Snow catching across the Celtic Sea

Alex Poulton, National Oceanography Centre

Picture 1. Snow Catcher going over the side of the ship. Photo: Jose Lozano.

Of particular interest during this cruise is the fate of the material that is produced in the upper part of the water column - this material sinks down through the water column as large particles called marine snow. Marine snow is formed in many different ways. Some is formed from phytoplankton sticking together to form large aggregates when growth conditions are not optimal in the surface ocean, for example when nutrients are limiting growth. Others are produced by zooplankton eating phytoplankton and then producing faecal pellets. These marine snow particles can sink through the water column at various speeds, with their sinking speeds linked to their composition and size. As they sink they act as a food source for zooplankton and other organisms that live in the lower depths of the water column.

Picture 2. Snow Catcher being deployed to 70 m. Photo: Jose Lozano.

Collecting marine snow is a challenging business. During this cruise we are using Marine Snow Catchers - large volume (100 L) water bottles which we send down to the depth of interest and then close, enclosing the sinking particles which we then bring back up onto the ship and allow to settle for an hour or two (pictures 1-4). After this settling period we can then remove the water from the Snow Catchers and examine the particles in the bottom of the Snow Catcher. 

Picture 3. Snow Catchers taking a rest. Photo: Jose Lozano.

These Snow Catchers have been used on multiple cruises from the Arctic to the Caribbean individually, but unique to the Celtic Sea is the deployment of not one or two, but four Snow Catchers twice - once in the upper 10 m and then again at 70 m. This is quite some operation, taking a large amount of organisation, (patience), timing and around five hours. Over the entire length of the cruise we will carry out this large-scale water collection and snow catching exercise at five different sites, including our Central Celtic Sea site (Candyfloss). Our hope is that as well as seeing changes in the surface community we will also see changes in the composition of the material leaving the upper sun lit ocean and sinking down to the seafloor.    

Picture 4. Team Snow Catcher celebrating success. Photo: Callum Whyte.

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.