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.
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.
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.
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.