AMT Blog

Time for another guest post, this time from Dr Gavin Tilstone from PML, giving a nice summary of the Atlantic gyres.

The Atlantic Gyres

Close to the equator we saw flying fish bouncing like skimmers over the surface of the ocean, periodically diving below the surface to feed on the micro-life that live just below. Either side of the equator there is a vast deep blue slab of ocean, which is barren and seemingly devoid of life. These open ocean regions are so deep (>4.5km) that nutrient rich waters that lie at depth, seldom reach the surface and the sunlit, upper ocean is fuelled by re-mineralized nutrients from grazing and breakdown of phytoplankton cells by zooplankton, bacteria and marine viruses.

After having spent the first 2 weeks in the North Atlantic Gyre we are now 1 week in to the South Atlantic Gyre. So what is a Gyre? It is a swirling vortex, which in the ocean is created by wind or currents. There are two main gyres in the Atlantic Ocean, which are created by currents; the northern Gyre which circulates clockwise and is created by the North Equatorial and North Atlantic currents and the southern Gyre which swirls anti-clockwise, created by the South Equatorial and Antarctic Circumpolar currents. These areas are the ocean deserts, which occupy 70% of the world’s oceans, are inhabited by the smallest of the marine phytoplankton, the cyanobacteria which live deep in the water column in the twilight zone where light and nutrients are just high enough to sustain their existence. So why are we here and why is it important to study these deserts? The Gyre ecosystems are a delicate balance between phytoplankton growth, zooplankton grazing and the release and re-mineralisation of nutrients. Any adverse affects on this tightly coupled food web, can have major consequences higher up the food chain on the fish and whales that graze either the phyto- or zoo-plankton.

The Cyanobacteria that inhabit the Gyres are evolutionary relics of the earliest plants. It is thought that the chloroplast of higher plants formed from an endosymbiotic relationship with cyanobacteria. The ability of this group of organisms to perform oxygenic photosynthesis is thought to have converted our atmosphere into a place habitable by respiring organisms (including man). Cyanobacteria live in the harshest environments and can be found in habitats as diverse as polar ice to desert rocks; from freshwater mud to the twilight zone of the deep ocean. They are arguably the most important contributors to global carbon and nitrogen budgets and account for 30% of the global carbon fixation. The Atlantic Meridional Transect therefore offers an unbroken long term time series of measurements to monitor changes in these delicately balanced Gyres and the cyanobacteria communities that dwell within them. In the past we found that the productivity of the Northern Gyre had decreased as a consequence of warmer, more stratified conditions and a decrease in the photosynthetic activity of the cyanobacteria. This year we have observed that the Northern Gyre is more productive than we have previously recorded due to increases in cyanobacteria productivity, which signifies a re-balancing of this ecosystem, which is good news for the higher forms of life that survive in these blue deserts.

Photo: CTD coming out of blue water by Rob Thomas, BODC, UK.

Sailing was delayed on 13 December due to ship maintenance and a local storm, which kicked water over the back deck and rocked the boat, even though we were tied up in port. We sailed out of Port Stanley on 14 December and anchored in Port Elizabeth to conduct a full muster to get those who had recently joined the ship familiar with emergency procedures of manning the life boats.

Views from Port Elizabeth

We arrived in Port Stanley at 10:00 a.m. on Monday 10 Nov. All of us tired and weary with the prospect of still having to pack our boxes of equipment into the containers. We eventually unloaded everything in the early evening which then gave us sometime to explore the town. It is spring in the Falklands and equivalent to May in the UK. My first impressions were it is windy, cold and barren place with low lying hills next to sweeping bays; something akin to the western isles of Scotland but colder. Stanley is a scattering of multi-coloured roofs sputtered against the shores of Port Elizabeth. There are just under 3000 people on the Falklands, 1800 of which live in Stanley. In the interior-countryside, people make a living from sheep farming. In Stanley there are 5 pubs, 1 restaurant, a couple of cafes, a couple of supermarkets, gift shops selling loads of tourist penguin memorabilia, a church, government building and swimming pool. My second impression is that most people in Stanley work in the service industry; the tourist trade must bring in a lot of revenue. Stanley is a small compact place. The people are very friendly and very patriotic; there are Union Jacks everywhere as a constant reminder of the Islands heritage and recent history.

The measurements we make on the ship are not the only data that we use to improve our understanding of Atlantic Ocean Dynamics. The Royal Research Ship James Clark Ross receives satellite images each day from the National Earth Observation Data Archive and Analysis Service (NEODAAS) at Plymouth Marine Laboratory (PML).

There are many instruments on board that we use to probe the mysteries of the ocean. From when we left Immingham to when we dock in the Falkland Islands, every minute of the day, 24 hrs a day, data is being collected along the ships track to assess the biological, physical and chemical properties of the ocean.

Since ‘crossing the line’ we have sampled eight stations in the southern hemisphere to find out what is going on in the Southern Gyre.

So what is a Gyre? It is a swirling vortex, which in the ocean is created by wind or currents. There are two main gyres in the Atlantic Ocean, which are created by currents; the northern Gyre which circulates clockwise and is created by the North Equatorial and North Atlantic currents and the southern Gyre which swirls anti-clockwise, created by the South Equatorial and Antarctic Circumpolar currents.

We have now moved from more productive waters back into oligotrophic conditions. The mixed layer has deepend to 60m, the deep chlorophyll maximum is sitting at 80m and just below the thermocline.

Today we spotted a single frigate bird and what was either a sunfish or turtle. The seas around the equator are calm and I looking forward to being rocked into a deep sleep before being abruptly awakened in less than 5hrs time for the next CTD.

From the conductivity-temperature-density (CTD) profile yesterday it appears that we have moved out of the Northern Gyre and into more productive waters. Last night at 22:30 I saw from the ships bow a magnificent display of marine phosphorescence. As the ship ploughed through the waves, tiny star like formations appeared in the spray, like a neon, night sky in the sea. This morning at the 04:00 CTD we saw squid and just after breakfast at 8:00a.m., a pod of 12 dolphins.

The sun is going down, so got to dash and filter my samples so they don’t start respiring the carbon they have fixed during the day.

On this vast deep blue slab of ocean, we have not seen any wildlife for days and it would appear on the surface that there is nothing out here except occasional white horses looming ferociously around the ship. Similarly the satellite ocean colour imagery that we receive daily from the National Earth Observation Data Archive and Analysis Service (NEODAAS) at Plymouth Marine Laboratory indicates that there is little phytoplankton in the surface layer of the ocean.

The Conductivity-Temperature-Density (CTD) trace operated by Dave and Terry and material from Chris’s zoo-plankton net hauls, however, paint a different picture. Below us between 80 and 130 meters, there is broad peak in the fluorescence profile indicating higher phytoplankton at depth which peaks at 110 meters. The sample that Chris takes from his zooplankton nets is teaming with tiny marine animals. The zooplankton become food for fish, such as anchovies, which Stuart and Jo have been detecting in the backscatter signal. They continually graze the phytoplankton crop, but are rather ‘sloppy feeders’ and as they feast on the phytoplankton cells, they release proteins, amino acids and nutrients back into the water, which Malcolm, Carolyn and Paul are measuring. These released nutrients, in turn, become food for marine bacteria and small phytoplankton. This process is known as the microbial loop whereby large zooplankton feed on small zooplankton which in turn feed on small phytoplankton. These open ocean regions are so deep (>4.5km) that nutrient rich deep waters seldom reach the surface and the sunlit, upper ocean is fuelled by re-mineralized nutrients from grazing and breakdown of phytoplankton cells by zooplankton, bacteria and marine viruses. The system is a delicate balance between phytoplankton growth, zooplankton grazing and the release and re-mineralisation of nutrients. Any adverse affects on this tightly coupled chain, can have major consequences higher up the food chain on the fish and whales that graze either the phyto- or zoo-plankton.

The sunlight is so intense at these latitudes that if the small pico and nano-phytoplankton, such as Syneccococus and Prochloroccocus, are carried to the surface, they can explode due to the intensity of the visible and ultra violet light. They therefore reside at the deeper layers of the ocean, surviving on low light and re-mineralised nutrients. Chris characterises the light field daily at solar noon. Glen measures the abundance of these small phytoplankton groups using flow cytometry and Mike, Manuela and Ross are looking at which phytoplankton groups are using which nutrients. Vas, Mario and I are measuring the fixation of carbon through photosynthesis in these different phytoplankton groups and the respiration and O2 consumption of the communities. As the mixed layer deepens, these organisms and other organic matter gets sheared upwards into high light and can be photo-chemically transformed into different components, which are either transformed into other nutrient pools or released as gases. Some of these photo-chemical processes are what Vas, Paul and myself are measuring. These vast blue deserts occupy 70% of the world’s oceans, and these tiny phytoplankton play a important role in maintaining the delicate food web that sustains life in these huge ocean ‘deserts’.

Better turn in soon less than 6 hrs until I have to get up for the next CTD.

RRS James Clark Ross Bridge-cam

Setting the scene

On 5th October after the force 9 storm, we started the science proper. There are 21 scientists from 7 institutes on board who make a broad spectrum of measurements that helps improve our understanding of the biological, chemical and physical dynamics of the Atlantic Ocean. The Atlantic Meridional Transect programme is hosted by Plymouth Marine Laboratory (PML) and is a national UK capability for long term multi-disciplinary oceanographic observations and marine research in the Atlantic. It has been running for 12 years; the first phase served as a platform for validating NASA satellite observations, the second, to study the microbial dynamics in the ocean deserts and this new and third phase is funded by the UK government under the programme Oceans 2025* as a time series of sustained observations in the Atlantic on the structure and biogeochemical properties of plankton ecosystems in the Atlantic Ocean. In a nutshell, to assess ‘the health and state of the Atlantic Ocean’.   

There are six scientists from the PML (Chris Gallienne, Carolyn Harris, Vasilis kitidis, Glen Tarran, Gavin Tilstone, Malcolm Woodward) who are measure everything from the optical properties, nutrients, photochemistry, phytoplankton community structure, phytoplankton respiration and carbon fixation and macro-zooplankton abundance to enhance our understanding of the biological, optical and chemical properties of the Atlantic Ocean. Paul Mann, a PhD student from the University of Newcastle is working alongside the PML nutrient and photo-chemistry teams. Mario Vera, a POGO fellow from Columbia, also works alongside the PML team on phytoplankton respiration.

There are five scientists from National Oceanography Centre (NOC) who also cover a wide spectrum of measurements studying the biology and physics of the Atlantic Ocean; Mike Zubkov’s team Ros Holland and Manuela Hartmann measure the abundance of marine bacteria, pico and nano-plankton (very small phytoplankton 0.2 to 2 microns) and micro-zooplankton and their uptake of nutrients. Stuart Painter and Jo Hopkins measure the physical properties (temperature and salinity) of Atlantic Seawater.

One scientist (Bruce Bowler) is from Bigelow Laboratory in the US. Bruce is a one man band measuring everything from the reflectance of the sea surface to absorption and backscatter, the biomass, calcium and carbon content of the microscopic calcareous, marine plants known as the Coccolithophores. Martin Ostrowski and John Pearman from the University of Warwick work alongside Mike’s team studying the genetic diversity of the phytoplankton and bacterial communities. Jeremy Young and Martine Couapel, from the Natural History Museum, study the abundance and genetic diversity of Coccolithophores.

All of this work is underpinned by Terry Edwards and Dave Teare from National Marine Facilities, Southampton who operate all of the overside instruments, Ben Tullis from the British Antarctic Survey, Cambridge who provides IT support and of course the crew and officers of the James Clark Ross who do everything from deploying our instruments, steering our course and turning out 3 course sumptuous meals three times a day.

* Oceans 2025 is a multi-disciplinary research program looking at changes in marine systems in a high CO2 world.