Is Southern Ocean organic carbon and biogenic silica export enhanced by iron-stimulated increases in biological production? Sediment trap results from SOIREE

During the first in situ, mesoscale iron fertilisation experiment conducted in the Southern Ocean (SOIREE, 61 degreesS 140 degreesE, austral summer, F ebruary 10-22, 1999), export processes beneath an iron-stimulated phytoplan kton bloom were monitored using free-drifting sediment traps. Duplicated ar rays were deployed during days T7-9 and T11-13 inside the iron-fertilised p atch with single "control" arrays deployed outside the patch on days T0-2 a nd T11-13. Average total mass and chlorophyll a flux at 110 m approximately doubled between TO-2 and T7-9 (395-735 mg dry weight m(-2) d(-1) and 50-11 0 mug chl a m(-2) d(-1), respectively), while particulate organic carbon (P OC), particulate organic nitrogen (PON) and phaeopigment fluxes increased o nly slightly (15-30%) and biogenic silica (BSi) fluxes were essentially con stant (90-100mgm(-2)d(-1)). Inside the patch between T7-9 and T11-13, mass flux remained at about 735-745 mg m(-2) d(-1) while other flux components i ncreased by 30-40%. Mean POC and silica fluxes inside the patch at T11-13 w ere about 185 and 150mgm(-2)d(-1), respectively, compared with 80 and 90 mg m(-2) d(-1) measured outside. However, flux variations of > 50% between the two "control" deployments at TO-2 and T11-13 confounded the trapping exper iment. POC and PON fluxes at deep traps were typically 30-50% of those at s hallow traps, whereas total mass and silica fluxes at 310 m were generally 70- > 90% of the flux at 110 m, indicating substantial remineralisation of POC, relative to silica, over the top 300 m of the water column. As also su ggested by other biogeochemical proxies (Th-234, delta C-13), POC and bioge nic silica export from the SOIREE patch did not increase measurably in resp onse to iron-stimulated increases in primary production and a floristic shi ft to large, heavily silicified, chain-forming diatoms. The temporal decoup ling between new and export production was accompanied by considerable alga l accumulation in the mixed layer, reductions in phytoplankton sinking rate s inside the patch, and low levels of mesozooplankton grazing during SOIREE . Since elevated satellite-derived algal concentrations persisted near the experimental site for 30-45 d after SOIREE, a conservative estimate of pote ntial total nux from the bloom is calculated (6-9 g C m(-2)). Considerable logistical constraints must be overcome, however, in order to quantify the magnitude of carbon export from iron-mediated blooms at high southern latit udes before we can establish links between Southern Ocean productivity and global climate change, as encapsulated in the "Iron Hypothesis". (C) 2001 E lsevier Science Ltd. All rights reserved.