Sunday 27th June 2010

Snow in June?

I know we have reported that adverse weather condition out here occasionally interrupt our sampling programme but I don’t think anyone has mentioned that it has been snowing every day we have been out here.  Well, not so you would have noticed if you looked outside – the view from my cabin at present is of grey, turbulent seas with a sky to match – but no snow!  That’s because we need to look below the surface.

Whenever we deploy our ROV, the cameras reveal a constant fall of snow – white particles drift past constantly, occasionally reaching blizzard intensity.  This ‘marine snow’, unlike its atmospheric namesake, is not made up of crystalline snowflakes but of amorphous (shapeless), often slimy aggregates. Not quite as glamorous as snow then!  However, as unattractive as it may sound, ‘marine snow’ has a vital role to play, both during its journey to the deep-sea floor and upon arrival.

A Chimera circles Grant’s amphipod trap

Tube sponges stand tall (wearing ophiuroid scarves?) while ‘snow’ falls all around them

Animals that live in the ocean depths do so without the benefit of sunlight. With the exception of fauna associated with hydrothermal vents, the majority of animals on the ocean floor rely directly, or indirectly, upon nutrients produced in the surface waters arriving as ‘marine snow’.  This is not exactly what you might call a ‘fast food’ service and dinner is often served much later on the sea floor than it is prepared in ‘kitchens’ at the surface.  Imagine being at the end of a long queue, down which a food trolley is travelling, hoping that there will be some left when your turn comes! As the ‘marine snow’ descends it is fed upon by animals along the way, animals such as salps and appendicularians, which drift around in the water column filtering water – quite literally for a living.  The ‘snow’ is also added to along the way – imagine the food trolley is also collecting leftovers as it moves down the line – so that when it finally reaches the sea bed, it is a mixed assemblage of ‘goodies’ that appears on the menu for the benthos (bottom dwellers).

As you might imagine, the food arriving on the deep-ocean floor is not particularly nutritious and you have to eat a lot of sediment to stay alive.  If you eat a lot, then you ‘poo’ a lot. The holothurian (sea cucumber) responsible for this little lot has clearly been feasting!

Holothurian poo

The coiled deposit in the photograph is the same size, approximately 12 cm, as the animal that produced it and the sea bed is littered with the aftermath of what is termed ‘deposit feeding’ activity; piles of poo, tracks, scrapes and mounds, evidence of animal activities that we refer to as ‘lebensspurren’ (‘life traces’).

Filtering the water is another option, allowing you to intercept your food before it arrives on the bottom.  To do this well it is useful to be raised off the sea floor on hard surfaces, as shown by these sponges.  The stalked crinoid, or ‘sea lily’, raises itself high above the surface to intercept food as it falls giving it a palm tree appearance (it looks like it might be growing out of the sediment but is actually standing on a small rock rather like the sponge you can see in the background).

Sponges on a basalt cliff

A stalked crinoid rises above the sea floor

In order to understand this supply chain and how it might influence the pattern of life on the deep-sea floor, we deploy sediment traps to intercept the falling ‘snow’ so that we can determine such important parameters as the timing, quantity and quality of the material heading down to the ocean depths.

At each of our four sites, either side of the Charlie-Gibbs Fracture Zone, we have recovered a ‘mooring’ that had been deployed on a previous cruise, in 2009. These moorings comprise a number of current meters, which log the speed and direction of water movements, and two sediment traps, one set at 100m and the other at 1000m above the sea bed (approximately 1500m below the surface).  The line to which these are attached is anchored to the sea-bed by ballast weights; recovery entails triggering a sonar activated release mechanism, causing the weights to be jettisoned and allowing the mooring to float slowly to the surface.

One of the current meters is brought on board

A sediment trap is recovered, its yellow and orange flotation buoys lie behind it on the deck.

Each sediment trap comprises a carousel of tubes into which falling sediments, and occasionally misguided shrimps, are directed by a large funnel.  The tubes contain preservative so that the contents do not decompose before we have the chance to examine them.  The carousel rotates to take sequential samples at monthly intervals allowing us to look for seasonal influences on the quantity and quality of our ‘marine snow’ supply chain. Comparison of the two traps provides information relating to changes in the nature of the ‘snow’ as it falls to the sea floor.  Studies of this nature have shown that the supply of food to the animals of the deep-sea floor is indeed seasonal, reflecting the seasonal productivity of sun-lit surface waters and imparting a rhythm to the growth and reproduction of many of the species that we are finding here.

Well, if all this talk of snow makes you feel cold, just remember that, while it’s not real snow that falls on the animals below us on the mid-Atlantic Ridge, they really are cold.  The temperature down there, according to our CTD readings, is 3.60C and they have no warm summer in the offing!

Geoff Wigham

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One Response to Sunday 27th June 2010

  1. Tek says:

    Snow in June sounds like poem.

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