Catching snow in the sea

Ocean research cruise blog of Jonathan Sharples

 

The last day on this station began with another 0500 early CTD, so that those scientists working on how fast the plankton are growing can start another set of experiments. During the afternoon we released another glider. This one has a special chemical sensor on it that has been designed at the National Oceanography Centre. It measures the amoung of nitrate in the water, a key nutrient required by the plankton. As with the glider yesterday, we are leaving this one in the water just while we are at sea; we aim to retrieve it just before we head back to Southampton in early December.

glider 2 deployed

We also had a go at using our “Marine Snow Catcher”. This large tube is designed to trap 400 litres of water at one depth. The tube is then brought back on deck, and all of the tiny particles in the water (plankton, bits of detritus)are allowed to settle in the tube. After 2.5 hours the scientists collect particles from near the top of the tube (which will be very tiny and will not have settled far), the middle of the tube and the bottom (containing the coarsest particles which settled quickly). We want to see how the organic matter in these different particles is being recycled by bacteria in the ocean; particularly we want to know if the bacteria recycle nutrients, such as nitrogen and phosphorus, more quickly than they recycle carbon.

snowcatcher

 
Our communications are still suffering. It looks like we may be down to a limited email connection for the rest of the trip, with the problem with the main system having been narrowed down to a component that we don’t have a spare of.

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Glider away….

Ocean research cruise blog of Jonathan Sharples

 

Two new pieces of equipment deployed yesterday. First, the Ocean Microstructure Glider (OMG). A glider does exactly what the name suggests – it glides through the sea. By making itself heavier than the water, and tilting its nose downward, it glides downwards. Then, when it gets to the depth at which it has been instructed to turn round, it makes itself lighter than the water, points the nose up and glides towards the surface. Inside a glider are instruments similar to those on the CTD – measuring water temperature, salt and plankton. The OMG also has some specialised instruments for measuring the amount of turbulence in the water. That’s what the “microstructure” part of the name refers to – the sensors measure tiny changes in water currents associated with turbulence. We are really interested in turbulence, as it mixes nutrients, plankton and carbon through the water. The really neat thing about gliders is that when they surface they can stick their tail end out of the water and communicate back to shore via a satellite link, transmitting data back and also receiving new instructions. Our gliders are not controlled by us on the ship, but by scientists back at the National Oceanography Centre in Southampton and in Liverpool.

wirewalker deployment

 Immediately the glider was away, we moved the ship clear and deployed a “wirewalker” mooring. This again has instruments for measuring temperature, salt and plankton, but it moves up and down a wire fixed to an anchor on the seabed and a buoy at the sea surface. The action of the waves on the buoy provides the energy that the wirewalker needs to ratchet itself down the wire (so, a note to my nephew Ben there – yes we do now have things that use the waves’ energy to power them! Your idea was spot on); it then releases its grip on the wire and floats back up to the surface. With decent waves (of which we’ve been having plenty) the wirewalker can profile up and down the cable every 15 minutes or so. Jo Hopkins for the National Oceanography Centre in Liverpool is running this instrument – she is keen to capture the details of how the water is mixing as the weather cools into winter.

omg glider deployment2

 
We’ve lost a lot of our communications at the moment – certainly internet and phones are out. Zoltan, the NMF computer tech, is working through all possible causes and he’ll be calling on the ships ELT tech as well. Hopefully we’ll be fixed soon. We still have access to the National Marine Facilities Webmail though, so I can get these posts through OK.

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omg glider off

Sampling iron

Fantastic weather today. Winds 5 – 10 knots, and we have lots of blue sky. Other than a long, 2 metre swell you’d have to describe the sea as calm.
We have a group on board, led by Maeve Lohan from the University of Plymouth, who are going to measure the amount of iron in the sea. Iron is a nutrient that the microbial plants in the ocean need. It occurs in the ocean in very small concentrations, and so is a real challenge to measure. Much of the challenge is because ocean scientists need to make the measurements from steel ships, so there is huge potential for contaminating the samples with iron from the ship or our equipment.

The instruments used by the iron scientists are all made from titanium or plastic, and they do all of their work is a special “clean lab” on the ship, into which the rest of us are forbidden to go. When their titanium instruments come on board after collecting water samples, Maeve and her colleagues rush forward with a bag of polythene gloves which are all put onto the taps of the sample bottles so that they don’t get contaminated while on deck.
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Salp soup

The weather deteriorated a bit after the first CTD cast. Wind reached about 40 knots, so we had to stop working as the motion of the ship was putting too much stress on some of the equipment hanging over the side. However, things have calmed down nicely for the afternoon. A group of 6 or so dolphins has been hanging around the ship, probably feeding on the fish that often congregate underneath us if we stay in one place for a while.

zooplankton net

Sari Giering (University of Aberdeen) has started her sampling of the zooplankton – the tiny animals that feed on the microbial plants (and on each other). There is general surprise that there is so much biological stuff in the water, given the time of year. The zooplankton net, which is hauled vertically upward through the water to catch any zooplankton on a mesh at the end of the net, has come back with all sorts of stuff. The latest haul had a large colonial salp – a gelatinous filer feeder about the size of your finger, but that lives in long connected ribbons of several dozen clones.
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bucket full of sulps

The first data

Right on schedule, at 0500 our main set of instruments hit the water to collect the first data. This package of instruments (the "CTD") is made up of several sensors that measure water temperature, saltiness, oxygen concentration, and also how many of the microscopic ocean plants there are. The long grey tubes around the outside are used to trap water (20 litres at a time) from depths where the scientists want samples for their experiments.

CTD into sea

The first data is exactly what I was hoping for. Throughout the summer this part of the sea would have had a warm, sunlit layer above deeper, colder water. As autumn and winter approach this surface layer cools and starts to get thicker, until eventually the whole of the water from the surface to the seabed (140 metres deep here) reaches the same temperature. The red lines in the left panel of the computer screen show the temperature. It’s about 14 deg C in the upper 40 metres, then drops to 12 deg C in the deeper water. In summer it would have been about 18 deg C in the upper 30 metres, and 11 deg C below. So, we’ve got here just in time to see the change in conditions towards winter.
These early morning "CTD casts" are sampled a lot by the scientists. There was a big meeting last night to discuss who required how much water from what depths. Also the order in which the samples are taken is really important (samples for dissolved gases need to be taken first, while samples for salt can wait until the end).

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orderly queue

Leaving Falmouth

Bang on time we left Falmouth Docks at 0830. The ship is so quiet many of us didn’t realise we’d started moving. We dropped the pilot off once clear of the docks. Ship pilots work for the port, rather than they ship. They know the sea in and around the port very well, so ships use them to guide into and out of the docks. Once we were clear of Falmouth, a fast boat came alongside us to pick up the pilot and take him back to shore.
The weather is remarkably sunny and calm. In fact we have some spare time as our first planned work is to start at 0500 tomorrow, and it’s about 15 hours to get to the work site. So, the ship’s crew have been testing one of the lifeboats – making sure that the davits (or cranes) that are used to lower it into the water work, and that the lifeboat’s engine is fine.
 

lifeboat recovery

Preparation of the laboratories continues, with the lab space gradually becoming clearer. Note in the picture below the typical fashion of the scientist at sea: lab coat, hard hat, lab groves, and also stell-toecapped boots. This is Matthew Bone (from the University of East Anglia); he will be working on how nutrients are released from the seabed at the start of winter.

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Matthew Bone

Everyone aboard

Everyone has made it aboard now. We had our safety briefing this afternoon, learning about where to go if there were to be an emergency, how to operate the watertight doors, as well as other practical information such as where the laundry is and what time meals are served.

Snowcatcher Discussion

The scientists have continued getting the gear ready, and then all strapped down so that once we get to sea things don’t start rolling around the deck. We have two enormous "marine snow catchers" on the aft deck. These are used to capture 400 litres of water from key depths, which is then brought back onto the ship and sampled to see what particles are in it. For instance tiny animals (zooplankton), or bits of sediment from the seabed, or – very importanly – bits of zooplankton poo. Particles in the ocean sink, taking with them lots of carbon which ultimately was removed from the atmosphere. It’s what happens to these particles, and the carbon that they carry, that forms the basis of a large component of our work. One of the mooring components was also completed and strapped down, ready to take out into the middle of the Celtic Sea and dropped onto the seabed. This seabed lander has two devices for measuring the water currents using pulses of sound. It will sit on the seabed measuring currents until March next year.

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