Optical imaging of mesopelagic particles indicates deep carbon flux beneath a natural iron‐fertilized bloom in the Southern Ocean

Abstract

We recorded vertical profiles of size distributions of particles (ranging from 0.052 to several mm in equivalent spherical diameter) in the natural iron-fertilized bloom southeast of Kerguelen Island (Southern Ocean) and in surrounding high-nutrient, low-chlorophyll (HNLC) waters with an Under Water Video Profiler during the Kerguelen Ocean and Plateau Compared Study cruise (Jan-Feb 2005). Total particle numerical abundance and total particle volume (TPV) in the 0-200-m layer were respectively 3-fold and 20-fold higher in the bloom, and integrated TPV was correlated to integrated chlorophyll concentration. The difference persisted well into the ocean interior with a 10-fold higher TPV at 400-m depth beneath the natural iron-fertilized bloom. Below 400 m, increases in TPV values at the bloom stations reflect the suspension of bottom sediments. Bloom waters had a greater proportion of large particles from the surface to 400 m and also exhibited an increase of this proportion with depth compared to HNLC waters. Multiple visits to the bloom reference Sta. A3, suggest preferential removal of large particles as the bloom declined. Comparing our particle abundance size spectra with those observed previously in polyacrylamide gel-filled sediment traps allows us to estimate mesopelagic particle sinking rates. These results suggest that particles sink faster in the HNLC waters than beneath the bloom. The fact that sinking speeds were not a simple monotonic function of particle size and varied spatially highlights the need to go beyond parameterizations of sinking rate as a function of size alone.

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

“…As shown by various studies on particle size distribution in different regions of the ocean (e.g. Jackson et al, 2005;Ebersbach and Trull, 2008;Guidi et al, 2008;Stemmann et al, 2008;Jouandet et al, 2011), the approach of determining particle size spectra provides insights in the context of carbon transport from the surface ocean into the mesopelagic zone. This is an important step towards prediction of carbon flux in response to changing environments.…”

Optical techniques for remote and in-situ characterization of particles pertinent to GEOTRACES

“…These are useful to estimate (1) macrozooplankton abundance (Checkley et al, 2008;Stemmann et al, 2008a), (2) particle flux through the water column (Guidi et al, 2008;Guidi et al, 2009;Jackson and Burd, 2002;Jouandet et al, 2011), and (3) particle size distribution in mesoscale spatial patterns (Guidi et al, 2012; see e.g. Fig.…”

“…These instruments have been used to study biogeochemical cycles combining particle information from camera systems and sediment trap measurements of particle flux in order to better estimate particle sinking rate, a characteristic influencing particle export (Asper, 1987;Asper et al, 1992;Diercks and Asper, 1997;Gardner and Walsh, 1990;Guidi et al, 2008;Iversen et al, 2010;Jackson et al, 1997;Jouandet et al, 2011;McDonnell and Buesseler, 2010;Peterson et al, 2005;Walsh and Hunter, 1992). Systems such as the In Situ Ichthyoplankton Imaging System ISIIS (Cowen and Guigand, 2008), the Zooplankton Visualization and Imaging System ZOOVIS (Benfield et al, 2007) and the Shadowed Image Particle Profiler and Evaluation Recorder (SIP-PER, Samson et al, 2001) allow counting and sizing of large zooplankton from images.…”

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

“…Temporal variability of flux in the Southern Ocean precludes extrapolation of discrete measurements to estimate seasonal or annual carbon export. However, seasonal export of POC can be derived from biogeochemical budgets Jouandet et al, 2011;Pollard et al, 2009) or be directly measured by moored sediment traps (e.g. Salter et al, 2012).…”

“…Picheral et al, 2010, Jouandet et al, 2011 or by directly collecting particles into surface-tethered sediment traps (e.g. Maiti et al, 2013 for a compilation in the Southern Ocean) or neutrally buoyant sediment traps (e.g.…”