Suspended particulate matter in a submarine canyon (Whittard Canyon, Bay of Biscay, NE Atlantic Ocean): Assessment of commonly used instruments to record turbidity

Nepheloid layers with elevated concentrations of suspended particulate matter (SPM) are found throughout the world???s oceans, generated by both natural processes, involving resuspension of seabed sediment by bottom currents, and anthropogenic sediment resuspension due to bottom trawling, dredging and in the future potentially due to deep-sea mining. These nepheloid layers represent pathways of lateral SPM transport, including lithogenic and biogenic sediment, organic matter, (trace) metals, organic pollutants and plastics. For assessment of the dispersion of these materials, it is essential that SPM concentrations can be accurately quantified. However, this is not straightforward as the detected turbidity signal not only depends on the concentration of particles, but also on physical characteristics of these particles, such as particle size. Here we present a comparative study of turbidity data to assess the potential implications different sensors have on the estimates of SPM concentration. Optical backscatter sensors (OBSs), transmissometers and both low- and high-frequency ADCPs were deployed simultaneously in the Whittard Canyon, and water samples were collected for quantification of SPM concentration and ex-situ particle size analysis. We found that SPM concentrations inferred from the transmissometer are easily overestimated in the biologically productive surface layer due to increased light absorption. Furthermore, we observed that depending on sensor type some particles are not, or less well, detected. This is due to differences in particle size sensitivities of these sensors towards the diverse range of particle sizes found in the Whittard Canyon. Whereby the low-frequency ADCP was most sensitive for coarse-grained material and the high-frequency ADCP and OBSs most sensitive for fine-grained material. In future studies, we suggest to use a combination of different sensors as it enabled us to identify a recurring cycle of resuspension, involving aggregation and disaggregation of SPM in the Whittard Canyon.