Author: Stein Kaartvedt (University of Oslo, Norway) and Cairistiona Anderson (University of St Andrews, UK)
The longest stretch of steaming since we arrived at the MAR has brought us close to the Portuguese 200 mile zone. During steaming, repeated whale sightings were made. The most unusual sighting was of two blue whales, while the most remarkable encounter was with three fin whales, foraging on plankton at the surface. By a very good piece of manoeuvring, the officers on the bridge were able to sneak G.O. Sars into the midst of this assemblage, giving us all a great experience (see picture) and the whale scientists an opportunity for tagging their targets (which regretfully failed on this occasion).
Close encounters with a fin whale. Photo: Henrik Skov. Click to enlarge.
One of the consequences of being at lower latitudes is a more pronounced and rapid shift between day and night. Shifts between daylight and darkness are of ultimate importance for most of the creatures living in the sea. As in any ecosystem, the primary food production (growth of algae) is fuelled by the sun. Light diminishes rapidly with increasing depth, and so does algal growth. The primary production in these waters may be limited to the upper 20-50 m. Production in this thin surface film is ultimately going to nourish all life throughout the many thousands of meters of the water column. So light is the basis for life, but light is also dangerous. Fishes use their eyes to locate prey and are very efficient predators in daylight. The smallest planktonic animals are not easily spotted, and can take the risk of foraging in surface waters even in daylight. For somewhat larger organisms, one way to resolve the contrasting needs of obtaining food and avoiding predators is to stay away from the upper, dangerous pastures during the day, and harvest the production of the surface layer in the shelter of darkness at night. Such daily migrations of organisms up and down the water column, known as diel vertical migrations (DVMs), are easily detected by the echosounders. As the shifts between day and night becomes more abrupt, migration speeds increase. This morning's echograms clearly displayed how some acoustic scattering layers (representing small fish or large zooplankton) were hurrying back to the relative safety of depth, descending nearly 400 m in the course of 90 minutes as surface irradiance increased in the morning (see picture).
Scientists working on their reports!
We have started on the last week of our cruise. On board, we are all becoming increasingly aware of our responsibilities with regard to reporting the initial results of the research we have been conducting. It is particularly important to do this by the end of this leg of the cruise, as it will provide a useful background for the research to be conducted by other scientists on the second leg. Parallel with continuing biological sampling, attention has become directed to summing up results, preparing cruise reports, and conducting discussions on procedures for the handling of data and publications in the future.
Echogram showing morning descent of small pelagic organisms away from the surface. Click to enlarge.
The weather is now calm and pleasant, with even some glimpses of sun. The sun is noticeably stronger now that we are further south, even if the breeze is still cool.
Tomorrows expected highlights
Tomorrow will bring us to the first sampling station in the middle of the ridge for the southern region. The last of the landers - an acoustic array that will do in situ studies of fish behaviour - will be deployed at ~900 m, on the pinnacle of a seamount.