Friday, 7 June 2013

6 - Catching the Ocean’s Snow!

Although it may seem odd, snow actually occurs a lot in the ocean! Not only do inputs from the atmosphere, like dust, sink down through the water column but so do many other particles from marine life in the surface ocean. Tiny marine plants grow using sunlight and these are grazed upon by small animals called zooplankton. Zooplankton often has the feeding behavior similar to that of a little child; a lot of it ends up “around” the mouth. Planktologists refer to it as “sloppy feeding”. This growing and grazing creates a lot of particles in the upper ocean, as does the zooplankton faecal pellets (aka poo) – which surprisingly is probably one of the most well measured things on this research cruise!

Marine Snow catcher
Some of particles are simply mixed in the surface of the ocean and reused, whilst others sink out of the surface layer and can start to form what we call marine snow. As the particles sink slowly, they can bump into other particles and become larger, or equally be broken apart. Not only this, but they are also being fed upon by other marine organisms, such as bacteria, who seem to have a taste for snow and zooplankton poo! The amount of material sinking out of the upper ocean and how this varies with depth is of particular interest because it is a way of pumping carbon from the atmosphere to the marine sediments where it is buried. This biological pump is a key part of the Earth’s carbon cycle, and its strength influences how much carbon dioxide there is in the atmosphere. But what particles are being removed via this biological pump, how much is sinking and does this change with depth, location or time of year? Well that’s exactly where the marine snow catcher comes in!

Anna at work
We’re using the marine snow catcher (MSC) like a giant water bottle - a 100 Litre water bottle no less – to do exactly what its name implies, catch marine snow. We can send the MSC down to different depths and bring back up a sample of water containing particles. It’s then possible to collect the particles that sink to the bottom of the MSC and conduct experiments on them to determine how fast they are sinking and how much carbon they contain. This information is useful when trying to calculate the "flux" or movement of material leaving the surface ocean. We can also measure different properties of the water in the MSC which has smaller particles suspended in it. By trying to ‘catch’ water and particles at different depths and times, and measuring a range of their properties, we can start to build up a picture of how the magnitude and composition of marine snow (and other sinking particles like zooplankton faecal pellets) varies in the ocean.

Marine snow size
(image of marine snow© Anna Belcher)
So, hopefully we will catch plenty of marine snow during this cruise and add one more piece to the puzzle of how carbon is transferred out of the surface ocean (exported) and help improve predictions of how this export might change with a changing climate.

Written by Anna Belcher

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