There are many instruments on board that we use to probe the mysteries of the ocean. From when we left Immingham to when we dock in the Falkland Islands, every minute of the day, 24 hrs a day, data is being collected along the ships track to assess the biological, physical and chemical properties of the ocean.
Perhaps the most important instrument that we use is the CTD which stands for Conductivity, Temperature and Density device. This is a stainless steel or titanium cage that houses temperature, conductivity (salinity), oxygen and fluorescence probes (phytoplankton biomass). At the top of the cage there is a transmissometer which measures the transmission of light through the ocean, a Fast Repitition Rate Flourometer which measures phytoplankton photosynthetic response and a pair of Acoustic Doppler Current Profiler (ADCPs) that measure particle backscatter to detect the size of particles in the water column. Around the perimeter of the cage there are 24 ‘bottles’ that hold between 10 and 20 litres of seawater. Each bottle is a cylindrical, PVC tube with rubber sprung caps that close over the top and bottom end of the tube. The CTD is operated by Dave and Terry and lowered into the water from a winch operated by the ships crew. As it descends, the profiles of salinity, temperature, fluorescence, oxygen are plotted on a computer screen in the control room. From these plots, we decide where to collect water and as the CTD is winched back up through the water column, it is settled at a desired depth, and bottles are ‘fired’ or closed using a computer software interface to capture the water from that depth. On AMT18, we generally deploy the CTD 3 times a day to 300 meters just below the light zone; one at 4 a.m. when Carolyn, Paul, myself, Glen, Vas, Mario and Manuela collect water. Another at 5:30 a.m. from which Bruce, Martin, John, Jeremy and Martine sample and the third at 12:30 a.m. from which nearly everyone samples. On one CTD cast we normally collect water from about 16 depths. The water is then drawn from the bottles into sampling vessels by the individual groups who perform an array of analyses from phytoplankton carbon fixation to genetic diversity (see blog entry ‘Setting the scene’). We have also been doing some deep casts to 4500+ meters, which take about 4 hrs. From these, physicists Jo and Stuart and nutrient chemist, Malcolm are able to identify the different water masses present in the Northern and Southern Gyres (see blog entry ‘The Atlantic Gyres’). During this cruise we will do between 100 to 120 CTD casts.
At the 4 a.m. CTD cast Chris also deploys bongo nets which have a 200 micrometer mesh used to ‘fish’ zooplankton. The net is deployed from the forecastle crane, early in the morning to capture the zooplankton which migrate closer to the ocean surface before dawn to feed on the phytoplankton. During the 12:30 CTD cast, Chris also deploys the optics rig which houses a number of instruments. There is a Fast Repetition Rate Flourometer (an FRRF) which flashes short pulses of blue light at the phytoplankton, which stimulates their photosynthesis causing them to emit a red “flash” back to the instrument. There is a WETLabs ac-9 which measures absorption (a) and attenuation (c) of the particles in the water column. Using these measurements we can measure the clarity of the water in the visible spectrum. There is also a Hobilabs backscatter meter which measures the backscattering of particles and a SATLANTIC UV sensor which measures the amount of ultra-violet (UV) light penetrating through the water column. To complement these, there are SATLANTIC radiometers (HyperSAS) on the bow and starboard mid-quarter which measure reflected light from the sea surface every 10 minutes. These data are used to sea truth satellite images. There is also a TRIOS UV sensor which records ambient UV. At the 12:30 CTD, Mike also deploys nets with mesh sizes ranging from 40 to 180 micrometers to capture micro-zooplankton (see blog entry ‘Blue Deserts?’).
When we are steaming between stations there are a number of instruments that continuously log data along the ships track. Towed behind the ship, is a Moving Vertical Profiler. This is a bomb shaped metal body that houses temperature, conductivity (salinity), oxygen and fluorescence sensors. As it is towed it undulates through the water column collecting profiles of these parameters. Terry and Dave spend a lot of their time monitoring and baby sitting the instrument, but it is worth it as the long track profiles they record are spectacular!!
Mounted on the hull of the ship, there is a larger Acoustic Doppler Current Profiler which uses a physical phenomenon called Doppler shift to measure how fast water is flowing. When a sound wave is reflected from something that moves, the frequency of the sound will shift slightly and from this shift in frequency Jo and Stuart can detect currents and also organisms.
Also mounted on the ships hull, there are several other types of acoustic instruments which were setup and running by Peter Enderlein & Co of BAS at the start of the cruise. The Multibeam echosounder maps the topography of the seafloor. The multibeam has transducers that both transmit and receive sound waves and send a cone of sound down to the seafloor, which reflects back to the ship. The returned echo is received by the transducer, amplified electronically, and recorded on graphic recorders. The time taken for the sound to travel through the ocean and back is then used to calculate water depths. The faster the sound waves return, the smaller the water depths and the higher the elevation of the seafloor. The EK60 echosounder estimates biomass and distribution of animals in the water column through backscattered sound waves. An Echogram from the EK60 echosounder, shows the distribution of fish and zooplankton in the water column as scattering layers.
Bruce has a neat array of instruments in his lab which are plumbed into the ships seawater supply. The seawater is sucked into the ship from the hull and flows into Bruce’s ac-9 and backscatter meter (described earlier), which he uses to detect Coccolithophores, tiny calcite shelled phytoplankton.
On the ‘Monkey Island’, the top deck of the ship, there is a meteoroligcal station recording wind speed and direction and ambient light. The University of East Anglia also have aerosol and particle samplers which are continuously collecting air particles from the atmosphere.
So even while I sit here in my cabin, tapping away at my laptop, a variety of data is being collected, which will hopefully reveal the secrets of the ocean.