Particles fluxes and moving fluids: experience from synchronous trap collection in the Sargassso sea

A series of synchronous, 24-h experiments using sensor-equipped sediment traps revealed that higher particle collection rates were associated with higher approach velocieties. Surface-tethered traps with variable drag configurations provided distinct differences in approach velocities for paired 400 m deployments and paired 1500 m deployments. Small-scale hot-film hydrodynamics sensors located both inside and outside the sediment traps detected flow cells within the traps with velocities between 50 and 100% of the external fluid approach velocities. In conjunction with laboratory flume simulations, these observations reveal that particles do not settle gravitationally across trap apertures. Intead, particles are swept advectively into traps at the downstream portion of trap apertures, and most are then expelled at the upstream portions of trap apertures. Fluid flows detected inside the drifting traps, which ranged from 1.2 to 31 cm s⁻¹,l probably overwhelm all but the strongest “swimmers” that interact with these sampling divices. At our two sampling horizons (400 and 1500 m), tether-line motions generated trap depth oscillations with a period of the order of 10 s and an amplitude of about 0.5 m. Such effects have not been accounted for in flume simulated of sediment traps collection experiments.