Export fluxes in a naturally iron-fertilized area of the Southern Ocean – Part 1: Seasonal dynamics of particulate organic carbon export from a moored sediment trap

摘要

A sediment trap moored in the naturally iron-fertilized Kerguelen Plateau inthe Southern Ocean provided an annual record of particulate organic carbonand nitrogen fluxes at 289 m. At the trap deployment depth, current speedswere typically low (~ 10 cm s?1) and primarilytidal-driven (M2 tidal component). Although advection was weak, the sedimenttrap may have been subject to hydrodynamical and biological (swimmer feedingon trap funnel) biases. Particulate organic carbon (POC) flux was generallylow (< 0.5 mmol m?2 d?1), although two episodic exportevents (< 14 days) of 1.5 mmol m?2 d?1 were recorded.These increases in flux occurred with a 1-month time lag from peaks insurface chlorophyll and together accounted for approximately 40% of theannual flux budget. The annual POC flux of 98.2 ± 4.4 mmol m?2 yr?1 was low considering the shallow deployment depth but comparableto independent estimates made at similar depths (~ 300 m) overthe plateau, and to deep-ocean (> 2 km) fluxes measured fromsimilarly productive iron-fertilized blooms. Although undertrapping cannotbe excluded in shallow moored sediment trap deployment, we hypothesize thatgrazing pressure, including mesozooplankton and mesopelagic fishes, may beresponsible for the low POC flux beneath the base of the winter mixed layer.The importance of plankton community structure in controlling the temporalvariability of export fluxes is addressed in a companion paper.