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25 May- Warming the old bones

RRS James Clark Ross Diary

Noon Position: 16° 35.7 S, 24° 59.7 W)
Distance Travelled since Grimsby: 36379.5 Nautical Miles
Air temperature @ Noon today: 26.1°C
Sea temperature @ Noon today : 26.5°C
Weather: Good, ENE, 4, 1013.7

Warming the old bones

Finally we are heading directly north and making good progress evidenced by the daily reduction in our latitude.  We are now officially in the tropics and so are enjoying fine weather every day.  The south east trades are a cool breeze to take the edge off the unrelenting sun and make working outside very pleasant.  Naked flesh is starting to be seen on the monkey island as the die hard sun worshippers make the most of the rays and even the odd bikini has been seen!

Going home - Click to enlarge Rebooting the system? - Click to enlarge

Above: The chart (left) and our IT expert reboots his system? Click the images to enlarge them.

Science is in full swing and we are stopping every day at 4am and 11am to take measurements from the water around us before sedately cruising north at 11 knots for the rest of the day (not fast enough for me - Ed).  Life is in a routine now and all the scientists are working very hard.  We are currently in a sub-tropic gyre which is the equivalent of an oceanic desert and so we are seeing little life around the ship.  There are no birds and only the odd flying fish to be seen but otherwise we are totally by ourselves in a very big ocean.  We said goodbye this week to all the Albatross that have been following the ship for the last 8 months. A highlight for the week was seeing another ship on the horizon one morning!  The other highlight was changing course.  This transect is so long and straight that we only change course about once a week!  It's tough for the lads on the bridge!  Below Mike and Anna take a well earned break from all the science. Click the image to enlarge it.

Anna and Mike - Click to enlarge

Everyone is getting excited about crossing the equator next week and we are currently looking for evidence of crimes committed by the lily-livered land-lubbers who will be crossing the line for the first time.  To this end a box has been place in the bar and people surreptitiously are slipping into it accusations that may be considered by the scrupulously fair and just Court of King Neptune.  Unfortunately it is the doctor who currently has the highest number of crimes against his name but then I have been onboard the longest!

Evidence please!!! - Click to enlarge
The evidence box
Click to enlarge

AMT 12 - Nutrients

This is the second is a series of articles that attempts to explain what the Atlantic Meridional Transect is all about and this week it is about nutrients.

A nutrient can be defined as any substance that feeds or helps to forward growth of an organism in any way.  So essentially it is all about phytoplankton food.  What they need to live and grow, where it comes from and what effects their feeding has on the surrounding ocean and the atmosphere above.  Phytoplankton can be thought of as tiny plants floating in the water and like the plants in your garden they need food and light to grow.

Phytoplankton like nothing better to feed on a lovely diet of:- NITRATE, NITRITE, SILICATE, PHOSPHATE and AMMONIUM.  Add a little carbon in the form of carbon dioxide and they are more than happy.  These compounds are the similar to those are found in fertilizers for the garden and are essential for growth and replication.  All are dissolved in the sea water either as molecules or small particles.  (Phytoplankton also need iron, but that is a story for another week).

These nutrients are at very, very low concentrations in this part of the ocean.

In the waters around the UK, nitrate is at a concentration of 15 micro moles but here it is at 1-2 nano moles.  That is about 7,000 times less prevalent!  This means that nutrients in this area are the things that control the rate of  growth of the phytoplankton.  Phytoplankton have plenty of light, warmth and carbon to grow but it is the lack of nutrients that slows the growth down and why this area is regarded as an 'oceanic desert'.

Malcolm and Katie are testing the quantity of nutrients in the water below the ship.  They are using 3 very sensitive machines that can detect the extremely low concentrations in the water.  Their liquid waveguide capillary cell colorimetric analyser is unique in the world and can sense 1 molecule of nitrate in 1 million molecules of water!  The high sensitivity is needed to detect the extremely low levels of nutrients in the water.  The water samples taken at different depths are studied to investigate which nutrients are present and where.

Nutrient analysis - Click to enlarge Mike and his toys - Click to enlarge

Above: Analysing nutrients in the lab. Click the images to enlarge them.

Warm water is less dense than cold water and floats above the cold water.

What does that have to do with things?, you ask.  The water we have been merrily sailing through this week is heated by the hot sun so that the surface layer warms up.  As the water is warm (>20°C) it stays on the surface because it is less dense than the colder water below and therefore is warmed further by the sun.  This means that the warm water on the surface does not mix with the water below and forms a 'skin' on the surface of the ocean.  The properties and nutrients of this 80m thick layer are different from the 5000m of water below it.  The layer forms the basis of the 'sub-tropical gyre' which is an area of the ocean above and below the equator where there is little water mixing.  On the chart below they can be seen as areas of blue, corresponding to low cholorphyll, demonstrating the low growth of phytoplankton. Click the image to enlarge it.

Sub tropical gyre - Click to enlarge

To illustrate this a further you'll see below a graph of the water column.  The axis on the left hand side shows the depth of water from the surface down to 300m.  The coloured lines represent from R to L, chlorophyll, oxygen concentration, salinity and temperature of the water.   The temperature trace reveals that there is approx 80m of water at 20°C before the temperature rapidly begins to cool.  This is the level at which there is a sharp division between the 'warm' water floating on the surface and the 'cold' water below.  Next look at the traces for both chlorophyll and oxygen.  These represent roughly the amount of phytoplankton in the water and the quantity of photosynthesis taking place.  These are both maximal at 90-100m.  Importantly most are not in the warm surface water, they are in the cold water below and phytoplankon have learnt to live at the junction between the two.

CTD trace - Click to enlarge

Why should they live in the cold dark water?  At that depth the light is only 0.1% of that at the surface.  Most plants grow towards the light but not these.  The reason is .............NUTRIENTS.  The surface water has few nutrients in it and because it does not mix with the water below these don't get replenished.  However in the cold water below there are nutrients (not vast amounts but more than the water above) and so the phytoplankton compromise on the light and temperature at that depth simply so that they can get their nitrates, ammonia, silicone etc from the cold deep water.

In essence the phytoplankton are forced to live below the layer of warm surface water in order to obtain the vital nutrients from the cold waters below.

Photosynthesis removes carbon dioxide from the environment.

The phytoplankton produce food for themselves using photosynthesis.  This process removes CO2 from the atmosphere and replaces it with oxygen.  One of the main aims of this cruise is to understand how much CO2 is absorbed by the phytoplankton in the ocean.  One of the ways to do this is to remove phytoplankton from the water at different depths and keep them alive on deck at the same temperature and with the same light levels as they would have in the sea.  A small amount of radioactive isotope, carbon called 14C can then be added to the water they are kept in.  Following incubation for 24 hours the phytoplankton can be removed, washed and then tested to see how much radioactive 14C  they have taken up.  If one knows how many phytoplankton live at each of the depths tested, a simple multiplication can work out how much carbon is taken up by phytoplankton over the whole ocean.  This figure can then be used in climate models.  A similar experiment using an isotope of nitrogen (15N) is also going on to determine the uptake of nitrate by the phytoplankton.  The tubes where the phytoplankton are incubated can be seen in the photo of the aft deck can be seen below. Click the image to enlarge it.

Beastie boxes - Click to enlarge

Some dead phytoplankton sink.

The larger phytoplankton sink when they die.  They take all the carbon and nitrogen that they contain with them as they sink down to deeper depths.  They effectively remove both carbon and nitrogen from the surface waters and this is known as export production.  Complicated experiments are taking place onboard to estimate the removal of carbon from the surface layer based on the ratio of thorium to uranium in the water.

It may be possible to have a better idea of the amount of carbon dioxide being taken up by the ocean from the atmosphere and removed by the action of phytoplankton in the water.  Carbon dioxide is an important gas in global temperature control and therefore phytoplankton may play an important role in world temperature regulation.

After all that complicated science you can soothe your brain with a few abstract pictures of the JCR as she enjoys the tropical heat. Click the images to enlarge them.

Paint me! Click to enlarge Please paint me! Click to enlarge RIB at sunset - Click to enlarge
Order? Click to enlarge Geometric order - Click to enlarge Hot ropes - Click to enlarge

Thankyou this week: to all the support from the Robst family.

Coming up next week: a line in the sea.

Alex Ramsden
Ships Doctor