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.