Saturday, 6 December 2008

THE GREAT EDDY HUNT (Moçambique Channel)

After a successful national launch of the ASCLME programme in Pemba (northern Moçambique), the fourth ASCLME / EAF Nansen research cruise got underway on the 28th November 2008. Before embarking on the cruise proper, the first day was used to successfully install a 1000m mooring just off the Pemba coast (see below for more details). By the evening, the sensors had been installed, the mooring team had been dropped off back in port and the Eddy hunt got underway.

So what is it all about?

Unlike other ASCLME cruises to the east of Madagascar, Voyage 2008409 in the Moçambique Channel is not purely exploratory in nature. Preliminary regional investigations into biodiversity, faunal standing stocks, and the oceanographic environment have previously been undertaken. Instead, the motivation for this cruise is to test a number of hypotheses on the role and importance of Mozambique Channel Eddies in supporting regional biological production and diversity.

And what are Eddies?

Eddies are rotating masses of water that, in the Moçambique Channel, travel primarily southwards along the Moçambique coast. At any one time, several eddies slowly make their way from the warm tropics towards the relatively colder southern Africa. There are anti-cyclonic eddies (or warm-core; rotating counter-clockwise) and cyclonic eddies (or cold-core; rotating clockwise). When these Eddies come into contact with the continental shelf, they often draw a large amount of phytoplankton, nutrients and coastal fish-larvae from the coast into the open ocean. It is this process that we are interested in! How important are eddies in subsidizing pelagic (open water) production of phytoplankton, zooplankton and ultimately fish? Do or can Eddies transport fish and invertebrate larvae across the Channel? How important are Eddies as feeding grounds for fish, birds and whales? These are some of the questions that that the multi-national and multi-disciplinary team of scientists on this cruise are trying to solve. Essentially: How important are eddies for regional fisheries and ecosystem-functioning in the Moçambique Channel?

Satellite data showing us the way:

AVISO surface height anomaly data (above left) show that the sea surface height of features A and C are lower than that of feature B. This is typical of cyclonic (A, C) and anti-cyclonic eddies (B). The chlorophyll image (above right), further indicates that phytoplankton (arrow) in the water is wrapped around feature B in an anti-clockwise direction. We can therefore be reasonably certain that we are looking at an anti-cyclonic eddy. Now all we have to do is steam to the right spot and start sampling …

The team: The team on this cruise consists of scientists from Moçambique, Madagascar, South Africa, France, Germany, Norway and Canada. On board are specialists in physical and chemical oceanography, fisheries, zooplankton, phytoplankton and food-web analysis. Back at their home bases, other scientists support the cruise by providing us with the latest satellite data and other important information.

Written by S Kaehler

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