Shelf Sea Biogeochemistry blog

Thursday, 20 August 2015

Mini-Flume Experiments

Sarah Reynolds. Senior Research Associate and Lesley Chapman-Greig, MRes student,  are Marine Biogeochemists with the University of Portsmouth, and their research is looking at how the processes in marine sediments can contribute to the carbon and nitrogen cycles in shelf seas.

For one of their experiments, sediment from the seabed is collected from a NIOZ core and brought up to deck, where it is stored in a mini-flume alongside water collected from the just above the bottom of the seabed by the CTD. The mini-flume simulates resuspension events on the seabed. The sediment lies at the bottom of the mini-flume, with the water from the CTD above, which is stirred by paddles of the flume to simulate the action of currents on the bottom of the seabed, which can disturb the seabed sediments causing them to be mixed (the scientific term is re-suspended) into the overlying water column. During resuspension events nutrients and carbon stored in the sediments can be released into the water column.

Mini Flume

Over the course of the experiment (~3 hours), samples of water are collected from the mini-flume at certain time points and collected for inorganic nutrients, dissolved organic carbon, particulate organic carbon and suspended particulate matter. These measurements can then be used to determine the concentration of nutrients and carbon that are released into the overlying water column. As the mini-flume experiment progresses the paddles of the flume are moved faster and faster until complete bed failure occurs.

The increases in speed of the water moving  above the sediment in the mini-flume, make it possible to measure how different current speeds close to the surface of the seabed may change the concentration of carbon and nutrients that are released from the sediments.

Sediment is collected for mini-flume experiments at three cohesive sediment sites, with each site having a different type of sediment, ranging from very muddy sediment with fine particles sizes to muddy sand and sandy mud. Depending on the type of sediment, the concentration of carbon and nutrients and the energy required to lift the sediment off the seabed varies, so by conducting this experiment with a variety of sediment types it is possible to discover how the concentration of carbon and nutrients mixed into the water column by seabed currents varies between different sediment types.

This cruise is final cruise in a yearlong project, where the same data have been collected at different stages of the seasonal cycle of the Celtic Sea. The data collected by this experiment can be used alongside other measurements, collected from the different sites at different times of the year, to get a good picture of how the suspension of sediments affects the carbon and nutrient cycles in the Celtic Sea, with the hope that these can be extrapolated to the Western European continental shelf.

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