Not a typical station, a unique oceanographic phenomenon


18 April 2011

Not a typical station, a unique oceanographic phenomenon

After 48 hours of sampling north of the South Pacific tropical front, Tara’s crew made a final, somewhat atypical immersion: a Nisken, one of the Rosette bottles, was filled with fruit cocktail! Wearing dresses and shirts, sailors and scientists plunged the bottle to 100 meters depth, before toasting this 100th station. Under an almost full moon, with music in the background, the team celebrated this event until 22:00 hours.  Although the evening was short, it was not due to fears of disturbing the neighbors, nor to the lack of conviviality on board, but simply because of the immense efforts made during the 2 days and nights had exhausted the crew.

During this penultimate station before the stopover at Guayaquil, Ecuador, the scientists made over 20 immersions of nets and rosette at the heart of a small cyclonic eddy. As in the upwelling zones, surface waters are pushed outwards which causes an upwelling of deep cold waters. In comparison with the composition of the oceanic desert, there is a greater quantity of nutritive salts here and the microorganisms are larger and more plentiful. In the wetlab, Montesserrat Coll Llado, the scientist responsible for viruses and bacteria, has seen her working hours extended, since the more particles in the water there are, the longer the filtration times. Céline Dimier, biologist responsible for protist sampling, has noticed the return of diatoms, which measure greater than 5 micrometers.

On Friday evening a juvenile Physalia, caught in one of the surface nets, was the main attraction for the whole team. Known as the Portuguese man-of-war, this gelatinous organism sports a transparent floater and blue tentacles with poisonous stinging cells. Thanks to its floater, which can measure 80cm in length, the Portuguese man-of-war travels at the speed of the wind throughout the warm waters of the world. Particularly dangerous to man, its presence near the coasts in great numbers often results in swimming prohibition.

But that’s not what accounts for the uniqueness of this station. The scientists have found an unusual and less studied phenomenon – an atypical concentration of chlorophyll at 180 meters depth. In other words, a second peak of chlorophyll has been observed in the abyss. It is less concentrated than the DCM (Deep Chlorophyll Maximum) found at 75 meters, but it signals the presence of phytoplankton at a depth where oxygen and light are practically non-existent. Normally, phytoplankton develop thanks to photosynthesis, a process requiring nutritive salts and light, resulting in oxygen production. Even though the data show nutritive salt metabolism at 180 meters, the lack of oxygen production suggests that photosynthesis is not driving development. The scientists do not know exactly what processes allow microorganisms to survive at these depths. These samplings from Tara will allow the genome scientists on land to investigate this subject.

To end this symbolic station, a band of dolphins has arrived to salute the sailors and scientists. Even though they’re keeping their distance from the schooner, these cetaceans have not stopped their friendly gestures: tail waving, and head nodding. It’s nice to think that these mammalian sailors have traveled away from their normal habitat, further north in warmer waters, to cheer the team’s accomplished work and to encourage the continuation of this unique scientific mission.

Anna Deniaud