2 January 2008Energy flux from the surface to the deep seaPolarstern ANT–XXIV–2This week we sailed north, still in a frozen ocean but leaving behind the thick pack ice of the last fortnight. The voyage Polarstern ANT 24-2 is studying the links between oceanography and the ecosystem, for comparison with future changes in climate. The part on System Coupling (SYSTCO) studies the energy flux between the life on the abyssal seafloor and in the waters above. SYSTCO leader, Angelika Brandt, said “Unlike the upper ocean, the abyssal seafloor gets no energy directly from the sun. The benthic animals must live on stored food and what comes from above.”
One of our major seafloor sampling stations at about 62°S, 3°W and water depth of 5,300 m had a sediment of uncompacted clay. This may have come from the land, deposited by the large-scale circulation of the Weddell Gyre. The Agassiz trawl came up with about 2 tonnes of mud containing holothurians, ophiuroids and crustaceans. The box corer and multiple corer did not deliver the usual undisturbed, quantitative sample of the sediment, although they operated correctly. Some secrets of seafloor life will remain hidden in the ocean depths!
A scavenger trap resting for about 18 hours near the seafloor caught 520 large amphipods, mostly Eurythenes cf. grillus about 20 mm long. These scavengers are efficient in utilising any dead matter that falls to the seafloor. The fish bait in the trap was picked clean and the amphipod has a swollen gut; the photos tell the story! How do so many amphipods find dead fish in a trap? How does the population travel from one meal to the next? These crustaceans brood their eggs and release live young; there is no dispersion by larvae. The ocean currents probably play a part in distribution of the amphipods, together with buoyancy regulation and their strong swimming legs.
We sampled the life in the upper 200 m of the ocean with a Rectangular Midwater Trawl, shown in the water with the smaller 1 m² net for zooplankton floating above the larger 8 m² net for micronekton (small swimming animals). Carefully timed tows to the target depth and a flowmeter enable us to compare the numbers in each catch. Again, the under-ice trawl performed well, catching actively-swimming marine life, including the krill shown in the aquarium. The pelagic polychaete Tomopteris in the photo mesmerised us with its wave-like contortions.
The pelagic nets caught five species of krill, in all stages of growth. As expected, the largest and most abundant was Euphausia superba. To study their distribution with depth, the Multinet operates five nets sequentially during a tow. The standard mesh is 0.100 mm but their prey is sampled in a finer 0.055 mm version. Identifying and counting all the animals from the samples is keeping us busy. Already our evening lectures are occupied with presenting the preliminary results, to get an overview of the energy flux between the life in the ocean and in the seafloor sediments.
Victoria Wadley |
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