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

Abstract

During the first in situ, mesoscale iron fertilisation experiment conducted in the Southern Ocean (SOIREE, 61°S 140°E, austral summer, February 10–22, 1999), export processes beneath an iron-stimulated phytoplankton bloom were monitored using free-drifting sediment traps. Duplicated arrays were deployed during days T7–9 and T11–13 inside the iron-fertilised patch with single “control” arrays deployed outside the patch on days T0–2 and T11–13. Average total mass and chlorophyll a flux at 110 m approximately doubled between T0–2 and T7–9 (395–735 mg dry weight m−2 d−1 and 50–110 μg chl a m−2 d−1, respectively), while particulate organic carbon (POC), particulate organic nitrogen (PON) and phaeopigment fluxes increased only slightly (15–30%) and biogenic silica (BSi) fluxes were essentially constant (90–100 mg m−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 increased by 30–40%. Mean POC and silica fluxes inside the patch at T11–13 were about 185 and 150 mg m−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 T0–2 and T11–13 confounded the trapping experiment. POC and PON fluxes at deep traps were typically 30–50% of those at shallow 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 suggested by other biogeochemical proxies (234Th, δ13C), POC and biogenic silica export from the SOIREE patch did not increase measurably in response to iron-stimulated increases in primary production and a floristic shift to large, heavily silicified, chain-forming diatoms. The temporal decoupling between new and export production was accompanied by considerable algal accumulation in the mixed layer, reductions in phytoplankton sinking rates 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 potential total flux 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 latitudes before we can establish links between Southern Ocean productivity and global climate change, as encapsulated in the “Iron Hypothesis”.