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.

Deploying the Cefas Lander and the SmartBuoy

Louis Byrne, British Oceanographic Data Centre, NOC

Wednesday saw the deployment of two moored instrument suites owned by Cefas. The first deployment was a lander similar to the NOC-L (National Oceanography Centre, Liverpool) Mini-STABLE lander deployed earlier in the cruise, although the instruments attached to the Cefas minilander are very different.

The Cefas lander has an ADCP (Acoustic Doppler Current Profiler), which uses the Doppler affect to measure current speed and direction through the water column. As well as the ADCP there is a water sampler collecting a sample of water in a plastic bag (to be analysed for nutrients on land after the mooring is retrieved) and other instruments measuring a variety of parameters including temperature, chlorophyll fluorescence and optical backscatter (a way of measuring how many particles are in the water, which is useful for determining how much sediment any storm events may mix into the water column).

 

A Cefas SmartBuoy on deck

The second one was a Cefas SmartBuoy which was deployed at the same location as the lander but instead of resting on the seabed it floats on the surface. The SmartBuoy has all the same instruments that are on the lander as well as a water sampler which will collect one sample of water each day for analysis back at the lab.

The Lander and the SmartBuoy are useful because they can provide long term high resolution background data. The overall UK SSB programme is a seasonal project, lasting one year, and repeatedly sampling the same sites to see how the processes affecting the carbon and nitrogen cycles vary between the seasons.

The seasonal changes in the Celtic Sea primarily revolve around the development of water column temperature stratification in spring, through to when it breaks down in late summer to early autumn (see the previous blog post for an explanation to  this process and the resulting phytoplankton blooms).

A Cefas SmartBuoy after being deployed in the Cetic Sea

The data collected by the SmartBuoy and minilander provide very useful data on the timing and magnitude of the development of stratification and the phytoplankton blooms. The chlorophyll fluorescence and oxygen sensors attached to the SmartBuoy on the sea surface can detect the start of the phytoplankton bloom as phytoplankton use chlorophyll to photosynthesise, a process which produces oxygen as a by-product.

Meanwhile on the seabed, when stratification develops there will be a decrease in oxygen. This is because aerobic bacteria and the countless other marine organisms which require oxygen will continue to use it, however, as this layer has now been cut off from the surface (by the thermocline) the oxygen diffusing into the  surface water from the atmosphere does not make it down to the water below the thermocline quick enough to replenish it. This decrease in oxygen will be picked up by the oxygen sensor attached to the Cefas minilander. The minilander is also able to detect when the phytoplankton bloom dies off, as the large influx of dead phytoplankton cells falling down through the water column (also known as Marine Snow) will cause a peak in chlorophyll and later a further decrease in oxygen, as the phytoplankton are consumed.

Large amount of marine particles or marine snow in suspension just above the sea floor. Image credit:
 https://phys.org/news/2013-07-marine-scientists-explore-biodiversity-ecosystems.html

By measuring the biogeochemical changes which revolve around the development and breakdown of stratification, the data from the Cefas minilander and SmartBuoy can help put the rest of the data collected during SSB into context, by placing the measurements taken during this cruise within the seasonal cycle that this region experiences.

Springtime phytoplankton blooms in the Celtic Sea

Louis Byrne, British Oceanographic Data Centre, NOC

The seasonal changes in the Celtic Sea primarily revolve around the development of water column stratification in spring and when it breaks down in late summer to early autumn.  Right now in March, the Celtic Sea is fully mixed, however with the days getting longer and warmer (we hope), the surface of the Celtic Sea is also warming. As the surface warms its density decreases and the water becomes lighter compared to the colder waters below which don’t have access to the suns heat. (Fig 1.) To help watch for these changes we have a daily set of sea surface chlorophyll and temperature satellite images sent from the NEODAAS team at PML to the ship, and any developments of blooms and changes to the temperature can be seen as they occur.

Fig. 1: Temperature profiles in the mid latitudes in the ocean. Dashed (- - - -) line is for the winter and the continuous line for the summer season.Source: https://nptel.ac.in/

This will eventually result in the creation of two distinct bodies of water, with a warm surface layer resting above a colder layer below, much like a cocktail which often have two or three coloured layers sitting on top of one another.

As well as causing the onset in stratification, the increase in temperature and sunlight also causes a truly massive increase in the number of phytoplankton in an event known as a plankton bloom [many plankton blooms are so large they can be seen from space! (see Fig.2)]. This results in a feeding frenzy as zooplankton (Fig. 3) numbers surge and they are in turn eaten by other organisms, passing the energy down the food web.

Fig. 2: Plankton Bloom in the Celtic Sea. Captured by the Envisat's Medium Resolution Imaging Spectrometer (MERIS) on 23 May 2010. Credits ESA

The phytoplankton bloom starts just before the onset of stratification, and then continues in the surface layer as the water there is warmer and receives much more sunlight. Eventually the phytoplankton will use all of the nutrients available in the surface layer and most of the plankton will die off. When this happens their cells will fall through the water column, causing a large increase in the biological material available on the seabed.

"Copepodkils". Licensed under CC BY-SA 3.0 via Wikimedia Commons - https://commons.wikimedia.org/wiki/File:Copepodkils.jpg#/media/File:Copepodkils.jpg

 

When stratification breaks down at the end of summer, the water column in the Celtic Sea is again fully mixed. The bottom layer of water is still nutrient rich and these nutrients are also mixed into the surface of the water column, and become available for photosynthesis. This causes a smaller phytoplankton bloom at the end of summer before the days darken, and the cycle is complete.