#05 2021

Cost-effective and efficient sampling in the deep sea

By Grant van der Heever and Dr Lara Atkinson (SAEON) & Dr Charles von der Meden (University of KwaZulu-Natal)

Sediment particle size has an important influence on the distribution and composition of animals that live on the seabed and is also frequently used in the classification of marine habitat types. 

In South Africa, benthic grabs (see images below) are regularly used to collect sediment samples for particle size analysis. However, grab sample failures due to misfiring (closing too early), incorrect or undesirable grab landings on the seafloor and sediment wash-out during retrieval are common and time-consuming, particularly in deep-sea work. These grab failures can also become costly with respect to additional time spent at sea, with estimate calculations, based on vessel costs per day at sea in 2017, suggesting monetary losses in the region of R4 000 per grab failure!

In South Africa, benthic grabs are regularly used to collect sediment samples for particle size analysis

The Van Veen grab before deployment

In an attempt to optimise data- and sample-collection opportunities at sea, a small cone dredge was attached to the rear-end of SAEON’s towed benthic camera system, SkiMonkey III (see image below), enabling a sediment sample to be collected from the seabed during each camera deployment. Sediment was successfully collected using the cone dredge attached to the towed camera and did not add additional at-sea sampling time.

To consider the cone dredge as a replacement or supplement to the grab when sampling sediment, researchers at SAEON’s Egagasini Node compared replicate samples collected using the custom-designed cone dredge and the conventional Van Veen grab at nine co-located stations, ranging from 355 to 508 m depth in a sandy ecosystem type (Southern Benguela Sandy Shelf Edge Ecosystem) off the west coast of South Africa (Figure 1).

Figure 1. Study area

Figure 2. Graph displaying the proportions of each grain size at each station

The researchers found no significant differences between the sediment properties collected using the cone dredge and the Van Veen grab (Figure 2), indicating that the cone dredge can be considered a cost-effective and efficient alternative for sampling benthic sediment at depth in this ecosystem type. Similar, additional testing should be conducted in other ecosystem types, especially mud habitats.

DOI for research paper: https://doi.org/10.2989/1814232X.2021.1902855

Cone dredge attached to SkiMonkey III