D341: Porcupine Abyssal Plain cruise 2009


Cruise blog

Friday 24 July 2009

Catching “snow” in the ocean by Jennifer Riley

The tiny plant cells in the ocean (phytoplankton) form the basis of marine food webs that include the fish we are familiar with in the fishmongers and larger organisms extending up to whales.   These tiny plant cells that occur in vast numbers in the sunlit upper surface ocean can also play an important role in helping to reduce the green house gas carbon dioxide in the atmosphere that is believed to be a major factor in global warming.

These tiny plants use carbon dioxide that has dissolved in seawater and convert it to organic carbon in their bodies.  When the plants die part of the carbon is eaten in the upper ocean and recycled back to carbon dioxide, whilst the remainder falls down to the deep oceana where the carbon that was once in the atmosphere is now locked away and will not be released back to the atmosphere for decades or even centuries. Trying to understand how big this movement of carbon to the deep waters is and the processes involved is the objective of our cruise.  The descending particles often come together to form fluffy masses of material (typically mm to cm in size) that is called snow (see the greenish mass in tube in the pciture, below right).  This marine snow is rich in bacteria and also feeds zooplankton (see below right), the shrimp-like organisms that inhabit the twilight zone beneath the sunlit surface waters.

Launching the Pelagra trap over the side of the ship

Critters from 300m down!

And that is where PELAGRA comes in. PELAGRA (see above, left) is a free-floating sediment trap that can be programmed to remain at a certain depth for a few days at a time. It has four funnels that intercept material sinking down through the water column, each with a removable pot at the bottom where the sinking particles settle. At the end of the sampling period, the pots close and PELAGRA floats to the surface where it can be recovered by the ship and the pot contents collected for analysis. So far we have had a relatively short test deployment of all five PELAGRA traps to make sure that everything is working as it should do. This consisted of the five traps being deployed for three days at depths of between 50 and 600m, pre-programmed for two days of sampling, before they resurfaced. Although two of the traps resurfaced too quickly due to a problem with ballasting, the other three traps provided a good haul of material on which to perform the various chemical analyses. One factor that needs to be considered when using sediment traps is that the collected biological material, sitting in the sample pot underwater for several days, can provide a concentrated source of food for many of the zooplankton that feed on marine snow, as mentioned above. It is common, when recovering samples, to find that several different kinds of

these organisms have found their way into the sample pots and, as they are not directly part of the sinking material in which we are interested, they need to be picked out of the samples. After our first deployment, adjustments have been made to correct the ballast problems from the trial run, and all five PELAGRA traps were once more deployed on Sunday. This time they have been programmed for a full five and a half day sample collection period, with each one sampling at a different depth. They are due to be recovered at the weekend, hopefully with another ample quantity of sinking particles.

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