Controls on mesopelagic particle fluxes in the Sub-Antarctic and Polar Frontal Zones in the Southern Ocean south of Australia in summer—Perspectives from free-drifting sediment traps

Abstract

The SAZ-Sense project examined ecosystem controls on Southern Ocean carbon export during austral summer (January–February 2007) at three locations: P1 in the low biomass Subantarctic Zone (SAZ) west of Tasmania, P3 in a region of elevated biomass in the SAZ east of Tasmania fuelled by enhanced iron supply, and P2 in High-Nutrient/Low Chlorophyll (HNLC) Polar Frontal Zone (PFZ) waters south of P1 and P3. Sinking particles were collected using (i) a cylindrical time-series (PPS3/3) trap for bulk geochemical fluxes, (ii) indented rotating sphere (IRS) traps operated as in-situ settling columns to determine the flux distribution across sinking-rate fractions, and (iii) cylindrical traps filled with polyacrylamide gels to obtain intact particles for image analysis. Particulate organic carbon (POC) flux at 150 m (PPS3/3 trap) was highest at P1, lower at P2, and lowest at P3 (3.3±1.8, 2.1±0.9, and 0.9±0.4 mmol m −2 d −1, respectively). Biogenic silica (BSi) flux was very low in the SAZ (0.2±0.2 and 0.02±0.005 mmol m −2 d −1 at P1 and P3, respectively) and much higher in the PFZ (2.3±0.5 mmol m −2 d −1 at P2). Hence, the high biomass site P3 did not exhibit a correspondingly high flux of either POC or BSi. Separation of sinking-rate fractions with the IRS traps (at 170 and 320 m depth) was only successful at the PFZ site P2, where a relatively uniform distribution of flux was observed with ∼1/3 of the POC sinking faster than 100 m d −1 and 1/3 sinking slower than 10 m d −1. Analysis of thousands of particles collected with the gel traps (at 140, 190, 240, and 290 m depth) enabled us to identify 5 different categories: fluff-aggregates (low-density porous or amorphous aggregates), faecal-aggregates (denser aggregates composed of different types of particles), cylindrical and ovoid faecal pellets, and isolated phyto-cells (chains and single cells). Faecal-aggregates dominated the flux at all sites, and were larger in size at P1 in comparison to P3. The PFZ site P2 differed strongly from both SAZ sites in having a much higher abundance of diatoms and relatively small-sized faecal-aggregates. Overall, the particle images suggest that grazing was an important influence on vertical export at all three sites, with differences in the extents of large aggregate formation and direct diatom export further influencing the differences among the sites.