ASCLME Nansen Cruise 2008

SCRATCHING THE SURFACE

2008-12-16T12:31:00.002+02:00

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

2008-12-16T11:35:00.006+02:00

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

CHASING EDDIES IN THE MOÇAMBIQUE CHANNEL

2008-12-11T18:53:00.004+02:00

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

Fishy affairs

2008-12-09T11:36:00.004+02:00

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

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 channel even 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 Ocean may 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)

Biological Observations

2008-12-08T07:09:00.004+02:00

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)

THE NAMING OF THE GUNGE

2008-12-06T14:34:00.005+02:00

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

2008-12-06T14:08:00.006+02:00

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)

2008-12-06T08:10:00.006+02:00

After a successful national launch of the ASCLME programme in Pemba (northern Moçambique), the fourth ASCLME / EAF Nansen research cruise got underway on the 28^th 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

Anchors Aweigh!

2008-11-26T09:30:00.002+02:00

Leg 2 of Cruise 3 of the ASCLME survey aboard the R/V Fridtjof Nansen gave NOAA’s Pacific Marine Environmental Laboratory (PMEL) in Seattle, Washington the opportunity to deploy two ATLAS moorings in the western Indian Ocean. The first mooring was installed at a nominal location of 8°S, 55°E on 21 November 2008 and the second at 12°S, 55°E on 22 November 2008. These moorings are part of the Research Moored Array of African-Asian-Australian Monsoon Analysis and Prediction (RAMA), which is a multi-national effort to provide key oceanographic and marine meteorological data sets for monsoon research and forecasting. The goal is to span the Indian Ocean with an array of 46 such moorings between 15°N to 25°S. The cruise also provided the opportunity to deploy 4 Argo floats along the cruise track between the moorings. These are first PMEL Argo floats in the Indian Ocean and they fill a significant hole in Argo data coverage.

PMEL had never deployed an ATLAS mooring from the Nansen, so there was an element of suspense as to how the difficult and time consuming the operations might be. However, the surprises in store for us were all pleasant. Both deployments proceeded flawlessly, and in each case it took less than 5 hours from the time the buoy was placed in the water until the time the anchor was dropped. These successful operations were the result of a great team effort involving Mooring Technician Steve Kunze from PMEL who supervised the deployments; Chief Bosun Helge Dahl and his crew who masterfully carried out the work on deck; Captain Kjell Sandøy and his officers who were superb at maneuvering the ship in changing winds and currents; Data and Information Consultant Lucy Scott who applied her GIS skills to map echo sounder data during our pre-deployment bathymetry surveys; Chief Acoustician Tore Mørk who provided Lucy the ship’s log files during the surveys; and all the scientists who eagerly pitched in to help out on deck.

ATLAS moorings were developed at PMEL in the early 1980s and have been updated over the years with improved sensors, materials, and design. The mooring consists of a 2.8 m donut shaped float on which is mounted a 4 m high tower for meteorological measurements. The upper 700 m of the mooring line is plastic jacketed steel cable to protect against shark bite and to provide a conductor for electronic transmission of subsurface data to the CPU on the surface float. Below the steel cable is 3-4 km of nylon rope attached to a 3 metric ton anchor made of old railroad car wheels. Just above the anchor is an acoustic release that when triggered will detach from the anchor for easy recovery of all the instruments and mooring line.

ATLAS moorings measure surface wind speed and direction, air temperature, relative humidity, solar radiation, rain rate, sea surface temperature and conductivity, temperature and conductivity at several depths in the upper 500 m, and ocean velocity at 10 m depth in the surface mixed layer. Data are transmitted to shore in real-time via NOAA’s polar weather satellites and are available to researchers and operational centers world wide. You can view and download these data at
http://www.pmel.noaa.gov/tao/disdel/disdel.html

Thanks to ASCLME and the R/V Dr Fridtjof Nansen, RAMA has increased from 20 to 22 moorings and the array is now nearly 50% complete (See Rama figure below). Also in place is a fruitful partnership between PMEL and ASCLME for interdisciplinary studies in ocean circulation, climate variability, and large marine ecosystems of the western Indian Ocean. This is oceanography at its best! Schematic of RAMA as of November 2008. Solid symbols indicate those sites occupied so far. Color coding indicates national support, with year of first involvement shown in the upper right box. Open symbols indicate sites that are not yet instrumented.

Plate captions:
Top: First ATLAS bouy going over the side
Middle: Steven Kunze, Tommy Bornman & Ryan Palmer attaching temperature sensors to the mooring line
Bottom: Acoustic release ready for deployment
Left: The ATLAS succesfully deployed
Right: Steven Kunze and Mike McPhaden deploying an ARGO float

Text by: Mike McPhaden, NOAA/PMEL

Photos by: Lucy Scott, Isabelle Ansorge & Tommy Bornman

Land ahoy!!

2008-11-25T10:35:00.003+02:00

The 41 day Mascarene Plateau Cruise finally came to an end on 16 November after sampling in amongst the islands of the Seychelles Group for more than a week! Passing close by lush tropical islands covered in palm trees with inviting golden sandy beaches was almost too much to bear!

All the scientists that participated in Leg 1 of Cruise 3, Mascarene Plateau, are shown in the image below. They are from left to right: Michelle Etienne; Oddgeir Alvheim; Helena Francourt; Denis Tweddle; Ole Sverre Fossheim; Marek Ostrowski; Tore Mork; Isabelle Ansorge (Local Chief Scientist); Vimal Ramchandar; Vikash Muibodhe; Vincent Lucas; Kim Bernard; Sven Kaehler; Riaan Cedras; Jackie Hill; Tommy Bornman; Morgane Perri; Rodney Govinden & Tore Stromme (Cruise Leader).

Most of the scientists and crew (minus the Seychellois participants that preferred to go home for a well deserved rest and local cuisine) went out that evening to paint Victoria red! I'm proud to say we partied like true sailors that have been at sea for too long & some only returned to the ship as the sun came up! The next day was spent exploring the beautiful Island of Mahe. Below are some of the photos taken in between eating crab curry & swimming at near pristine beaches.

The R/V Dr Fridtjof Nansen remained in port for 3 nights to bunker fuel, change ship and scientific crew, take on supplies and officially launch the ASCLME Programme in Seychelles. We conducted tours of the ship for school groups, scientists from local research institutions and local dignitaries. Due to heavy rain the launching ceremony was held at the Convention Centre and it was a great success! Our sincere thanks goes to the UNDP office in Seychelles for organising the event and also to the local Seychellois scientists (Michelle, Helena, Vincent and Rodney) for making the whole day a success!

Text and photos by Tommy Bornman

Interesting results!

2008-11-25T09:12:00.003+02:00

Having now surveyed the entire Mascarene Plateau we have a better understanding of its influence on the surrounding ocean environment. What have we learnt..

It appears that the SEC is displaced southwards from its mean position of 10°-16°S by the obstruction caused by the shallow bathymetry of the Mascarene Plateau. Upstream of this Plateau the SEC exists as a broad (~650 km in width) shallow (~1000 m in depth) current with speeds averaging 0.30 ms-1. On approaching the Mascarene Plateau the SEC splits into separate cores centered near 18°, 12° and 8°S. Once passed the Plateau it seems likely that these cores continue westwards towards the Madagascar coast at 50°E and there form the North East and South East Madagascar Currents.

Deep channels separating individual banks (see past blog) act as choke points funneling the flow of the SEC from east to west.

Atmospheric circulation as a result of the SE trade wind field results in a gradual shoaling of water masses between 15°-5°S. The thermocline depth changed from 250 m close to Mauritius to just under 30 m on the Seychelles Bank. Since nutrients increase with depth, it would be expected that nutrient levels would gradually increase with distance north thus influencing the biological productivity of the surrounding region. This probably explains improved fish catches as we moved closer to the Seychelles!

Finally, the presence of an eastward flow between 6° - 2°S (blue box) can be related to the position of the eastward flowing South Equatorial Counter Current. This current lies north of the SEC and flows in the opposite direction. This proved to be extremely interesting with salty warm water being swept into the Seychelles region from as far away as the Arabian sea!

Image captions:

Top Image: General circulation over the Mascarene Plateau (The 1000 m isobaths is shown in red). Note the influence of the deeper channels speeding up the surface flow downstream of the plateau (highlighted by the green boxes)– exactly what we had hoped to find! The blue box shows the prevailing eastward flow associated with the South Equatorial Counter Current.

Bottom Inage: After 55 days this is what the drifter data looks like! See how they all race westwards through the main gaps with the South Equatorial Current. The really interesting pattern is actually just south of the seychelles where two drifters seem to be caught up in the eastward flowing South Equatorial Counter Current.

Text by Isabelle Ansorge

Copepods run the world!

2008-11-22T21:47:00.003+02:00

Between trawling for new fish species, photographing seabirds and whale watching, it’s easy to forget that some of the oceans most interesting creatures are only seen clearly through the lens of a microscope. In fact much of the oceans diversity can be found within the group of animals we refer to as zooplankton. Copepods, krill, amphipods, tiny fish larvae, baby decapods, polychaete worms and pelagic snails are only a few of the organisms that we have pulled from the water column by use of a bongo net, and of all these animals the copepod stands supreme. The copepod (see photos below) is a small crustacean with a tear drop shaped body, long antennae and an armoured exoskeleton and although there are a number of carnivorous species, most copepods are predominantly herbivorous. In terms of biomass, copepods are rivaled only by Antarctic krill (Euphausia superba) and contribute an enormous amount of protein to oceanic food webs. Herbivorous copepods form one of the most important trophic links in the oceans, because they graze on minute algae and in turn are fed on by larger zooplankton and fish, making the energy found in phytoplankton (which is fueled by nutrient inputs and the sun) available to rest of the oceanic food web. Without copepods, we would have no whales, dolphins, seals or fish and so, in essence, copepods run the world.

Images: Left: Pontella sp. Middle: Euchaeta sp. Right: Sapphirina sp.

Text & photos by Jackie Hill

Size does matter!

2008-11-11T10:12:00.003+02:00

Indeed it does, and whoever says otherwise is no fisherman. After 31 days of zigzagging across the Mascarene Plateau, we finally made it onto the Seychelles Bank, the northernmost bank of this crescent shaped plateau. It was at 10:57a.m. on Friday 7th November when we cast our first trawl in the crystal clear turquoise waters of the Seychelles. The trawl was hauled 30 minutes later and one could not help but glow with pride once its content were scattered onto the deck. Even from afar one could not have mistaken those reddish coloured fish trashing about. 5 huge red snappers (Lutjanus sebae); the smallest weighing 7.2Kg and the biggest over 12Kg. Enough to please any fisherman. Another interesting component of this catch was an outstanding grouper (Epinephelus sp.), weighing 7.2Kg. The catch consisted of a variety of fishes of all sizes and colours of the rainbow. Compared to previous hauls done on the Nazareth and Saya de Mahla banks, this one won a myriad of attention from everyone onboard both for its size and diversity.

After being out at sea for so long, and seeing those snappers, a well sought delicacy in the Seychelles, we couldn’t help our mouths from watering as we started to discuss the best way to savour those “Bourzwa”, as they are known locally. Having one on the barbecue grill during a sunday afternoon picnic on one of our many sandy white beaches seemed to win most votes.

Today Sunday 9th November has been a memorable day. Firstly, seeing the captain raising our national flag while sailing in our waters was really nostalgic! Furthermore, seeing the inner islands on the horizon welcoming their children home were surely those moments that really make you appreciate where you come from. We stand tall today as we are fortunate to be part of this scientific team carrying out the first exhaustive research expedition of the Mascarene Plateau. Although the practical side of the research is almost over, the task has just begun as we move to the data analysis phase and get those pens onto papers to prepare numerous reports.
It is a long and tiring cruise but so far it has been a great working and learning adventure.

Rodney Govinden and Vincent Lucas (Fisheries Scientists, Seychelles Fishing Authority)

Helena Francourt and Michelle Etienne (Research Officers, SCMRT- MPA)

SWIM DEEP-SEE BIG

2008-11-10T20:23:00.004+02:00

From marine scientist to oceanography
What a transition!!!!! I work in the Marine Science Division at the Albion Fisheries Research Centre viz. marine chemistry and bacteriology laboratory (coastal water pollution) and monitoring of corals, coral reef fishes, and marine invertebrates, study on seagrass ecosystem, and other associated research work around the beautiful island of Mauritius. Then came a moment of transition when I was selected to attend a training course on Oceanography at the University of Cape Town, July 2008, consequently I ended up on R/V Nansen for a 45 days research cruise, October 2008 onwards.

Never had I thought of being on such a long cruise but believe me, it becomes really very interesting after a few days of adjustment (sea-sickness, that what’s make the difference between being at sea from land) and which is inevitable. Never mind, I am constantly learning from each and every person here on-board.

What is it like to be on ASCLME, R/V Nansen Research Cruise 2008?
After having obtained an intensive one month (July 2008) training course on Oceanography at the University of Cape Town by professionals and having been exposed to various oceanographers and scientists, being on R/V Nansen have been always pleasant. Learning has been the engine for motivation and below is some of my personal interpretations of major observations as an amateur oceanographer:

What is oceanography?
It is the study of the ocean or part of the ocean by relating the physics, chemistry and the biology to understand the inter-connected dynamics and sustainability of life. In general it is the connectivity between the earth, ocean and atmosphere.

What you need to be an oceanographer?
Physically fit, good observer, good knowledge of science, computer literate, dedication to work non-stop, eager to learn, above all love for the ocean, and the ability to spend weeks or months on board research vessels, at times, face harsh weather conditions, are the main characteristics that one need to have to be a committed and successful oceanographer.

W hat should you expect on-board a research vessel?
Be ready to meet crew members, technicians, scientists and oceanographers from different countries, with different accent and different sense of humour, however all dedicated professionals are united with only one goal which is to make the cruise a successful one and achieve the scientific objectives.

What would you see in an oceanographic research cruise?
Beside the blue sky, the burning sun and the blue sea, you will always find computers in every corner, paper filled with numbers, maps and plots, colorful graphical interpretations, bathymetric maps, filtration apparatus (phytoplankton, zooplankton, meso-zooplankton), chemicals, formalin, acetone, fish specimens in most part of the working places. Also ODV-based graphical interpretations stuck on the laboratory walls together with altimetry data (ocean current patterns) make the laboratory much more colorful.
Vikash Munbodhe (AFRC)

Importance of computing Knowledge in Oceanography
In this world of Information Technology, there exists no science without the use of computer, software and programming. Hours I spend in the acoustic to retrieve information from satellite data, data collected from the echo sounder and current pattern from the Acoustic Doppler Current Profiler from the vessel mounted equipment. All these have only been achieved from the proper training I received on-board the Nansen from professional programmers.

I became fascinated by the real-time data which are amazing when just after playing with the keyboards, one can get so much information in form of colorful graphical interpretations, depth profiling and current patterns. Acoustics is important in ocean studies as major changes in the current pattern, temperature and real time bathymetric data are obtained with the proper interpretation of the data obtained from satellite and other sources and interestingly I am able to do it now.

In short these are the activities that are taking place on ASCLME Nansen Research Cruise: Acoustic, CTD, Bongo, Multinet and trawling are the core activities for the research cruise. Acoustic is mainly related to the mapping of seabed using echo sounder, taking measure of the ocean current by the use of Vessel-mounted Acoustic Doppler Current Profiler and data collection from satellite drifters, deployment of CTD at particular station.

CTD is an instrument which is deployed at a specific point to a certain depth whereby the sensors collect data on physical characteristics such as conductivity, temperature, dissolved oxygen, water density, and others. The CTD also has 12 Niskin bottles which are triggered from the Acoustic Centre to collect water samples for biological and chemical analyses.

Vimal Ramchandur (MOI)

Hey no need to get panicked!!!!!!!!!!!!
You also get good delicious food and cakes (and a gym to get rid of it again), movie time, time to socialize, BBQs as well as internet connection, so guys we are close to you and permanently updated. You will constantly be in contact with whales, sharks and dolphins and with lovely colorful fishes following demersal trawls, however at times you can also see very scary and bizarre ones.

So don’t be a loser, join us, see life differently, ASCLME & R/V Dr Fridtjof Nansen, Exploring Mascarene Plateau, Indian Ocean 2008.

OOPS!!!!!!!!!!!!!!! Hey HAPPY BIRTHDAY VIMAL, my country mate is getting one year older today

Vikash & Vimal
Mauritius

New fish species

2008-11-05T08:22:00.005+02:00

Since writing my earlier account of the first few days of trawling we have been very busy sorting and identifying catches from the 24 trawls carried out to date. We are now reaching the stage where, although every trawl has brought up at least one extra species for the collection, we are becoming very familiar with the fauna as a whole. I am therefore able to spend more time on collecting, pinning out fins, and preserving fishes to obtain good representative samples of a size range of each species. I have been in regular email contact with Phil and Elaine Heemstra, the editors of the forthcoming Fishes of the Western Indian Ocean books, and delight in presenting them with photos of fish that are proving difficult to identify. They, in turn, are sending these photos to their various expert contributors to get their opinions. It certainly looks like there will be several new species to be described, and here are just a few of the more interesting fishes we are struggling with.

Nemipterus sp., resembles Nemipterus bipunctata which occurs off Mozambique, but that species lacks the distinctive yellow facial markings of this fish. These markings are reported as important distinguishing characters of this genus in the Indonesian/Australian seas where numerous species occur.

A species caught in one trawl only, does not resemble any known species from the Indian Ocean.

A large (50 cm+) snapper, Lethrinus sp., with very distinctive lips. We caught two in one trawl. The species does not appear in the FAO guide to the snappers of the world and the photo has been sent to the expert on these fishes for comment.

A stingray with completely smooth skin and uniform colouration (markings on back are trawl damage). These features do not fit any known Dasyatis species.

A large, 40 cm+, sparid bream provisionally assigned to the genus Dentex. We caught three of these from two trawl stations.

This very delicately coloured flatfish belongs to the genus Parabothus, but will have to be sent to an expert to determine its true identity. There is a strong possibility that it is a new species. These are the only two specimens collected.

In addition to apparently new species, we have lots of interesting species to sort later in the laboratory back in Grahamstown. In pelagic trawls near the surface we catch lots of small juvenile fishes that are difficult to link with adult stages. The remarkable photo is of a completely transparent flatfish caught in a trawl at the surface, where the total depth was 2800 m. Flatfishes are bottom-dwelling species once they have metamorphosed into the adult flatfish form with both eyes on one side of the head, but here we caught several of these fishes completely metamorphosed into adult forms and still living a pelagic life at lengths of 3-4 cm.

The snake blenny, Xipharias stelifer, is a remarkable fish that lives in burrows in the sand. We caught four at one site.

These are just a few examples of the biodiversity in this part of the Indian Ocean. We haven’t counted the number of fishes collected on this survey yet but the total is now well in excess of 150 species.

By Denis Tweddle & Oddgeir Alvheim

Suntan? What suntan?

2008-11-04T11:15:00.002+02:00

Pas de cétacés en vue…..

2008-11-04T07:20:00.002+02:00

Après plus de trois semaines de croisière, les cétacés se font malheureusement toujours désirer. En effet, ont pu être observés depuis le depart de l’Ile Maurice, uniquement: un groupe de baleines à bosse (Megaptera novaeangliae); deux baleines, reperées par leur souffle mais malheureusement trop éloignées pour pouvoir être identifiées; et un groupe de grand dauphins (Tursiops truncatus) venus jouer à l’étrave du bateau. Il semblerai que les zones prospectées ne fassent pas partie de leurs zones de prédilections. Les conditions météorologiques ne facilitent pas non plus les observations. La mer est souvent agitée et il pleut régulierement, ce qui limite considérablement la visibilité.

Still no cetaceans in view….
After more than three weeks of cruising, we are still hoping to find cetaceans. Since leaving Mauritius, we have observed only a group of humpback whales (Megaptera novaeangliae); two baleen whales (unfortunately too far away to be identified) and a group of bottlenose dolphins (Tursiops truncatus), which came to ride the bow waves. It seems that the areas under investigation are not some of their favourite. Weather conditions also don’t help for observations; the sea is often rough and it is regularly raining, limiting the visibility.

Text by: Morgane Perri

Photo: Tommy Bornman

Small is beautiful (well ... sometimes)

2008-10-31T12:20:00.008+02:00

While to date, much of the cruise blog has dealt with all things BIG, lets not forget about the small creatures of the ocean. Because of their small size, it is easy to forget about the zooplankton. Yet, in the oceans, it is these small animals that convert photosynthetic biomass into the animal biomass that ultimately feeds and sustains all of the larger predators. Without the zooplankton, most of the larger fish, whales and sea-birds could not survive!

Below are shown some of the strange and alien-looking creatures that are being picked up in our samples. (left: amphipod releasing egg; right: copepod; bottom left: ctenophore; bottom center: pelagic polycheate; bottom right: salp).

At the moment, zooplankton samples are being collected both by multi-nets (which can sample at different depths) and by Bongo (which collects larger amounts of zooplankton, but from the whole water column). Samples are being used to determine community composition, the geographical and depth distribution of different species as well as for stable isotope studies that investigate the importance of different zooplankton taxa as a food source for fish and squid.

In addition to being an important food source, zooplankton also harbor the juvenile stages of many commercially and ecologically important fish species. These are called the ichthyoplankton. During the life-cycle of most fish, it is the early juvenile stages that are most at risk from their environment. Due to their small size, the ichthyoplankton are at the mercy of the currents and individual larvae cannot protect themselves against predators. This is why the majority of fish die when they are very young (often >99%). It is frequently the success or failure of this planktonic life-stage that determines the potential success of adult fish stocks.

(left: fish larvae; center: fish larva; right: cephalopod larva)

text and images by: Sven Kaehler

If its Monday .. we must be heading east ...again!!

2008-10-27T16:08:00.005+02:00

So what are we doing and where are we?

We are slowly moving northwards towards the gap between the Nazareth and the Saya de Malha Bank (see a previous blog). The last 2 weeks have seen us sailing back and forward and forward and back along acoustic lines, which have been set up to study the fish stocks over the plateau as well as improve our knowledge of the seafloor in this region. It’s pretty amazing to see the seafloor rise from 3000 m to 50 m in about 8 miles! Think of climbing Table Mountain from Cape Town centre and then triple the distance up!!.

As we move northwards we have been measuring sea surface temperatures and you can see from the map on the right that there is a gradual increase from 24 to 26° C with distance north. Dotted along the plateau are sampling stations where we collect bottles of water for oxygen, salinity, nutrient, chlorophyll and zooplankton concentrations from top to bottom...in some places this takes minutes because the seafloor is only 30 m deep!.

..and last week we started receiving data from the drifters thrown in at the start of the cruise. The 2 southern drifters certainly seem to be in pole position! travelling over 50 cm per second westwards we suspect that they have been caught up in a strong current channelled by the gap to the north of Mauritius.

The other 2 drifters on the east side of the plateau were only deployed at the end of last week and we think that they will move south rather than over the plateau.. and that’s an interesting fact because it suggests that the Mascarene Plateau acts as a huge barrier to all surrounding flow. This certainly raises questions on whether the ecosystem varies between the deep ocean and the plateau and how water masses may differ from one region to another. ..hopefully by next week we will have some more data to prove or disprove various theories.. .. so keep posted and we’ll soon update the blog on our progress north!

Cures from the Deep: The Search for New Pharmaceuticals from Deep Water Western Indian Ocean Marine Sponges.

2008-10-25T18:18:00.006+02:00

The search for new chemical entities with exploitable medicinal properties is the cornerstone of modern drug discovery. Traditionally, these new chemical entities have been procured from several sources including the vast natural product reservoirs characteristic of terrestrial and marine ecosystems. Many marine organisms (invertebrates e.g. sponges; algae and micro-organisms) produce natural products (biomolecules) as chemical defence agents against predation or in a chemically mediated response to, inter alia, inter-species competition for limited resources (e.g. space on a reef or nutrients) and intra-species communication (e.g. larval settling cues). Surprisingly, many of these marine natural products also possess medicinal properties and internationally several marine derived chemical compounds are currently in development as new anti-cancer and anti-inflammatory drugs.

The coastline of southern Africa, and the deeper waters off its shores, sustains a unique diversity of endemic marine fauna and flora that can offer rich rewards for marine natural products chemists in search of novel bioactive marine natural products with medicinal properties. For the last fifteen years Professor Mike Davies-Coleman’s marine natural products research group at Rhodes University has been systematically searching for new marine natural products, with medicinal potential, from the marine invertebrates occurring off the coast of South Africa, Mozambique and Marion Island in the Southern Ocean. While the southern African inshore benthic communities are reasonably accessible with the aid of SCUBA, accessing offshore deepwater invertebrate communities is logistically problematic, and the marine sponge material collected as part of the Macarene Leg of the ASCLME cruise will be crucial in enhancing our knowledge of Western Indian Ocean marine biomolecular diversity.

The Rhodes University research group is part of an international collaboration working together to discover possible new marine medicines and has strong research links to the National Cancer Institute in the USA and marine natural product research groups in the USA, Canada, Australia and New Zealand. Marine sponge material collected during this ASCLME cruise will be screened for natural products that are able to either kill oesophageal cancer cells or activate enzymes that halt the onset of arthritis and osteoporosis. The Eastern Cape Province of South Africa has the highest reported incidence of oesophageal and cervical cancer in South Africa and the identification of marine natural products exhibiting cytotoxicity towards the former cancer is an important component of marine drug discovery efforts at Rhodes University.

Text by Professor Mike Davies-Coleman

Images by Kim Bernard

Tropical storm approaching

2008-10-18T11:40:00.005+02:00

A tropical depression has been evolving in the vicinity of Diego Garcia over the past week. The non-frontal synoptic scale low-pressure system definitely has some indications of cyclonic wind circulation as can be seen in the adjacent synoptic chart and satellite image courtesy of the Mauritius Meteorological Service. Winds circulate clockwise around low-pressure and cyclonic systems in the southern hemisphere. Gusts associated with tropical depressions are generally less than 90 km per hour whereas in a true tropical cyclone winds can range from 150 to over 300 km per hour. The red cross in the satellite image indicates our approximate position and we expect to receive the full force of the storm tomorrow evening followed by two days of strong winds and heavy showers. We will spend some time tomorrow battening down the hatches and ensuring that all the equipment in the labs is securely fastened! If physically possible I will add some photos of the cyclone and the Dr Fridtjof Nansen weathering the storm!

Barbeque on the high seas

2008-10-17T10:32:00.008+02:00

From 14-15 October we were sampling stations in the vicinity of the Cargados Carajos Shoals (see adjacent Google Earth Image). The shoals consist of 16 small islands (and some 34 cays) and are classified as a dependency of Mauritius. The shoals used to be a volcanic island that over time eroded until it became submerged and a coral atoll was left behind. The length of the Mascarene Plateau cruise and the lack of any port calls over the five week period will place strain on both scientists and crew. It was therefore decided at the start of the cruise that we will have two barbeque (braai) parties at locations where we could safely anchor the ship to rest and recuperate. The Cargados Carajos Shoals presented just such an opportunity and it was decided, after completing the last station, to anchor off Coco Island (île aux Cocos at 16°50'S, 59°30'E).

We anchored in 30 m of water and immediately started the fires. In the bottom left image Tom and Bjorn are busy grilling T-bone steaks, fillet steaks, spareribs & chicken. Supper was an "al fresco" affair on the trawl deck. The food was excellent and the evening gave everyone a chance to mingle and get to know one another better!. The night ended with some of the scientists and crew strutting their stuff to the sounds of the Norwegian Oompapa band followed by rhthymic dancing and ethnic knee jerking to the sounds of the Swahili Melaika Marumba band. For those not keen to shake their stuff a fishing group had assembled on the back deck hoping to catch more samples.

Demersal Trawling

2008-10-16T06:52:00.012+02:00

The demersal trawling programme is producing some very colourful fishes of fascinating variety. We caught 38 species in our first half-hour trawl and the total is now up to nearly 130 species. Five of our bottom trawls so far have been in about 20-30 m depth and the species found at this depth are now becoming very familiar to us, so sorting is becoming quicker and easier. I was able to name all of the fish in this morning’s sample in a very short time. Smith’s Sea Fishes is proving an excellent guide to identify these fishes even though we are well away from the sea area covered by this famous book. This illustrates the role of the Agulhas current in spreading these species down the south east African coast.

Before I left Grahamstown, Eric Anderson, SAIAB’s expert on deeper water fishes, told me that the deeper waters around the Mascarene Plateau were very poorly explored and that many species caught in waters around 300 m deep are likely to be new. I’m not sure how true that comment will prove to be, but I’m certainly having difficulty identifying some apparently common large species from deeper water trawls in 200 m and 300 m. I’m still searching the literature and thus won’t put pictures of these on the web yet to ask for help, but may do so before the end of the cruise.

Among the more interesting species landed was a black snoek, Thyrsitoides marleyi, not far short of 2 m long, held up in the photo by Jackie Hill, and also a sawshark, Pliotrema warreni, whose toothy snout became so entangled in the netting that it had to be cut free. Other common fishes are a variety of trigger fishes, fusiliers, butterfly fishes and other tropical reef fishes. The opening of the codend at the end of each trawl continues to be eagerly anticipated, to see what new species spill out on to the deck

Image captions (top to bottom): A collection of fishes from the trawl awaiting sampling of muscle tissue for genetic analysis; Balistoides conspicillum; Lactoria fornasini; Apolemichthys trimaculatus; Sargocentron spiniferum; A very colourful mantis shrimp trawled from 50 m depth; (bottom left): Jackie holding a large black snoek trawled at 300 m depth; and (bottom middle): A sawshark, Pliotrema warreni, caught at 200 m depth.

Text and images by Denis Tweddle & Oddgeir Alvheim

What are we doing here?

2008-10-15T12:59:00.003+02:00

A little bit about the Mascarene Plateau

The Mascarene Plateau, in the Southwestern Indian Ocean, is about 2200 km in
length, running from the Seychelles Bank at 4°S to the island of Mauritius at
20°S. As you can see on the left its oriented roughly north–south similar to a crescent. It is characterised by a series of islands, banks and shoals separated by deeper ridges and channels. The main banks are called the Seychelles Plateau, the Saya de Malha Bank, the Nazareth Bank and the Cargados-Carajos Bank. These are typically 20-100 m deep, coral topped and sometimes break the surface to form small islands – in fact we will anchor off Coco island later today! The Plateau is surrounded by steeply descending slopes so that water depths rapidly increase to 3000-4000 m on either side of the plateau - as you can see from the figure below. The nature of this long fractured plateau is thought to form a barrier to the surrounding ocean circulation. Some of the shoals are more than 250 km wide. The Mascarene Plateau is a rare example of an extensive shallow-shelf sea completely detached from land boundaries and is, except for the Shoals of Capricorn Marine Programme and Darwin Initiative in 2000 – 2001, a largely unexplored marine ecosystem.

The red line in the picture above relates to the bathymetry figure on your left which shows the steep rise associated with the Mascarene Plateau. We think that the flow moves up onto the plateau in the east bringing with it (up and over the plateau) an increase in nutrients.

So what do we hope to see?

The most northerly part of the study area is under the influence of the northeast
monsoon from December to February, but during the remainder of the year the whole area is under the influence of the Southeast Trades and the South Equatorial Current (SEC). Recent studies in this area suggest that the shallow Mascarene Plateau acts as a barrier to the predominantly westward zonal flow of the SEC causing it to split into a number of tributaries, which then become channelled through the deep gaps separating the shoals. This creates some interesting questions as to the effect this channeling has on the plateau's ecosystem.

The figure left shows the position of the banks and the velocity of the surface circulation. What we can see really clearly here is how these deep gaps influence the surface circulation as the flow associated with the South Equatorial Current is channelled through each gap...

Key Questions and a cruise track!

1. What is the influence of the South Equatorial Current on the waters and ecosystem over the Mascarene Plateau?

2. In what way is the flow of the South Equatorial Current affected by the gaps in the Mascarene Plateau?

3. Is the Mascarene Plateau characterised by an increased diversity in habitats and biota?

4. What are the main components in the Mascarene Plateau pelagic ecosystem, its distribution and abundance?

5. What are the biodiversity of the pelagic ecosystem, and the main fauna of the demersal fish community?

6. Can the Mascarene Plateau be considered a Large Marine Ecosystem on its own?

....and that’s what we are doing here.. to answer the questions we are deploying a network of CTD stations, multinet stations, bongos and demersal trawls similar to what you see in the track above....

Keep posted for some answers!

posted by Isabelle Ansorge - here at 59°S, 16°30' E

Drifting around in the Indian Ocean

2008-10-13T11:01:00.004+02:00

Late last week we deployed two drifters at 18.20S, 59.01E. The drifters were deployed in the deep channel north of Mauritius. A number of oceanographers have suggested that a strong current flows from east to west through this channel. Looking at a schematic of the general circulation of this region, you can see that the current flowing westwards is in fact a major tributary of the South Equatorial Current (SEC), which eventually on its travels passes to the north of Madagascar.

This survey is providing us with a wealth of hydrographic data it’s only a snap shot and what happens downstream of the survey or 2 months after the survey has been completed is not known. So drifters give us that missing information.

Once deployed they drift with the surface current collecting data such as temperature, current speed and direction, air temperature, air pressure and relay the information back to satellite.

History

For many years, ocean currents have been estimated by how they carry objects. For example, sailors measured the speed of their ship through the water using the ship log. They measured their absolute position by celestial navigation (in the good old days, pre-GPS!). The difference between the absolute speed and the speed through the water gave the speed of the currents. Very strong currents, such as the Agulhas Current made a big difference in how long it takes to travel. More recently, researchers began tracking objects while they were drifting. This tracking was first done visually either from a coastline or anchored ship, then using radio, and most recently using satellites. During the 1970s, when satellite tracking became possible, many competing drifter designs were proposed, built and deployed in various studies around the world.

How does a drifter work?
The modern drifter is a high-tech version of the "message in a bottle". It consists of a surface buoy and a subsurface drogue attached by a long, thin tether.

The buoy measures temperature and other properties, and has a transmitter to send the data to passing satellites. The drogue dominates the total area of the instrument and extends 20 m below the sea surface. Here is a picture of a drifter in a cold clockwise eddy in the southern ocean. You can see how the drifter rotates round and round in the eddy!!

Drifter Design
Drifters basically have a surface float and attached to that a long holey sock known as a drogue. The surface float is on average 40 cm in diameter. It contains: batteries; a transmitter, which relays all the information on a 6-hourly basis to satellite; a thermistor to measure sea surface temperature; a barometer to measure atmospheric pressure, wind speed and direction, as well as a GPS to measure position and the time taken for the drifter to move from one position to another – this way you can measure the surface current speed and direction.

The drogue is centered at 15 meters beneath the surface to “anchor” it to the surface – this prevents strong winds from pushing the drifter around. The outer surface of the drogue is made of nylon cloth. Throughout the drogue, rigid rings support the drogue's cylindrical shape. The drogue is a "holey-sock": each drogue section contains two opposing holes, which allow water to flush through and prevent the drogue from getting “wound up” in strong surface currents.

Once deployed, a drifter lives an average of 400 days before ceasing transmission. Occasionally, drifters are picked up by fishermen or lose their drogue and run aground.

Deploying a drifter
Drifters weigh about 25 kg each. Before deployment, the drogue and tether are bound with paper tape which dissolves in the water, and the tether is sometimes wrapped around a water-soluble cardboard tube to protect it from kinking.

The drifter is deployed by throwing it from the stern of a vessel, preferably from the lowest deck and within 10 m of the sea surface. After deployment, it may take up to an hour for the paper tape to dissolve and trapped air bubbles to be released, so that the drogue sinks to the target depth (15 m).

How much do they cost
Drifters range in price depending on the number of sensors you have. On average you can expect t pay about $2500 per drifter.

Keep posted for more cruise stories!

Does my bum look BIG in this?

by Isabelle Ansorge