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

a b s t r a c tThe 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 71.8, 2.1 7 0.9, and 0.9 70.4 mmol m À 2 d À 1 , respectively). Biogenic silica (BSi) flux was very low in the SAZ (0.2 7 0.2 and 0.02 7 0.005 mmol m À 2 d À 1 at P1 and P3, respectively) and much higher in the PFZ (2.3 7 0.5 mmol m À 2 d À 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 À 1 and 1/3 sinking slower than 10 m d À 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 faecalaggregates. 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.

Development and deployment of a point‐source digital inline holographic microscope for the study of plankton and particles to a depth of 6000 m

“…Compact fecal pellets whether ovoid or cylindrical in shape will sink at a much faster speed through the water column than large amorphous and porous marine snow (Ebersbach et al 2011). In this study, the separation of particles into fecal pellet-type particles and "others" (including plankton and marine snow) reveal interesting patterns.…”

Projected decreases in future marine export production: the role of the carbon flux through the upper ocean ecosystem

“…In the Southern Ocean, phytodetrital aggre-gates can contribute up to 30 % of total carbon export (measured during the initiation of the spring bloom by Laurenceau et al (2015)). The available observations suggest that the contributions of phyto-and zooplankton to particle formation are both temporally and spatially variable in the Southern Ocean (Ebersbach and Trull, 2008;Bowie et al, 2011;Ebersbach et al, 2011;Smetacek et al, 2012;Quéguiner, 2013;Laurenceau et al, 2015), making it difficult to constrain the contribution of phytoplankton aggregation to particle formation on coarser temporal and spatial scales. Recently published metagenomic data and data on particle size distributions might be an important step forward in elucidating the complicated interplay between different members of the planktonic ecosystem and the carbon flux to depth (Guidi et al, 2016).…”

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

“…The b exponent in this formulation has been reported to range from 0.4 to 1.7 (Buesseler et al, 2007b;Lampitt et al, 2008;Henson et al, 2012) in the global ocean. Nevertheless, a change in the upper mesopelagic community structure (Lam et al, 2011) and, more precisely, an increasing contribution of mesozooplankton (Lam and Bishop, 2007;Ebersbach et al, 2011) could lead to a shift toward higher POC flux attenuation with depth.…”

“…The UVP (underwater video profiler) provides high-resolution images of particles (> 52 µm), and the particle size distribution is then converted to carbon fluxes using an empirical relationship (Guidi et al, 2008;Picheral et al, 2010). Drifting gel traps allow for the collection, preservation and imaging of sinking particles (> 71 µm) that are converted to carbon fluxes using empirical volume-carbon relationship (Ebersbach and Trull, 2008;Ebersbach et al, 2011;Laurenceau-Cornec et al, 2015). Finally, drifting sediment traps are conceptually similar to moored sediment traps but avoid most of the hydrodynamic biases associated with this technique (Buesseler et al, 2007a).…”