PAP Cruise : Tuesday 12th August 2009

August 12th, 2009

psiAs the cruise draws to a close it is time to reflect on what we have achieved. The observations we have made, described in the preceding blog entries, will allow us, in the fullness of time, to describe far better than has been hitherto possible the downward flux of organic carbon through the twilight zone and the processes responsible for the attenuation of this flux in the NE Atlantic during summer.

However we are a long way from this stage. We now face a period of at least six months data synthesis and sample analysis before we can begin to understand and place in context what we have observed. This is particularly important because a recent US programme known as Vertigo completed similar analyses at two sites in the Pacific. The power of our observations here will therefore be multiplied enormously via comparisons with Vertigo, building on the novel insights they obtained.

On a personal level it has been a great pleasure to sail on this cruise.
The supportive and comradely nature of the scientific party, ships staff and technical support personnel  has given me strength in my hours of need and some of the wildlife sightings have been spectacular.

As we head home through the Irish Sea it remains for me to hope that you have enjoyed the blog entries and that you have gained some insight from them into oceanography as both a career and a lifestyle. Finally many thanks to Jennifer Riley and Charlotte Marcinko who have masterminded the content of the blogs, all those people who wrote them, to Martin Bridger our software expert on the ship who made it all possible. And you would not be reading this if it were not for the Classroom@sea and Eurosites websites who are hosting them.

PAP Cruise : Monday 11th August 2009

August 11th, 2009

R HollandMy name is Ross Holland and I’m the Marine Flow Cytometry technician at the National Oceanography Centre, Southampton. This is my eleventh cruise on the RRS Discovery, and it’s been fun as always! Time is running out for me to write an entry for the blog, as I just finished analysing my last sample of the cruise just before dinner today! I thought it was about time I got involved by telling you a little bit about what I have been doing for the last five weeks.

Flow cytometers, the machines I am responsible for, are instruments used by Marine Microbiologists to study the very smallest forms of life in the oceans, the bacteria, protozoans and the pico-phytoplankton, many of which are less than one thousandth of a millimetre in size. These groups of organisms may be very small, but they are the most abundant life forms in the oceans, and so it is very important to know what they are up to! There may be as many as 3 million of them per millilitre of seawater, don’t let this put you off swimming in the sea though, marine bacteria are perfectly harmless.

During this cruise the focus of our efforts has been on these organisms and their abundance and function at previously little-studied depths.

Our standard flow cytometer works by passing a sample of seawater through a laser beam. Each cell that passes through the laser beam, scatters light and fluoresces in a specific way, and by detecting the exact way in which the particle reacts, we can assign it to a different group of organisms. In addition, the cytometer can actually physically ‘pick out’ groups of cells we are interested in. By incubating them with different labelled substances, we can sort a few tens of thousands of cells out and then measure the rate of uptake of these substances, and get an idea about how active each group of cells is.

Also on this cruise I have been involved in using the sophisticated new size fractionating plankton net developed by Mike Zubkov and Kev Saw. This has been used to catch large numbers of much bigger phytoplankton and zooplankton for us to count on our newest flow cytometer at NOCS, the Flow Cam. This cunning device actually takes a photograph of every cell it counts and can recognise the group of organisms it belongs to just from the photograph. From this we can tell a great deal about the ecology of the waters we are studying around the PAP site!

Ross Holland

PAP Cruise : Monday 10th August 2009

August 10th, 2009

Lat 50 26.27 N
Long 11 41.20 W

Radioactive tracers to estimate carbon fluxes

The biological pump is a major component of the global carbon cycle which transports approximately 10 gigatonnes carbon per year from the atmosphere to the oceans interior. For it, to effectively sequester carbon in the deep ocean, it must export organic carbon to depths below that of winter mixing. This occurs via the sinking of organic carbon aggregated into particles, large enough to attain high sinking velocities. The aim of our work in the D341 is to estimate the POC fluxes from the upper layers to the twilight zone in the PAP site using two different and complementary techniques based on the disequilibria of the radioactive pairs 234Th-238U and 210Po-210Pb.

IMG_0729

Some of the elements that comprise the earth are radioactive. Those natural radionuclides are ideal tracers of a great variety of environmental processes for several reasons. First of all they are ubiquitous in the environment; secondly, their radioactive decay allow us to “easily” measure them (or we can try to measure  1mBq/L of 210Po or 6·10-9 ng/L using other techniques!)

Finally, equilibriums between the activities of the parents and the daughters of the radioactive chains are established in close environmental compartments. As the compartments are rarely close  in the environment, these equilibriums are broken and the measurement of the correspondent desequilibrium allows us to establish different properties of the compartment of interest.

Is this last property the one that we use to estimate carbon fluxes from 234Th and 238U and 210Po and 210Pb disequilibria.

234Th (T1/2=24,1d), daughter of 238U (T1/2=4,47.109y), can be used to estimate how much POC is exported into the deep ocean. 238U is conservative in the seawater. But unlike 238U, 234Th is particle reactive in the water column. As particles sink through the water column, 234Th is scavenged with them and the secular equilibrium between 234Th and 238U is broken, the subsequent disequilibrium can thus be used to quantify carbon export fluxes.

IMG_0750

First, 234Th export fluxes are calculated. Moreover, if we know the ratio POC/234Th, PIC/234Th or BSi/234Th (that can be measured in the sinking particles using in situ pumps, SAPS), it is possible to estimate the rate of carbon and biominerals export from the surface ocean.

In a similar way, a disequilibrium through the water column it is found between 210Pb (T1/2=22y) and its daughter 210Po (T1/2=138d). However, this disequilibrium has different characteristics than that of the pair 234Th-238U. 234Th is attached to the surface of the particles, however, 210Po it is assimilated by the organic matter and incorporated to the cell as a substituent of sulphur. This is an important difference because it is expected that 210Po-210Pb disequilibrium allow us to better estimate POC fluxes whereas 234Th will be used to estimate particle scavenging.

Furthermore, the different half lives of 234Th (days) and 210Po (half a year) would allow us to study different timescales, fast changes in fluxes (234Th) and also seasonal variations (210Pb).

During the cruise our aim is to measure 210Po, 210Pb and 234Th in the station in as much depths as possible (high resolution profiles if we want to integrate the disequilibrium properly!). For this reason, there where always so many carboys spread around our lab, and some of them flying all around during the storms!

After collecting the water, radionuclides are precipitated following different radiochemical procedures. Afterwards, their radioactive decay will be measured and the concentrations of the radionuclides are obtained.

Maria Villa

Fred Le Moigne

PAP Cruise : Sunday 9th August 2009

August 9th, 2009

The story of a mass murderer

Zooplankton samples

Zooplankton samples

‘Aren’t they cute?’ – The answers I get to this question vary strongly when I let the people on this ship have a glance through my microscope – from ‘Yuck, they look horrible’ to ‘Aww look, it’s fighting the other one’. I never got a ‘They look really tasty!’ as an answer. But that may be as I never asked a fish, whale or other carnivorous zooplankton. What I am looking at are copepods: small shrimp-like animals that occupy a key role in the marine food web.

To get my sample, I get up at horribly early hours in the morning, because these little shrimp-like creatures know quite well how tasty they are. To avoid predation, they migrate into the deeper waters during daytime, when the sunlight is shining through the upper water layers and fuels the phytoplanktonic photosynthesis. At night time however, in the safety of darkness, copepods and other zooplankton migrate upwards into the food-rich surface layer to feed. They haven’t expected me!

Sarah and her zooplankton net

Sarah and her zooplankton net

So there I stand with a 200 μm-mesh plankton net at 3 am. After I got my sample, I choose the animals I am interested in and sort them according to size and species. This sounds rather simple. In reality though, I pick each animal individually using a microscope and tweezers. The copepods I am looking at are sometimes as small as 0.4 mm and are just about visible with the naked eye. On a ship that is constantly moving, and in a crowded lab with a table that is vibrating from the filtration rig of my neighbour scientist, this can be a rather tough call. But I am young and patient, and so I count myself lucky when I finally got all the animals needed at about 7 am: It is breakfast time!

It is breakfast time for me, and for the copepods, too! I am about to set up my feeding experiment. I am interested in how much and what these little animals eat. As I cannot watch them actively eat like my fellow scientists at the breakfast table, I need to use an indirect approach. Imagine two identical meadows, and assume that the grass is growing at the same rate on both meadows. Now a farmer brings 10 cows onto one of the meadows and leaves them for 24h. If you mow both meadows after those 24 h and weigh the grass, you can see from the difference how much the 10 cows ate in 24 h. This is exactly what I am doing. I put 10 copepods in 2.2 L glass bottles containing phytoplankton-rich water that has been collected for me from about 25m depth using a CTD rosette. Simultaneously, I set up three control bottles without animals. The bottles are screwed onto a plankton wheel, which is slowly rotating the bottles to keep animals and food in suspension. After 24 h, I take water samples from experiment and control bottles, and fix the sample with the preservative Lugol’s iodine. When I am home again, I will count the phytoplankton in each sample and calculate how much these small copepods munched. This information will help us to understand the role of mesozooplankton in the carbon cycle. Even though a single copepod is tiny, the high abundance of these little grazers results in an enormous turn-over of carbon. Because of their high grazing rates and daily vertical migration, copepods may transport high amounts of particles through the water column. Together with the zooplankton sampler ARIES, my feeding experiments will give us a better idea of how much these wee marine beasties contribute to the biological carbon pump.

Sari Giering

So, what happens to the animals after the experiment and all the other animals that I do not use? Yes, you are right: they get flushed down the sink. And yes, I do feel horrible about it even though they are minute. But dear fellow scientists: Imagine how much zooplankton a single 20 tonnes baleen whale eats every day!!!

PAP Cruise : Friday 7th August 2009

August 7th, 2009

Lat: 48 55.88N
Long: 16 32.24W

Under pressure…

Our Lab

Our Lab

This is not only the fabulous title of a song but it is also the condition of life for so many organisms inhabiting the Ocean. Indeed, Ocean is deep with a mean depth of 3,800m. Here, in the PAP site, the depth is over 4,500m. As you maybe know, the hydrostatic pressure increases with depth (1 bar every 10 m), that means that at atmospheric pressure (pressure where we are living, ~1 bar), the pressure is equal to ~0.1 kg per centimeter square but at 1,000m-depth the pressure is equivalent to 100 kg/cm²!!

One of the main topics of this cruise is to better understand how the organic matter is consumed by organisms. We focus our attention on bacteria with a special consideration to their environmental condition of life. We are considering two kinds of pressure effects on organic mineralization:

1)     Bacteria attached to particles sinking through the water column. These bacteria coming from the surface waters are submitted to an increase of pressure (and a decrease of temperature) that limits their metabolism and then their degradation of organic particle.

2)     Free-living bacteria in deep-sea environments. These kinds of bacteria are adapted to their environment and then they are more active under pressure than after decompression of the sample.

To study these microscopic organisms, we have developed original (and heavy) high pressure devices together using high pressure bottles (HPBs). These HPBs are used in the first case to simulate the increase of pressure that bacteria attached-to-particles experiences (PASS: PArticles Sinking Simulator) and in the second case to maintain their in situ conditions all the time during our experiments (HPSS: High Pressure Serial Sampler and not French CTD as it is call here!!). On board the RSS Discovery, we are occupying a whole container (and many more…).

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During this cruise, we have used particles recovered from PELAGRA (see blog of the 31st July) in order to estimate their degradation by bacteria simulating a sinking velocity of 200m/day.

Also, we are measuring deep-sea microbial activity under in situ pressure conditions. Joined with the IODA6000 data (see blog of the 3rd August), we will expect to better estimate the bacterial carbon demand of bacteria living in the dark ocean.

Christian Tamborini and Mehdi Boutrif