Shelf Sea Biogeochemistry blog

Thursday, 23 April 2015

Snow catching across the Celtic Sea

Alex Poulton, National Oceanography Centre

Picture 1. Snow Catcher going over the side of the ship. Photo: Jose Lozano.
Of particular interest during this cruise is the fate of the material that is produced in the upper part of the water column - this material sinks down through the water column as large particles called marine snow. Marine snow is formed in many different ways. Some is formed from phytoplankton sticking together to form large aggregates when growth conditions are not optimal in the surface ocean, for example when nutrients are limiting growth. Others are produced by zooplankton eating phytoplankton and then producing faecal pellets. These marine snow particles can sink through the water column at various speeds, with their sinking speeds linked to their composition and size. As they sink they act as a food source for zooplankton and other organisms that live in the lower depths of the water column.

Picture 2. Snow Catcher being deployed to 70 m. Photo: Jose Lozano.
Collecting marine snow is a challenging business. During this cruise we are using Marine Snow Catchers - large volume (100 L) water bottles which we send down to the depth of interest and then close, enclosing the sinking particles which we then bring back up onto the ship and allow to settle for an hour or two (pictures 1-4). After this settling period we can then remove the water from the Snow Catchers and examine the particles in the bottom of the Snow Catcher. 

Picture 3. Snow Catchers taking a rest. Photo: Jose Lozano.
These Snow Catchers have been used on multiple cruises from the Arctic to the Caribbean individually, but unique to the Celtic Sea is the deployment of not one or two, but four Snow Catchers twice - once in the upper 10 m and then again at 70 m. This is quite some operation, taking a large amount of organisation, (patience), timing and around five hours. Over the entire length of the cruise we will carry out this large-scale water collection and snow catching exercise at five different sites, including our Central Celtic Sea site (Candyfloss). Our hope is that as well as seeing changes in the surface community we will also see changes in the composition of the material leaving the upper sun lit ocean and sinking down to the seafloor.    

Picture 4. Team Snow Catcher celebrating success. Photo: Callum Whyte.

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