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.