Abstract

Gravitational sinking of particles is a key pathway for the transport of particulate organic carbon (POC) to the deep ocean. Particle size and composition influence particle sinking velocity and thus play a critical role in controlling particle flux. Canonically, sinking particles that reach the mesopelagic are expected to be either large, or ballasted by minerals. However, the presence of transparent exopolymer particles (TEP), which are positively buoyant, may also influence particle sinking velocity. We investigated the relationship between particle composition and sinking velocity during the EXPORTS campaign (Export Processes in the Ocean from RemoTe Sensing) in the Northeast Pacific Ocean using Marine Snow Catchers. Suspended and sinking particles were sized using the FlowCam and their biogeochemical composition was assessed by measuring the concentration of particulate organic carbon (POC) and nitrogen, particulate inorganic carbon, biogenic and lithogenic silica and TEP. Sinking fluxes were also calculated. Overall, suspended and sinking particles were small (< 51 μm, diameter) in this late summer, oligotrophic system. Contrary to expectation, the ratio of ballast minerals to POC was higher for suspended particles than sinking particles. Further, suspended particles showed TEP-to-POC ratios three times higher than sinking particles. This suggests that TEP content dictated the sinking behavior of the two particle fractions. Fluxes of POC averaged to 4.3 ± 2.5 mmol C m-2d-1 at 50 m and decreased to 3.1 ± 1.1 mmol C m-2 d-1 at 300–500 m. These flux estimates were slightly higher than fluxes measured during EXPORTS with drifting sediment traps and Thorium-234. A comparison between these approaches illustrates that small sinking particles were an important component of the POC flux in the mesopelagic of this late summer oligotrophic system.