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Sampling the seafloor

Whilst ROVs can be used to find out what large animals (megafauna) live on the seafloor, many animals are too small to see with the ROV or live hidden within the seafloor sediments. Benthic samplers are used to collect these smaller animals.

When a benthic sampler hits the seafloor it takes a sample of the uppermost layers of seafloor sediment, which can be stored for further processing in the lab. A sieve with a specific mesh size will be selected depending on the size of the animal we are interested in. For example, if we are interested in the macrofauna, we will pick a mesh size that is smaller than 2 mm. Smaller animals and fine sediments will pass through the sieve, while the macrofauna will stay behind on the mesh to be identified.

Geologists also sample the seafloor using coring devices, but these go much deeper into the seabed; sediment sampling for biological analyses typically only samples the top c. 20cm of the seafloor.

Choosing where to sample the seafloor is important – it needs to be carefully targeted so that a representative sample is obtained. During the iMirabilis expedition sampling locations will be chosen, where possible, based on the areas explored with the ROV. Visual imagery from the ROV can be used to highlight particular areas of the seafloor that are of interest. 

Box corer

There are different types of benthic samplers. A box corer can collect large samples in soft sediments and is lowered from a research vessel on a wire. Once it hits the seafloor, it can penetrate as deep as 0.5m and closes up before it is winched back on board. When sample collection is successful, bottom water will be present above the sediment sample and can also be collected for analyses. Box core samples can also be used to study the sediments themselves and geochemical processes. More importantly, they can be used to carry out quantitative studies of the benthic micro- to macrofauna.

Above: A box corer being deployed. Note the shovel-like closure mechanism on the left is open - when triggered, this plate swings down and slides across the bottom of the square box, trapping a cube of the uppermost seafloor sediment and overlying seawater inside. Image courtesy JM Roberts/JC073.
Above: A loaded box corer being brought back on board the ship - note the closure mechanism is activated, trapping the sample inside. Image courtesy L.De Clippele/DY108
Above: The top of the box core is full of water - this is the layer of seawater that lies just above the seafloor, so it's very valuable sample material and is siphoned off for analysis. Image courtesy L.De Clippele/DY108
Above: And then, lots of careful sieving of the solid material in the bottom of the box core to analyse what the sediment is composed of, and what lives in it. Image courtesy L.De Clippele/DY108
Multicorer and push cores

As the name suggests, a multicorer will take multiple cores at once. Relatively small plastic tubes are pushed into soft sediments and can collect (for example) 8 replicate cores at once. Like the box corer, the multicore allows the layer of seawater just above the seafloor to be sampled too, so you have a complete view of what the seafloor environment is like. Typically these samples are used to study the meiofauna.

On some missions, an ROV is used to take small, very precisely located push cores of the seafloor sediment. Just as it sounds, the ROV’s manipulator arm is used to push a small plastic cylinder into the seafloor to remove a plug of sediment, in a similar fashion to samples taken using a multicorer.

Recovery of the multicorer (left), with the closeup above showing the coring tubes filled with a sample of the uppermost layer of the seafloor sediment and the seawater lying immediately above it. Images courtesy NOC/JC010.
Van Veen grab sampler


Van Veen grabs are relatively lightweight and low tech, making then easy to deploy during research expeditions. When deploying the Van Veen grab, the two levers with buckets at their ends are spread open like a pair of scissors. The moment the grab hits the seafloor, these levers are unlocked and the grab closes around a scoop of the seafloor sediment. Van Veen grabs are especially useful in areas with coarser sediments or when we want to collect coral rubble or framework, but they do not preserve the sediment-water interface like the box corer or the multicorer. They are therefore a quick and easy way to sample but are not as precise as other benthic sampling methods.

Above: Van Veen grab being deployed. When triggered, the two scoops close around a sample of sediment at the seafloor. Image courtesy JNCC/S Pearson/CEFAS
Written by Laurence De Clippele

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EU

This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 818123 (iAtlantic). This output reflects only the author’s view and the European Union cannot be held responsible for any use that may be  made of the information contained therein.