).
Although each person in the group had a specific task, these were rotated. Different instruments were used to collect samples and data, take measurements and perform analysis.
Activities
These included collection of temperature and salinity profiles through the water column; collection of atmospheric and meteorological data using the Automatic Weather Stations on board; measuring surface and underway surface bio-geochemical parameters along the cruise track; measuring and collecting chemical samples of nutrients, dissolved oxygen and chlorophyll-a along the cruise track; collecting phytoplankton samples; and deploying buoy drifters, Argo floats and sea sliders.
Measuring conductivity, temperature and depth
CTD is an oceanographic instrument used to measure conductivity, temperature and depth. It was deployed twice a day to a depth of 2 000–3 000 metres depending on the weather, recording data all the way down from the sea surface. The ship had to be stationary during the deployment. Water samples were taken to determine if the results would correlate with CTD sensor results.
Oxygen samples were taken and analysed at the chemistry laboratory on board the ship. Chlorophyll, salinity and nutrient samples were collected and stored in a freezer to be analysed ashore. Longitude, latitudes, air temperature, wind speed and direction were recorded every time a sampling was made. Some data from the CTD profiles from Cape Town to Tristan da Cunha Island are found in the figure below.
Figure 1. The Temperature – Depth cross section from Cape Town to Tristan da Cunha. The station depth was initially 2 000 m but due to time constraints, the station depth was reduced to 1 000 m for subsequent stations. Warm waters are depicted by red colours, which are characteristic of subtropical Indian Ocean Waters. (Image: Mthuthuzeli Gulekana)
Sampling
Salinity samples were collected every four hours. In the case of chlorophyll, a 500 ml water sample of sea water was collected using a plastic bottle. This water would be filtered through a 2.5 cm micro filter paper using a chlorophyll filter machine. The filter would be wrapped in foil, placed in a plastic container and stored in the refrigerator to be analysed on shore.
Nutrient samples were taken every two hours and frozen in the refrigerator to be sent to a laboratory for analysis.
Argo floats
An Argo float is a robotic observation system for the Earth's oceans that provides real-time data for use in climate, weather, oceanographic and fisheries research. It operates on a 10-day cycle and transmits information to a satellite. It can float as deep as 2 000 metres, measuring temperature, depth and salinity. It has a lifespan of about 3-5 years. Six of these probes were deployed.
Sea gliders
Sea gliders were deployed in the Southern Oceans for the first time. A sea glider is a robotic instrument that typically takes measurements of temperature, conductivity (to calculate salinity), current, speed and direction, chlorophyll fluorescence and bottom depth. It navigates by programming its GPS position. The tilting sensors as well as the magnetic compass enable it to move from an initial point to a determined distribution.
Two gliders were deployed. Their movement was controlled by operators on shore.
Continuous Plankton Recorder (CPR)
A CPR is used to collect plankton samples from the ocean surface while it is towed by the ship. It records temperature, depth and salinity as it collects the samples. A cassette would be inserted on the instrument to collect plankton. Each cassette can collect samples for about 500 miles, after which it is replaced with another.
Five cassettes were used for this whole voyage and the samples collected will be analysed on shore. The coordinates were recorded every time the machine was deployed.
Buoy drifters
Buoy drifters are used to measure GPS positions, temperature and atmospheric pressure. This is done to detect weather patterns, as the sea drifters follow the current as they drift with the current. They are deployed by simply throwing the drifter off the stern of a vessel into the sea.
Acknowledgements
I would like to thank the Department of Science and Technology, the National Research Foundation, SAEON and DEA for the wonderful opportunity – it was truly an experience of a lifetime. I would also like to thank the science team that I worked with on board the SA Agulhas II for making it a most memorable trip, as well as the officers and crew of the SA Agulhas II.
