D341: Porcupine Abyssal Plain cruise 2009


Cruise blog

Monday 27 July 2009

A salty tale...

You may be wondering why a biological cruise has physicists on board. The reason - and here I risk being cast adrift in a lifeboat – is that life in the ocean is dependent on physics. The base of the oceanic food web is provided by phytoplankton, minute single-celled organisms that, just like land plants, require sunlight and nutrients to grow. The trouble is that the two resources tend not to overlap. Sunlight is absorbed by water; so readily in fact, that there is only sufficient light for phytoplankton to grow in the top 100m or so of an ocean typically 5km deep. In this sense, phytoplankton scratch out a living on the very margin of the ocean. Their existence is made more difficult by the fact that marine nutrients are predominantly formed by all the crud (more technically known as the marine snow of Jennifer's research) sinking out of the surface, so most nutrients are found deeper in the water than sunlight can penetrate. This situation is exacerbated by the tendency of phytoplankton to grow rapidly whenever conditions allow, rapidly eating themselves into starvation by stripping surface waters of the little nutrients they have. So, how do the ocean’s phytoplankton find sufficient nutrients to allow them to match the cumulative efforts of rain forests and prairies in extracting carbon dioxide from the atmosphere each year? The answer is simple: physics. The physical circulation of the ocean acts like a rotorvator (ED: please check spelling), 'turning over' waters and bringing nutrient-laden deeper waters to the surface. Without the currents of the ocean, life within it would be very different and possibly much sparser than we know it. Hence, physicists play an important part in biological cruises in providing information on the context within which the observed life can exist. You are, perhaps, asking yourself how we do this? What exciting measurements do we have to compare to those of the biologists who are busy watching the moon-blue flashes of bioluminescence and delicate sinking aggregates or rummaging in abyssal mud for critters? We measure how warm and salty the water is. I admit it, it doesn’t sound very exciting, but even these basic scraps of information allow us to infer a great deal about how the ocean circulates. A lot of oceanic currents are driven by sharp horizontal changes in the density of sea water. Since temperature and the amount of salts dissolved in sea water control its density it becomes how apparent how these rather unglamorous basic measurements nevertheless enable us to say a great deal about how the phytoplankton are being sustained. We’re currently involved in mapping the temperature and saltiness of a region covering 10,000 square kilometres centred on our main study site. Already, simply from temperature and salinity measurements, we have found an eddy, a rotating body of water tens of kilometres across, nearly a thousand metres deep, whirling in the northwest corner of the area. The powerful and complex flows associated with such features are increasingly recognised as having a major influence on plankton ecology. Not bad detective work with just a thermometer and its salty equivalent to work with.

A delicate instrument used to collect
salinity samples about to be deployed
by a highly-skilled operative.

This figure shows how, on one of the transects of our survey, seawater density varies as a function of horizontal distance and depth (for which pressure is a proxy). Each coloured band corresponds to a particular density, increasing from from blue to red. The bulge in density lines (isopycnals) centred at 600dbar at the right hand side, reminiscent of a python after a large lunch, is the eddy core. The distortion of the surrounding isopycnals is what drives the circular motion of the eddy.

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