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.

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.

Oxygen concentration in the sediments and the effects of filter coffee in human behaviour

Louis Byrne, British Oceanographic Data Centre, NOC

Thursday was a quieter day on board the RRS Discovery and we managed to have some time to relax (and catch up on some much needed sleep). In the morning we all learnt a valuable lesson about what happens when you give a certain SAMS research scientist a filtered coffee before noon - it seems to be roughly equivalent to feeding a gremlin after midnight. Luckily Natalie had calmed down enough by the evening to give Steve, the CPO(s) [Chief Petty Officer (science)] a haircut, with the finishing touches being applied by Eva McQuillan, the Irish Observer on this cruise.

Earlier in the blog in the post titled ‘What is happening in the benthos?” we looked at the work of Natalie and SAMS (Scottish Association of Marine Science) in examining carbon cycling and storage in different types of marine sediment. In addition to the measurements outlined in that post, Natalie is also taking separate core samples and measuring them for oxygen consumption and depth in the sediment.

 

Fig. 1: Sediment core being profiled for oxygen

One type of measurement involves using a very fine oxygen probe (microelectrode) to find out how deeply oxygen penetrates into the sediment. This probe is lowered into a sediment core like the one pictured, and as it goes down the core it measures how the oxygen concentration changes as you descend deeper into the sediment.  As you go down deeper into the sediment the oxygen concentration decreases quickly, as the oxygen is being used by bacteria and other organisms living in the sediment quicker than it is being mixed back into the sediment. 

 

This decline is not the same for all types of sediment, as the more sandy a sediment is, the deeper oxygen can penetrate into the sediment. This is for a couple of reasons. The first is that muddy sediments have smaller grains which can fit together more tightly meaning the sediment can hold less water between the grains and the oxygen in that water gets used up quicker.

The second is because muddy sediments can hold more organic matter giving the aerobic bacteria (bacteria that respire using oxygen) in the sediment more organic matter to consume. In consuming the extra food they will use more oxygen in the sediment. The picture below (Fig. 2), shows oxygen profiles from one of the sediment cores collected during this cruise (the sediment type is sandy mud which is mud with a little bit of sand).  By just one centimetre (1000 micro metres =1 mm) below the surface of the sediment, all of the oxygen has been used up. If this was an oxygen profile from sandy sediment, the oxygen would penetrate to depths of five centimetres or more.

 

Fig. 2: Oxygen profile from that sediment core

This particular sediment core also beautifully illustrates how some marine animals have adapted strategies to cope with the low oxygen concentrations. The burrow which you can see in Fig. 3 is that of a polychaete worm, and it creates a flow of oxygen from the surface of the sediment down to a depth of several centimetres by moving its body (this is known as bioirrigation). The process of moving sediment (e.g. to create burrows) is known as ‘bioturbation’. This flow of oxygen from the water above the sediment allows the worm to live in the oxygen poor mud and also allows oxygen to penetrate deeper into the mud than it would normally be able to do. This can then affect the chemistry within the sediments and the overlying water, and alter the oxygen penetration depth.

 

Fig. 3: Polychaete worm in its burrow.