Shelf Seas Biogeochemistry – A short introduction

By Louis Byrne, British Oceanographic Data Centre, NOC

We woke up on Monday to a sea which was perhaps even worse than Sunday. We were still a fair distance away from site A and were not scheduled to reach site A till approximately 2100 Monday evening. Due to the rough seas I spent the majority of the day hugging my toilet bowl, but not before making the rookie mistake of blocking my sink with the remains of my breakfast, which Geoff the Steward was not too happy about.  Due to a day spent in transit not much happened, and due to my sea-sickness I was not around to see what did, therefore I thought it would be a good time to introduce the reason why we’re rushing towards the Celtic Sea at a slow and steady speed of seven knots.

Although shelf seas make up only 5% of the ocean surface, they have been estimated to be the most valuable biome on earth, with high levels of primary productivity supporting diverse ecosystems. High concentrations of nutrients support the growth of phytoplankton, which are single celled marine organisms that photosynthesise like plants on land. Like plants on land, Phytoplankton are the base of the marine food web and they provide a diverse food source for many marine creatures, such as zooplankton.
 

Phytoplankton are the foundation of the oceanic food chain.

Zooplankton are tiny marine animals which are food for fish and countless other marine organisms, that are then in turn eaten by others. It is in this way that the sun’s energy fixed by phytoplankton on the surface of the water column is distributed throughout the marine ecosystem, underpinning more than 90% of global fisheries and offering many other important ecosystem services.

In addition to supporting the entire marine food web, the photosynthesis carried out by phytoplankton also removes significant amounts of carbon dioxide from our atmosphere.  Although tiny, phytoplankton have a disproportionately massive effect on our atmosphere, and are responsible for creating as much as half of the oxygen that we breathe, removing an equally large amount of carbon dioxide as they do it. Some of the carbon extracted by the phytoplankton will sink to the sea floor and be stored in the sediments (often for thousands of years!), reducing the overall concentration of carbon dioxide in our atmosphere.

In order for the shelf seas to sustain these high levels of production, the phytoplankton must be supplied with nutrients, but where do these nutrients come from? It is the need for us to better understand the role of shelf seas in the global nutrient cycle, how this supply of nutrients determines the shelf’s primary and secondary production and how this affects other processes such as carbon storage which has led to the Shelf Seas Biogeochemistry programme.

At 2100 on Monday night we reached site A and decided that the seas were too rough to sample that night. Therefore, an 0600 hours CTD cast was scheduled for the following morning, and we were hopeful that our cruise was about to get its first piece of data.

For those of you wishing to see the answer to yesterday’s question, the answer is Richard Cooke of the National Oceanography Centre, Liverpool.

Expeditions of Discovery find the secrets of the shelf seas

Shelf Sea Biogeochemistry and the recent pelagic cruise (DY018) were featured in an article 'The secrets of the shelf seas – one of Earth’s most important ecosystems' by The Observer newspaper. 

The sea off our coasts teems with microscopic life that breaks down the carbon dioxide we pump into the air. Now a series of expeditions aims to find out more.

Colony of salps floating under the RRS Discovery during DY018. Underwater photography courtesy of Claire Ostle (University of East Anglia).

Docked

Ocean research cruise blog of Jonathan Sharples

 

We finished up all of the sampling during yesterday afternoon, and headed in past the Needles lighthouse on the west corner of the Isle of Wight. The pilot was picked up just before Calshot Spit, and we steamed up Southampton Water. It was bitterly cold! Probably the coldest weather we had experienced all cruise.

The ship docked in Empress Dock, in front of the Oceanography Centre, just after 1700. As soon as the gangway was in place, and we’d got the announcement that the ship had been cleared by customs, off we all went – the entire science group headed off through the docks to the Platform Tavern.

And that’s it. A very busy morning ahead as we unload the ship, but normally we are able to get away by noon. The end of a very productive cruise, with remarkable weather allowing us to do a lot more than we expected.

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The Needles

Land sighted

Ocean research cruise blog of Jonathan Sharples 

 

We steamed along the south coast overnight, and at breakfast this morning we passed Lulworth Cove and then Swanage. There’s just one last bit of science left to do. We are crawling slowly into Poole Bay and Christchurch Bay, taking surface samples of seawater. Clare Davis, from the University of Liverpool, is processing these water samples for a couple of the Liverpool University PhD students. The students are researching the dispersion of organic matter from estuaries out into the ocean, and also looking at the relative supplies of nutrients from rivers and from the deep ocean to the shelf seas. These samples are also tying our work into another research project focussed on land catchments and river nutrients. Anouska Panton, a researcher working at the University of Southampton, will be carrying out fieldwork in Christchurch harbour today so that later we can link the data together with what we are collecting to get a broader picture of river-supplied nutrients and their fate in the autumnal shelf sea.

One important job we managed to clear yesterday was the cruise photo. We picked the right time for it, sat 20 miles off Plymouth with nice, sunny weather. Today wouldn’t have been as good – it’s windy and grey outside. But at least we can now see land, for the first time in three and a half weeks.

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DY018 people

Visitng E1

Ocean research cruise blog of Jonathan Sharples

 

We arrived at position E1, south of the Eddystone, at about 0600. This is a site regularly sampled by the Plymouth Marine Laboratory (PML), generally about once per month but more frequently recently in collaboration with the project we are working on. Scientists and technical staff at PML maintain a data-gathering buoy out here. We carried out 6 seabed cores this morning, and were then met by the two PML boats. Coring the seabed from the PML boats is difficult, so they are very happy that we can stop here for a few hours to collect these samples for them, and transfer the samples to their boats to be taken back to PML and analysed.

pml explorer alonside

It was also our last CTD profile here at E1, at 0630. And it was fully mixed from the surface down to the seabed! Not too surprising as E1 is fairly close to the permanently-mixed water of the English Channel, and it’s only 75 metres deep. So we expect it to become mixed relatively early in autumn. We’ll do some more zooplankton nets this afternoon – Sari Giering is keen to have a lst go at collecting some more of the trichodesmium nitrogen-fixing bacteria, this time to get some samples for some DNA analysis.

Nick shows us the engine room

The clear-up of the labs has begun. The ship has a fast turn-around in Southampton, so we need to be ready when we arrive tomorrow evening to get some of the larger bits of equipment and container labs off. Some of the scientists took some time to go on a tour of the ship’s engines. Nick, the 2nd enginner, showed us around those normally hidden parts of the ship that power us through the water, provide fine-control of the ship’s position when we are working a station, as well as powering all our instruments, making our freshwater, ventilating the ship, and treating the sewerage. Remember there are about 50 people living on this 100m-long metal box for several weeks at a time: the ship is like a small, very independent village.

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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