Tuesday, 16 December 2008
SCRATCHING THE SURFACE
Our first task was to define the eddies using satellite tracked drifters (see previous blog page). The next step was to answer the questions; how can we characterize an eddie when looking downward at its hydrological properties and: how can we investigate its potential impact on the living realm? Our sampling strategy provided a partial but quasi systematic survey of part of the eddy field. The survey consisted of hydrographic stations (black dots on the SLA map below) along a north – south transect. At each station, temperature, salinity, dissolved oxygen and fluorescence were recorded continuously from surface to 1000 m - the nominal maximum depth chosen for this study - focusing in on the biological signature of the eddies (sections plotted to 250 m). Sea water samples were collected at various depths. Analyses (at the laboratory) of nutrients, total chlorophyll, primary production, particulate organic matter, pigments and absorption will produce valuable information to relate the physics to the biology of the ocean.
For the moment, let’s have a look at what the “physics” say and let’s comment on what the vertical sections tell us. Keep in mind that this transect crossed two cylonic (clockwise) and one anti-cyclonic (anti-clockwise) eddies. Theoretically, a cyclonic eddy is an area of divergence at the surface (basically, due to the earth rotation and the so-called “Coriolis force”), while convergence occurs at the centre of an anticyclone. Surface water missing at the centre of a cyclone has to be replaced from the subsurface layers. This upwelling brings colder water towards the surface, which is clearly shown by the “doming” of the isotherms in the first diagram (see at 150 m depth and 620 km distance). The same signature is also found in the oxygen distribution (water is oxygen depleted relative to the surface at depth) and - interestingly for biology - in the fluorescence distribution. This means that the whole water column (or at least the upper 250 m) upwells toward the surface. This is important in terms of biological productivity. Indeed, deeper water is also richer in nutrients (nutrients are exhausted at the surface due to consumption by the primary production). Upwelling brings nutrients into the upper euphotic zone (i.e. where light is available), so primary production can occur and the whole trophic chain can potentially take benefit from the fertilization! Note that the fluorescence maximum values are higher and shallower in the centre of the two cyclones. Contrary to the above scenario, the convergence at the centre of an anticyclone pushes down the lower layers (downwelling). Again, the temperature, oxygen - and even fluorescence - vertical distribution perfectly illustrate this downward movement (middle of the transect at 400 km). In terms of its biological signature, the anticyclone will tend to suppress productivity, resulting in fewer catches when trawling within a cyclone! Last but not least – at least for an oceanographer – the salinity distribution also illustrates these up and down vertical displacements. Essentially, downwelling in the centre of the anticyclone vertically expands the intermediate layer which is characterized in this part of the Mozambique Channel by a salinity subsurface minimum. In contrast, cyclonic eddy upwelling “compresses” the intermediate layer, which expels the low salinity water around the eddy. Both processes result in a beautiful “bubble” of low salinity water, right in the middle of the anticyclone!written by: Jean-Francois Ternon and Tammy Morris
The post-hunt party
After a successful survey of Mozambique Channel Eddies, work is now drawing to a close and a final social gathering was held before the data analysis starts and before steaming back to
By popular demand Tor's lyrics are provided below (yes … we have all been at sea too long … :)
The Eddie hunt song (by Tor Gammelsrød)
Women on cruises bring favourite pets
For instance did Jenny bring a huge multinet
Keshnee brought chemicals, and got high now and then
Tammy brought drifters, Jackie brought Sven
REF: Nansen Cruise, ocean blue
Fantastic food and an excellent crew
Jackie and Sven they have courage and will
doing production is their favourite skill
always during night, seldom during day
what kind of production they don’t want to say
Jenny loves animals, particularly small
But flirting with her? She’s not interested at all
Says she: if you want fancy me use multi-net
That is the way to get my trousers wet
Both females and males they have sexual drifts
for some a nuisance, for others a gift
For Tammy the pressure got hard, oh my Lord,
she decided to throw her drifts overboard
Bjørn and André they studied Keshnee
Is it possible to get that beauty on my knee?
but she did not want to use them as her pets
She was busy reading books and smoking cigarettes
Trawling for fish for survival is a must
Jean Francois and Doris in acoustics they trust
Records they broke which long stand they will
catch per unit effort was just below nil
The Longliner should help us with catch after all
But she did not meet us. Scandal! said Pascal
This is not only a scandal said furious Michel
It’s a criminal act!; c’est merde!, Quelle bordell
Bevin turned the light in the lab way down low
Avelino and Dino gave a sexy floor show
The lab ended finally up as discotheque
The samples got drunken the filters got leak
The party to night has a scientific aim
so for Bruce the whole cruise should not be in vain
For our patient and dedicated top spectator
finally may study us top predators
Bruce will determine our sex and our age
Sexual maturity, our length and our weight
But I think I will move ashore in a tent
When he starts to sample the stomach content
written by: Sven Kaehler
Thursday, 11 December 2008
CHASING EDDIES IN THE MOÇAMBIQUE CHANNEL
As mentioned previously, the aim of Leg 4 of the ASCLME / EAF Nansen research cruise is to survey eddies in the Moçambique Channel. In order to do so, their location needs to be identified first, and near-real time satellite observations help in providing an idea of what is going on in this very chaotic, or even turbulent part of the ocean.
The problem with near-real time satellite observations is just that, they are near-real time. Sea surface height measurements from altimeters are processed by AVISO (www.aviso.oceanobs.com), and in order to provide a map of the currents at any particular day (see figure below), data from a host of satellites needs to be incorporated and interpolated. This takes time, generally the data is made available with a 7 day delay. So, in effect, we have to guess what the eddy is going to do, based on 7 day old information, adjust our sampling strategy accordingly, and hope not much has changed in the last week!!
To help us make a more informed guess as to the whereabouts of the eddies, we deployed surface drifters during the first (north-south) transect of our cruise. Surface drifters are essentially buoys attached to a 5 m long sock (for more information go to www.oceanafrica.com/drogues/drogues.html). These drogues drift in the ocean following the currents and transmit their longitude and latitude positions to satellites, which are relayed to our support team in
Comparing their drift patterns to the week old geostrophic currents (see above), one can see that in some cases these follow the current patter described by the delayed satellite observations, but in many cases they do not. This highlights the difficulty in surveying Moçambique Channel eddies accurately given the tools at our disposal! On one particular day we arrived on station expecting westward currents, but the ship board current meters were showing eastward currents!!
The drifter tracks show that the current system itself is indeed very chaotic and turbulent, with drifter tracks criss-crossing, over-lapping and splitting from each other. Modern thinking tends toward describing the flow dynamics in the Moçambique Channel as “eddy-driven”, and our drifters very neatly show this to be the case.
Written by: Bjorn Backeberg and Tammy Morris
Tuesday, 09 December 2008
Since our departure from
- At night, a stratification of the communities is observed, myctophids being deeper in the DSL than the flying squids and the cigarfish.
- Coral reef fish larvae and juveniles are observed all over the
Mozambique channeleven far away from the coasts. This is an interesting observation that suggests that reef-fish populations from the east coast of Africa and the islands of the western Indian Oceanmay be connected.
- It must be noted than shrimps and euphausids are absent of the trawls.
Cigar fish during night-time surface trawlsOne of the many coastal reef species caught during day-time surface trawls
Red flying squid that are prevalent during night-time trawls
written by: Michel Potier (IRD, France)
Monday, 08 December 2008
The dominant bird species on the cruise is the Sooty Tern Sterna fuscata. Almost 90 hours of observation have been completed so far producing almost 2000 birds. In the north Common Noddy Anous stolidus, threatened some competition, but this waned as the survey progressed south. Tropical (formerly Audobon’s) Shearwater Puffinus Iherminieri and Parasitic Jeager Stercorarius parasiticus (harassing Sooty Terns) was of interest in the north.
Several feeding aggregations of terns were observed throughout the survey (but not the 6 December), the group size varying from 50 – 480 birds. All the aggregations were associated with feeding activity of small tuna. Some Sooty Terns were seen to grab small slender silver fish. A bird aboard at night regurgitated several small squid, a species taken frequently in trawls conducted to date. Other species were few and far between, but the most productive area for diversity seemed to be on the approaches to
Cetaceans were scarce, but when seen exhibited a reluctance to be remotely close to the vessel, which severely inhibited their identification. This behaviour may result from the acoustic equipment used for fish location. However, despite this Sperm Whale Physeter macrocephalus, Minke Whale Balaenoptera acutororostrata, Cuvier’s Beaked Whale Ziphius cavirostris, Short-finned Pilot Whale Globicephala macrorhynchus, False Killer Whale Pseudorca crassidens, Bottle-nosed Dolphin Tursiops truncatus, Common Dolphin Delphinus delphis and Spinner Dolphin Stenella longirostris, have been positively identified. I might add that only the Minke Whale sightings have been relatively close to the vessel.False Killer Whales
written by Bruce Dyer – MCM, Top Predator observer (photos: S Kaehler)
Saturday, 06 December 2008
THE NAMING OF THE GUNGE
Trichodesmium is commonly called "sea saw-dust" because its colonies and large brown blooms have been mistaken as sandbars by ships in the ocean (including Captain Cook who wrote the first documentation of Trichodesmium over 200 years ago). These photosynthetic cyanobacteria can be found as filaments (trichomae) comprised of 10's-100's of cells or in colonies 1-10 mm in length. The fact that these colonies can be seen by the naked eye is what gave Trichodesmium its name - the Greek word "trichoma" for hair and "desmus" for bonded = "bonded-hair," which is how Trichodesmium colonies look to the human eye. The colonies can be yellowish-brown to deep red in color due to their primary light harvesting pigment, phycoerythrin. They are buoyant and able to regulate their position in the water column due to large gas-filled vacuoles or vesicles in each individual cell. Trichodesmium blooms are surface aggregations that can be 10-1000's of km wide. They occur during periods of low wind stress and warm temperatures. Some of these blooms are so vast that they are visible from space. However, the bacteria on the surface do not generally survive for extended periods of time for various reasons, including UV damage. [Source: Wikipedia]
Written by: Jenny Huggett (DEAT: Marine & Coastal Management,
Strong Currents near Pemba, northern Mozambique
Experimental oil drilling at 1000m depth is planned in the area, which is another reason to learn about the local ocean currents. So a team of scientists from the Instituto de Investigacao Pesqueira, Moçambique and the Universitetet i Bergen, Norway deployed a current meter mooring at about 1000m depth (photo: preparing and deploying the mooring).
Written by Tor Gammelsrød (photos: Tammy Morris)
THE GREAT EDDY HUNT (Moçambique Channel)
After a successful national launch of the ASCLME programme in
Satellite data showing us the way:
The team: The team on this cruise consists of scientists from
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