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.

Moving sediments at the bottom of the ocean

On Monday we got to see another sediment re-suspension experiment carried out by Charlie Thompson of the University of Southampton (UoS), and Sarah Reynolds of Portsmouth University. 

Charlie Thompson (UoS) has written the following about this experiment which was carried out using a piece of equipment called a benthic flume:

The seabed isn’t solid, but is instead made of mobile sediments (muds, sands and gravels), which are moved around by the action of waves and tides. Charlie and Sarah’s work looks at what happens to nutrients, carbon and the sediments themselves when they begin to be re-suspended and moved around by the tides and water currents.

Benthic flume sitting on the deck

 One way to do this is using a benthic flume (see photo above , which sits on the seabed and uses a motor driven paddle system to allow us to recreate strong currents onto the actual sediment at the bottom of the ocean, when and where we want them. By doing this, we are able to measure when a specific type of sediment (e.g. Sand or mud) starts to move – which in turn allows us to predict how often that sediment might be re-suspended by natural water flows over the seabed. We can also determine how the fluxes of nutrients and carbon into or out of the bed are changed compared to on a stationary bed from water samples taken at time intervals during the experiments.

Benthic flume being deployed over the side of the RRS Discovery