Shelf Sea Biogeochemistry blog

Monday, 10 August 2015

Measuring the metabolism of the seafloor

By  Megan Williams, National Oceanography Centre

Today we recovered our benthic lander. The frame had been deployed for two days and has nine instruments measuring a range of parameters including water velocity, nutrients, suspended sediment, sediment particle sizes, and benthic oxygen consumption. Our first deployment was at a site with sandy sediments.

Recovery of the benthic lander 

 The steps toward our first recovery were many (see pictures): after driving the instruments and frame down from the National Oceanography Centre in Liverpool to our sister location in Southampton, we built the frame and started attaching instruments, batteries, and routing cables. When the frame was in a state it could be moved (with fragile instruments not yet installed), the frame was driven to the mobilization dock and loaded onto the RRS Discovery. Once on the ship, we could install the fragile water sampler (which will be used for nutrients and suspended sediment measurements) and the eddy correlation system (which makes fast oxygen and velocity measurements near the bed). The eddy correlation system measures subtle turbulent currents (eddies) just above the seafloor with both up and downward elements as they move past the sensor as swirls of water 'rolling' over the seabed. The sum of the upward (positive) and downward (negative) movement of dissolved oxygen gives a measure of how much oxygen the seafloor is using (i.e. the metabolism of the seafloor).

With a planned deployment time, we programmed instruments to start, did last minute calibrations, and set up the mooring. The frame was then slowly lowered 100 meters (m) to the sea bed, a ground line was set out, and a weight and buoy are connected 300 m away so as to not interfere with measurements.

All has gone well so far! We have the frame back on the ship this afternoon. We have now started to collect all the data off the lander, changing batteries, and preparing for another deployment of the instruments at a site with muddy sediment.

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