Is our understanding of particles near the ocean surface distorted by the narrow lens of ocean optics measurements?

Emlyn Davies, SINTEF Ocean, Norway, David McKee, University of Strathclyde, Physics Department, Glasgow, United Kingdom and Sünnje Linnéa Basedow, The Arctic University of Norway, Tromsø, Norway
The optical properties of seawater are influenced, to some degree, by scattering and absorption by all suspended material in the water column. Particulate material may be composed of inorganic or organic matter in the form of a floc or marine snow, or it may be an organism in its own right. Particle size and composition plays a significant role in its transport, and thus in controlling particle fluxes that are tightly coupled to the global biogeochemical cycle.

We present a theoretical assessment of the relevant size ranges for determining inherent optical properties, based on simple optical calculations that encapsulate the known range of sizes and composition of particles in the ocean. When considered in the context of commercially available in-situ sensors, a clear gap is evident between the size ranges of optical significance for ocean color and those sizes captured by in-situ optical sensors and other particle measurement technologies. This discrepancy suggests a need for more holistic approaches to monitor particles in the surface ocean, in order to more closely couple optical signals with biogeochemical fluxes.

Combined optical and acoustical sensing solutions are proposed in order to properly characterise the entire spectra of particles and their role in the carbon cycle. This is presented via demonstration of combined sensor deployments onboard small autonomous underwater vehicles. In analysing such complex multi-sensor datasets, we explore new solutions to AI-supported data mining whereby in-situ particle imagery can characterise the size and type of the most significant contributors to the particle population and fluxes.