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

Abstract

Abstract.A sediment trap moored in the naturally ironfertilized Kerguelen Plateau in the Southern Ocean provided an annual record of particulate organic carbon and nitrogen fluxes at 289 m. At the trap deployment depth, current speeds were typically low ( ∼ 10 cm s −1 ) and primarily tidaldriven (M2 tidal component). Although advection was weak, the sediment trap may have been subject to hydrodynamical and biological (swimmer feeding on trap funnel) biases. Particulate organic carbon (POC) flux was generally low (< 0.5 mmol m −2 d −1 ), although two episodic export events (< 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 in surface chlorophyll and together accounted for approximately 40 % of the annual flux budget. The annual POC flux of 98.2 ± 4.4 mmol m −2 yr −1 was low considering the shallow deployment depth but comparable to independent estimates made at similar depths (∼ 300 m) over the plateau, and to deep-ocean (> 2 km) fluxes measured from similarly productive iron-fertilized blooms. Although undertrapping cannot be excluded in shallow moored sediment trap deployment, we hypothesize that grazing pressure, including mesozooplankton and mesopelagic fishes, may be responsible for the low POC flux beneath the base of the winter mixed layer. The importance of plankton community structure in controlling the temporal variability of export fluxes is addressed in a companion paper.

Planktic foraminifer and coccolith contribution to carbonate export fluxes over the central Kerguelen Plateau

“…The carrousel comprised 12 sampling cups (250 mL) containing 5% formalin hypersaline solution buffered with sodium tetraborate (pH¼8). A detailed description of the sample processing and particulate organic carbon (POC) analyses are provided in Rembauville et al (2015b). Briefly, swimmers (zooplanktonic organisms actively entering the trap) were manually removed, samples were freeze-dried and the carbonate fraction was dissolved by the addition of acid before the organic carbon content was measured with a CHN analyzer.…”

“…At the A3 station, the circulation is weak (o3 cm s À 1 ) and primarily tidal-driven (Park et al, 2008). Physical data acquired during the sediment trap deployment suggest the record was not subject to major hydrodynamic biases (Rembauville et al, 2015b), allowing a detailed and quantitative discussion of the export fluxes.…”

“…For the fine fractions (20-63 mm and o20 mm) Ca analyses, the original 1/8 split samples (Rembauville et al, 2015b) were further split into 1/80 aliquots with a rotary wet-splitter (McLane WSD-10) using purified water (Elix by Millipore purification system) buffered with ammonia as a rinse solution. Coccoliths in sinking particles captured in sediment trap samples may be contained in faecal pellets and/or phytoplankton aggregates.…”

“…Only one empty shell of pteropod (Limacina helicina) was found in the samples and therefore pteropod's contribution to the passive carbonate flux was considered negligible. However, numerous pteropods were found as swimmers (distinguished by well preserved organic material) actively entering the trap in late summer (Rembauville et al, 2015b). Those shells were withdrawn from the samples as they were considered not to contribute to the passive flux.…”

“…The annual POC export (98.2 mmol m À 2 yr À 1 , Rembauville et al, 2015b) and the annual PIC export (6.6 mmol m À 2 yr À 1 ) at station A3 allow us to estimate the strength of the carbonate counter pump: the reduction of the CO 2 drawdown by the biological pump due to the CO 2 production during the calcification process in the mixed layer (Frankignoulle et al, 1994;Zeebe, 2012;Salter et al, 2014). As the trap depth (289 m) was close to the winter mixed layer depth (220 m in this region of the Southern Ocean (Park et al, 1998;de Boyer Montégut et al, 2004), POC fluxes were not corrected for attenuation with depth.…”