Tuesday, 16 December 2008

SCRATCHING THE SURFACE

What is happening below 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 South Africa. During our stop-over and braai just off the coast from Vilankulos (Mozambique), our cruise leader Tor turned into a bard and Keshnee Pillay graciously accepted the role of "golden-haired" Santa Lucia and joined the pre-Christmas festivities.



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

Nansen cruise…

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

Nansen cruise…

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

Nansen cruise…

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

Nansen cruise…

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

Nansen cruise…

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

Nansen cruise…

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

Nansen cruise…

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

Nansen cruise…

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

Nansen cruise…


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 Cape Town and then sent to us. Below right is an image of the near-real time geostophic current velocities derived from sea surface height measurements from altimetry with the successive positions of the surface drifters overlaid. The numbers represent the I.D. of the drifters and the positions where they were thrown overboard. We have lovingly given them nicknames such as “Bob” or “Amper Vergeeten” (in this case as the name states).



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

Fishy affairs

Since our departure from Pemba, 10 pelagic trawls have been performed. Four during daytime hours and six during the night when the DSL (deep scattering layer) has finished its ascent. The DSL, is a sound reflecting layer in the water column that can be located with echo-sounders and is caused by aggregations of zooplankton or fish.

At daytime, the surface layer of the water (10m deep) and aggregations present between 200-250m have been trawled. Near the surface, catches consisted almost exclusively of juveniles of coastal coral-reef fish and of tunas. Such fish are often found in the stomach contents of top predators foraging at the surface (some birds and tunas). The deeper water aggregations trawled during the day typically consisted of mono-specific (i.e. single species) schools of mesopelagic fish such as the myctophid (lantern fish) Diaphus richardsoni.

A night, most of the trawls were conducted between 10 to 20m depth. They were charactherized by a high catch of flying squids (Ommastrepidae, essentially Sthenoteuthis oualaniensis) and cigarfish (Nomeidae, Cubiceps pauciradiatus). Myctophids were represented by several species. In deeper trawls (40m depth) the catch of flying squids decreases and the catch of myctophids increases in terms of diversity.

First results


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

Biological Observations

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 Madagascar and northward towards Juan da Nova Island. Of interest in this region was Great Frigatebird Fregata minor, Red-footed Booby Sula sula, White-tailed Tropicbird Phaethon lepturus, Wedge-tailed Shearwater Puffinis pacificus, Jouanin’s Petrel Bulwaria fallax, Black-bellied Storm Petrel Fregetta tropica and Wilson’s Storm Petrel Oceanites oceanicus. The survey moved across to the Mozambican coastline and the steady flow of birds dried up. We await a clearer picture to see where and why the birds numbers improve as the survey progresses. Oddities included Madagascar Squacco Heron and Cattle Egret.

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
Sooty Tern

written by Bruce Dyer – MCM, Top Predator observer (photos: S Kaehler)


Saturday, 06 December 2008

THE NAMING OF THE GUNGE

Finally we have a name for the brown gunge that has been visible in broad streaks at the ocean surface since entering the first cyclonic eddy, and which has been clogging our plankton nets and slowing down the filtering of our numerous water samples - Trichodesmium. In our samples it appears as masses of loose fibres and fuzzy clumps. It occurs in nutrient poor tropical and subtropical ocean waters, and is an important marine nitrogen-fixing bacterium that is being studied extensively for its role in nutrient cycling in the ocean. It forms blooms and provides a substrate for many small oceanic organisms, such as other bacteria, diatoms, dinoflagellates, protozoa, and copepods. The species Trichodesmium erythraeum is endemic to the Indian Ocean.

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]

Figures: (a) Streaks of Trichodesmium seen from the Nansen; (b) a Trichodesmium bloom visible from space (from AIMS Research.); (c) thick Trichodesmium “gunge” from the bongo net haul; (d) Trichodesmium sp. as seen under a microscope.

Written by: Jenny Huggett (DEAT: Marine & Coastal Management, South Africa)

Strong Currents near Pemba, northern Mozambique

Ocean currents typically range between 10 – 50 cm/s. Outside Pemba, however, vessels have experienced strong south-flowing currents when trying to enter the port. These could not be explained as tidal currents, because the bottom depth is more than 1000m and therefore tidal currents are weak.

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).

This instrument has an internal data recorder, so we have to pick the mooring up again next year to get access to the data.

During the deployment we recorded current speeds using the ship mounted current meter (ADCP) of almost 2 m/s (= 4 knots, almost the speed of a fishing vessel)! If this turns out to be a typical current speed this is bad news for the oil companies, because the currents would cause too high a load on both the drilling platforms and the 1000m long pipes from the surface to the bottom, for present technology to cope with. We will know next year.

Written by Tor Gammelsrød (photos: Tammy Morris)



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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