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

Abstract

Nepheloid layers with elevated concentrations of suspended particulate matter (SPM) are found throughout the world's oceans. They are 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 mass concentrations can be accurately quantified. However, this is not straightforward as the detected turbidity signal, which is used as a proxy for SPM mass concentration, not only depends on the concentration of particles, but also on physical characteristics of these particles, such as particle size, substance and shape. Here we present a comparative study of turbidity data to assess the potential implications different sensors have on the estimates of SPM mass concentration. Optical backscatter sensors (OBSs), transmissometers and both low- and high-frequency ADCPs were deployed simultaneously in the Whittard Canyon (North Atlantic Ocean), and water samples were collected for quantification of SPM mass concentration and ex-situ particle size analysis. We found that SPM mass concentrations inferred from the transmissometer are easily overestimated in the biologically productive surface layer due to higher light absorption by chlorophyll-bearing phytoplankton, compared to suspended detritic particles. 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 the use of only one type of sensor could potentially lead to misinterpretation and mis-quantification of particle transport processes and fluxes.