Particle size distributions in the upper 100 m water column and their implications for animal feeding in the plankton

We deployed autonomous particle-sensing SOLOPC floats more than eight times during five cruises, amassing almost 400 profiles of particle size (d>90μm) and abundance between the ocean surface and 100 m. The profiles consistently had subsurface maxima in particle volume. The median (by volume) equivalent spherical diameter for the particle distribution was 0.4–0.8 mm and increased with depth in a manner similar to that observed in coagulation simulations. There was a sharp cutoff at the bottom of the high particle concentration region. Estimation of particle fluxes made using the size distributions show an increasing downward movement through the particle field above the sharp particle cutoff. The increase of particle flux with depth through the euphotic zone implies a partial spatial separation of production and consumption. The sharp drop in particle volume and flux implies that the base of the particle-rich zone is a region of active particle consumption, possibly by zooplankton flux feeding. Our data show greater concentrations of zooplankton-type particles relative to marine snow-type particles below the particle maximum. Such behavior could explain why zooplankton are frequently observed at and immediately below the particle maximum rather than the productivity maximum and suggests an important role for flux feeding in carbon and nutrient cycling at the base of the particle maximum. This implies that zooplankton act as gatekeepers for the movement of organic matter to the mesopelagic. The ability of the SOLOPC to sample hourly with high resolution in the upper 100 m of the ocean provides a powerful complement for the study of particles where it has been difficult to use sediment traps.

Research highlights
► Particles in the upper 100 m of the ocean have mean diameters ranging from 0.4–2 mm.
► Mean particle diameter increases with depth.
► There is a sharp decrease in particle volume at about 50 m.
► Particle decrease should be associated with decrease in vertical flux.
► We infer zooplankton grazing is regulating vertical flux out of surface layer.