Last of the Snowcatchers

Ocean research cruise blog of Jonathan Sharples

 

The weather eased off very quickly during yesterday, ending up with winds less than 10 knots. We arrived back at the mooring site in the central Celtic Sea and began a last set of sample collection and experiments, mainly focused on the zooplankton and on the particles settling down through the water.

The Marine Snowcatcher worked well. We’re getting better at operating it, though we think that is mainly a result of calmer weather. We are still not completely convinced that the deeper samples collected with the Snowcatcher are always from the depth that we think we have triggered the catcher to shut – if the ship is pitching at all it’s possible for the catcher to shut while it is being lowered through the water to the sample depth. However, we can solve that by collecting nutrient and salt samples from the catcher and comparing those with what we see in the CTD data to tell us the depth that the Snowcatcher sample was really taken.

last snowcatcher

 
Elena Garcia-Martin, from the University of East Anglia, and Darren Clark, from the Plymouth Marine Laboratory, are working on last these samples. They are measuring how the different sizes of particles, and their different components (carbon, nitrogen phosphorus), and being recycled by bacteria. The deep bacteria are acclimatised to darkness, so they have to be collected after sunset, extracted from the Snowcatcher carefully so that they don’t get fried by the ship’s deck lights, and taken into a darkroom laboratory for analysis.

Elena and Clare sampling particles

One last set of measurements to do here this morning, then we head off towards Plymouth. Should arrive just south of the Eddystone lighthouse about 0600 tomorrow.

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Glorious mud

Ocean research cruise blog of Jonathan Sharples

 

We reached the northern-most station by about 7 pm last night. There was great excitement watching the data from the CTD as it was lowered through the water. If any site was going to have reached the fully mixed winter state by now, it was going to be this one. About a dozen of the scientists were crowded around the CTD computer in the main lab, willing the temperature of the water to stay the same as the CTD went lower. But there was a collective groan as a thermocline appeared at 66 metres below the surface. It’s a bit disappointing that we are not going to be out here to see that final transition to the winter mixed water, but I’m pleased that I appear to have generated so much enthusiasm for shelf sea physics amongst the crowd of biogeochemists on board.

box corer

Matthew Bone, from the University of East Anglia, is interested in the muddy seabed at this site. We collected 4 cores from the seabed using a large “box corer”. This is a large steel cylinder that is lowered down onto the seabed, and then pushed into the seabed by the large weights above it. When it is pulled out, a core of the seabed mud is held within the cylinder and brought on board. Matt has been working on measuring how the mud releases nutrients back into the water. This muddy area of seabed, in an area called the Celtic Deep, is an important fishing ground for a scampi that lives on, and burrows into, the mud. At one point last night the radar was showing 12 fishing vessels around us, within a distance of about 10 miles. One of the cores caught a scampi. It seems happy enough in the lab, busily shifting mud around the top of the core and tending a burrow. The plan is to release it later today when we pass over another area where we have in the past seen scampi on the seabed.

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mephrops

A windy morning

Ocean research cruise blog of Jonathan Sharples

 

A bit of weather more typical of November today and last night. We finished over-the-side work at about 7 pm yesterday, with winds of about 40 knots about to make use of the iron-free CTD unfeasible. The wind has dropped a little this morning, 30 knots of so, but the sea and wind are giving us a fairly good list to port as we steam between stations.

windy morning

We had to cancel the work planned for the first site this morning, as a fishing boat close by suddenly decided that the spot we had been sat on all night was exactly where he needed to drag his nets. Once we’d cleared away from where the fishing was, the winch that lowers the iron-free CTD suddenly threw us an error. The ship’s engineers are working on it now, and I decided that we’d lost enough time waiting around that site and should just head up to the next one. Timing is a bit tight today. Ideally we need to get up to our most northerly site by about 6 pm so that we can do some seabed sampling up to about midnight. That should give us time to head back for one last set of measurements back at the mooring site before we start to make our way into the English Channel.

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Heading north

Ocean research cruise blog of Jonathan Sharples

 

Another successful day yesterday, with the wirewalker mooring and both of the gliders recovered very quickly. Jo Hopkins immediately removed all of the instruments from the wirewalker, and strapped them to the CTD ready for the next time we lowered it through the water. This allows Jo to calibrate the wirewalker data with the data collected by the CTD, with the CTD data all calibrated against analysis of samples we collect in the sample bottles. Every profile of data we collect through the water with the CTD involves samples being collected for salt concentration, dissolved oxygen and chlorophyll. These samples are analysed against known, internationally-recognised standards and lab techniques, so that we can calibrate the sensors on the CTD and estimate the error associated with their measurements. This is a vital part of any science: no other scientist would allow us to publish our results if we couldn’t demonstrate that our measurements achieved acceptable standards.

omg glider recovery

We can measure salt concentration to within about 2 thousandths of a gramme in 1 kg of seawater. We need to know salt to this level of accuracy because it has, along with temperature, a big influence on how dense the seawater is. The sea is always attempting to sort itself out so that less dense water floats above denser water, so knowing salt and temperature can tell us a lot about how the water will be moving. I’ve mentioned dissolved oxygen before in the context of Chata’s work – biology both produces oxygen (when the microbial plants are glowing) and consumes oxygen (when bacteria break down the organic matter), so accurate data on the oxygen in the water tells us a lot about how the biology is operating. Chlorophyll in the ocean is the same green stuff that you see in leaves and grass – the chemical that plants use to collect energy from sunlight. Chlorophyll is particularly good for plants that live in the ocean. Sunlight is absorbed very quickly as it passes downward from the sea surface. All of the red light from the sun is absorbed within the first 1 metre below the sea surface. Blue light travels the deepest in the sea, and chlorophyll is well suited to capturing energy from blue light. Clearly this is an advantage for the microbial plants in the sea, as they are mixed through the upper few 10s of metres and need to maximise their chances of collecting the sun’s energy. But why should land-based plants use chlorophyll when they don’t have the problem of metres of ocean absorbing the light? Photosynthesis first evolved in the ocean. Land-based plants haven’t bothered to evolve a form of photosynthesis more suited to life above the sea, instead they just highjacked the system that the ocean’s microbial plants had developed. Quite literally. At the heart of the photosynthesising biochemical machinery in every leaf lies a light-capturing system that can be genetically traced right back to photosynthesising marine bacteria.

Billy does the salts

We’ve started to head north through the Celtic Sea now, stopping every 25 km or so to lower the CTD through the water and collect more information. The wind has picked up, with about 25-30 knots now. The sea is looking rough, but it’ll take a few hours for the swell to pick up and start to move us about.

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November weather

Ocean research cruise blog of Jonathan Sharples

 

The remarkable weather continued yesterday as we continued a series of measurements and zooplankton nets next to the moorings. A couple of scientists were even spotted sunbathing between net hauls. The wind continued to drop, and the sea finally reached a glassy state by sunset. Pretty good for November in the Celtic Sea.

The winning picture of the salps in the process of releasing faecal material into the water is below: look at the streaks of back trailing from the curl of colonial salps in the lower left of the picture. Some of these salp groups are reaching lengths close to 2 metres.

chain of salps

salps cought pooing

 This morning just as the sun came up we carried out one vertical profile with the CTD just next to the wirewalker mooring. That will provide Jo Hopkins with vital data for her to calibrate the instruments on the mooring. We are now pulling up the wirewalker, and will then head off to collect the 2 gliders that neeed to come back with us. The glider “pilot” back at the Oceanography Centre has sent instructions to the gliders to meet as at a specific location, so the gliders will have dutifully reached that position this morning and will now be bobbing about on the surface waiting for us.

ctd at down

The weather is due to close in tomorrow, with 25-30 knots of wind expected from mid afternoon through to mid afternoon on Friday. But the longer term forecast is suggesting a return to these calm, sunny conditions. Feels strange for this time of year, but none of us are complaining.

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More jellies

Ocean research cruise blog of Jonathan Sharples

 

The children at Churchtown Primary School are I gather busy working on the questions we asked them about sinking salp poo. The zooplankton group on board are getting very excited about their results, and already planning the scientific papers that they want to write. We collected more of the zooplankton yesterday so that we can make better estimates of the rate at which they eat and the rate at which they release the faecal pellets. In an attempt to get an idea of what these delicate organisms look like in the ocean we attached a few waterproof cameras to the CTD, and lowered them into the sea surface to record pictures for half an hour or so. I set the challenge to get a picture of a jellyfish or salp in the process of releasing faecal pellets into the water. There was a clear winner (Clare Ostle, from the University of East Anglia), but she was working very early this morning and is currently in bed – so I’ll get the photo for tomorrow.

an interesting bucket of jellies

Meanwhile, to help the kids at Churchtown think about this problem, the picture below has some good examples of the salps (the long, tubular jellies, connected in spirals) and the tiny jellyfish. Another rally interesting organism in this photo can also be seen, just about. The photo looks like it has a fine sprinkling of sawdust in it. These are tiny colonies of a photosynthesising bacteria called trichodesmium. It’s special in the ocean because it is a nitrogen fixer – it is able to use nitrogen gas dissolved in seawater, rather than the form of inorganic nitrogen (nitrate) that most phytoplankton need. That means they can grow in areas where nitrate is in very low concentrations, such as the large areas of open ocean in the sub-tropics. Finding them here is odd, because there is enough nitrate around and so the trichodesmium should not have any advantage compared to other phytoplankton. I’ll find out a bit more about them for another blog entry.


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salps and tiny jellyfish

The importance of zooplankton poo

Ocean research cruise blog of Jonathan Sharples

 

At dawn this morning we reached the end of the iron sampling transect, crossing onto the edge of the continental shelf at a depth of about 250 metres. Quite a stunning sunrise, with flat calm seas. Not what you’d expect for November. The dreadful-looking forecast for the end of the week also appears to have dissipated, so we might be able to push our work further north into the Celtic Sea.

end of iron transect

We are about to head southeast for an hour or so, to return to the shelf edge site that we spent 3 days on earlier in the cruise. We need to repeat some of the Snowcatcher work there, and also the zooplankton biologists on board want to find some more salps and jellyfish to try out some experiments to determine how much they are eating and also what happens to the waste material that they excrete. I’ve asked the children at Churchtown Primary School in Southport to have a think about this problem – how quickly does a salp waste pellet (i.e. a salp poo) sink through the sea? It’s an important thing for us to know about. A fast sinking particle doesn’t give the bacteria in the water much time to breakdown the organic material before the pellet reaches the seabed. A slow-sinking pellet can be broken down into inorganic material before it reaches the seabed, and that inorganic material is then returned to the water where it is accessible to the phytoplankton. Also, sinking quickly means that the carbon in the pellet is removed from the ocean surface (and the atmosphere) very quickly – you could argue that the stability of Earth’s climate owes a great deal to zooplankton poo.

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