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.

Over the past 6 weeks we have each collected quite a lot of samples, which when processed will provide an awful lot of data. But what happens with all this data once it is processed? Obviously we will each have first access to our own data to do what we want with it, be it publish it in an article or present at scientific meetings. But the nature of the cruise means everyone’s data is in some way related, which when compared can provide a much bigger picture of biological, chemical and physical ocean processes. So just sitting on your own data and hiding it away would be a real waste, if you were to do this you may miss some patterns that would become apparent if you were to compare each other’s results. Whilst crucial to get the most out of the cruise, comparing data isn’t as simple as just emailing everyone your results; it takes some real co-ordination and organisation. Because of this we have had our very own data manager assigned to AMT21. Therefore I introduce Rob Thomas, he is our data manager on board, and is going to explain a little about his role.

Following the article on the BBC World website we have recieved and answered questions from the general public. I have added a selection of them below.

The Bridge is the command centre of the ship. The officers on watch have responsibility for driving the ship, navigation and keeping track of deck operations. The day is split into 6 shifts, with each officer taking two of them. Chief officer Richard works the 4-8, 2nd officer Malcolm works 12-4, and Euan 3rd officer is on watch between 8-12. This is a 24 hour operation, with the Master, Peter, on the bridge when necessary, normally during busy periods such as CTD operations.

Earlier this week I was sticking up the blog in the corridor outside the labs. When asked ‘What are you doing?’ I simply replied ‘putting the blog on the wall’ to which I got ‘there are no walls on ships!’ hollered at me! Walls are in fact called bulkheads on ships. So it got me thinking, what are the other terms out there with a maritime alternative. One coffee break later with the deck crew and engineers I was well on my way learning the maritime lingo!

Today we have a very special blog entry. I hold great pleasure in handing over to the Master of RRS James Cook, Peter Sarjeant. As the Master, or Captain, Peter holds a positon of great responsibility, overseeing all the operations on board the ship. It is because of him, and his fantastic team that we are here in the middle of the South Atlantic. I will now hand over…

Monday 15th November 2010, I will start at the beginning of the day.

T’was a Friday, and all was calm. Breakfast was unusually quiet, what was wrong!? The science technicians on the other hand had been having a busy morning. We had reached the site of the South Atlantic Gyre (SAG) mooring site. This meant no pre-dawn CTD, but a hectic day on the aft deck.

Thursday 11th November, just two weeks until we reach Punta, and science is back in full flow. An early start for all; now we only have one pre-dawn CTD at 4.30am, so no more 6am lie ins to sample the second one!

Satellites are part of our everyday life, for mobile phone conversations, sat nav, weather forecasts… the list is endless really. Out here we receive three sets of images a week for sea surface temperature (SST) and chlorophyll from the National Earth Observation Data Archive and Analysis Service (NEODAAS) at Plymouth Marine Laboratories. The SST image is awash with colour, painting the Atlantic Ocean as a rainbow. However, the chlorophyll is a complete contrast, making the ocean look like a blue desert. If that’s really the case then why are we spending 6 weeks, sampling for biology in a desert!?