High biomass, low export regimes in the Southern Ocean

Phoebe J. Lam and James. K. B. Bishop

Deep-Sea Research II
54 (2007) 601-638

Abstract. This paper investigates ballasting and remineralization controls of carbon sedimentation in the Twilight Zone (100-1000 m) of the Southern Ocean. Size-fractionated (<1 um, 1-51 um, >51 um) suspended particulate matter was collected by large-volume in-situ filtration from the upper 1000 m in the Subantarctic (55 S, 172 W) and Antarctic (66 S, 172 W) zones of the Southern Ocean during the Southern Ocean Iron Experiment (SOFeX) in JanuaryFebruary 2002. Particles were analyzed for major chemical constituents (POC, P, biogenic Si, CaCO3), and digital and SEM image analyses of particles were used to aid in the interpretation of the chemical profiles.

Twilight Zone waters at 66 S in the Antarctic had a steeper decrease in POC with depth than at 55 S in the Subantarctic, with lower POC concentrations in all size fractions at 661S than at 551S, despite up to an-order-of magnitude higher POC in surface waters at 661S. The decay length scale of <51-um POC was significantly shorter in the upper Twilight Zone at 66 S (delta_e = 26 m) compared to 55 S (delta_e = 81 m).

Particles in the carbonate-producing 55 S did not have higher excess densities than particles from the diatom-dominated 66 S, indicating that there was no direct ballast effect that accounted for deeper POC penetration at 55 S. An indirect ballast effect due to differences in particle packaging and porosities cannot be ruled out, however, as aggregate porosities were high (~97%) and variable.

Image analyses point to the importance of particle loss rates from zooplankton grazing and remineralization as determining factors for the difference in Twilight Zone POC concentrations at 55 S and 66 S, with stronger and more focused shallow remineralization at 66 S. At 66 S, an abundance of large (several mm long) fecal pellets from the surface to 150 m, and almost total removal of large aggregates by 200 m, reflected the actions of a single or few zooplankton species capable of grazing diatoms in the euphotic zone, coupled with a more diverse particle-feeding zooplankton community immediately below.

Surface waters with high biomass levels and high proportion of biomass in the large-size fraction were associated with low particle loading at depth, with all indications implying conditions of low export. The 66 S region exhibits this high biomass, low export (HBLE) condition, with very high >51-um POC concentrations at the surface (~2.1 uM POC), but low concentrations below 200 m (<0.07 uM POC). The 66 S region remained HBLE after iron fertilization. Iron addition at 55 S caused a 10 fold increase in >51-mm biomass concentrations in the euphotic zone, bringing surface POC concentrations to levels found at 66 S (~3.8 uM), and a concurrent decrease in POC concentrations below 200 m. The 55 S region, which began with moderate levels of biomass and stronger particle export, transitioned to being HBLE after iron fertilization. We propose that iron addition to already HBLE waters will not cause mass sedimentation events. The stability of an iron-induced HBLE condition is unknown. Better understanding of biological pump processes in non- HBLE Subantarctic waters is needed.

Figure: excerpts from Figure 6 Lam and Bishop (2007). Upper Digital reflected light images of >51-um MULVFS filters at 55 S cast 1; (Lower) and at 66 S cast 3. Samples collected from the upper "Twilight Zone". Images taken against black and white backgrounds highlight fluffy white and dense pigmented particles that contribute to carbon sedimentation. The square grid marks are approximately 12 mm spacing.

55 S (LOW BIOMASS ABOVE) Note visibly more sinking material at depth

66 S (HIGH BIOMASS ABOVE) Note little aggregate material penetrates below 136 m

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