Shelf Sea Biogeochemistry blog

Showing posts with label oxygen concentration. Show all posts
Showing posts with label oxygen concentration. Show all posts

Monday, 13 April 2015

The breath of the ocean

My name is Jose Lozano and I am a PhD student from the University of Vigo, Spain. In this cruise (DY029), I work with  Elena Garcia, post-doc at the University of East Anglia, taking samples and doing  measurements of oxygen (O2) respiration in the Celtic Sea (Candyfloss) by using different methods, Optodes (optical sensor devices, which is designed to measure absolute oxygen concentration and % saturation), Electron Transport System and Winkler (a test used to determine the concentration of dissolved oxygen in water samples).

Net community production (NCP) is a measure of the net amount of carbon removed from the atmosphere, which represents the difference between Gross Primary Production (carried out by phytoplankton through the photosynthesis) and Dark Community Respiration (from both phyto and zooplankton). Plankton found in the world’s oceans are crucial to much of life on Earth. They are the foundation of the bountiful marine food web, produce half the world’s oxygen and suck up harmful carbon dioxide.  It is therefore vital for scientists to closely observe the oceanographic and biological variables related with these little buoyant organisms, temperature, nutrient content, light extinction or partial pressure existing in the water column.

During the cruise we have very busy schedules, not only the scientists but also the crew and  the technicians. They all work constantly, making the practice of science much easier, by cleaning, cooking, creating tools, or fixing devices. We, the scientists, couldn't make it without their support.

Dolphins, Photo: Jose Lozano

When you spend 24 hours a day in an oceanographic vessel, even in hours of rest, you feel very tempted to go on deck to chill out and breathe the fresh air at the stern. In a good day you can feel the ocean breathing gently and musically through the waves, the cool wind blowing on your face, you can observe the wildlife, the terns and the gannets flying over your head and families of common dolphins jumping playful just few meters away from the vessel. You can even see some land animals, such as owls, garden birds or little spiders, which are travelling with us on the ship. All these organisms, from the smallest diatom to the biggest marine mammal, breathe oxygen (though in the case of archaea or bacteria, other molecules may be used) in order to obtain energy from organic matter, so to be able to keep going.

Sandwich tern. Photo: Jose Lozano

Thursday, 12 March 2015

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.