Relationship between particle size distribution and flux in the mesopelagic zone

Large aggregates commonly named “marine snow” are difficult to collect and study because of their fragile nature, but they make up the largest fraction of vertical carbon flux in the ocean. Developments in imaging sensors and computer systems have facilitated the development of in situ image acquisition systems that can be used to produce profiles of aggregate size distribution and abundance. However, it is difficult to collect information on the different properties of particles, such as their composition, from in situ images. In this paper, we relate sediment trap data to particle size (d) distributions to estimate the vertical fluxes (F) of mass, particulate organic carbon (POC), particulate inorganic carbon (PIC) and particulate organic nitrogen (PON) using simple power relationships ( F = A d b ). Mean aggregate fractal dimension of 2.3 and a size-dependent settling speed are determined from the flux estimations. We have used these relationships to map the distribution of mass flux along 180°W in the equatorial Pacific. Similar mass fluxes below the euphotic zone have been reported along 150°W for the same period with conventional sediment traps, supporting the accuracy of these relationships. The high spatial resolution of sedimentation processes studied in situ with the Underwater Video Profiler allowed us to undertake a detailed study of the role of physical processes in vertical fluxes.