Shelf Sea Biogeochemistry blog

Tuesday, 18 November 2014

Shelf edge station begins




Ocean research cruise blog of Jonathan Sharples

 

Work at the shelf edge has started well. One big difference between the work here and the work that we did at the first station on the cruise is that we have no moored instruments here. The shelf edge is the most heavily fished part of the seas around NW Europe, so long-term deployments of moored instruments tend to be unsuccessful as the chance of moorings being snagged by fishing gear is very high. For the duration of our work at this site we instead hang a chain of instruments from the ship. Jo Hopkins and Chris Balfour, from the National Oceanography Centre in Liverpool, spent the previous day setting up about 40 temperature, salt and chlorophyll loggers so that their clocks were all synchronised and they all take measurements at the same rate (once per minute). The instruments were then clamped every 2.5 metres on a 200 metre wire lowered over the stern, with a 300 kg ball of lead on the end of the wire keeping it vertical in the water.

Jo with chain instruments

We are using this chain of instruments to track a particular feature of the shelf edge. As the tide moves onto and off the shelf, the steep slope in the seabed causes the tide to push the thermocline up (tide flowing onto the shelf) and down (tide flowing off the shelf). This up and down motion generates waves on the thermocline that move away from the shelf edge, both onto the shelf and away into the deep ocean. These underwater waves can be very large, 100 metres from peak to trough and 15 km long. They are important because they result in a lot of mixing at the shelf edge, bringing nutrients from the deeper water up towards the surface. Our chain of instruments will track this up and down motion of the thermocline wave, so we have a picture of how rapidly the physics of the water below us is changing as we collect all of the biological and chemical samples
Original post 

t-chain deployment

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