Abstract
During the second KErguelen Ocean and Plateau compared Study (KEOPS2) in OctoberNovember 2011, marine snow was formed in roller tanks by physical aggregation of phytoplankton assemblages sampled at 6 stations over and downstream of the Kerguelen Plateau. Sinking velocities, morphology, bulk composition (transparent exopolymer particles, biogenic sil- ica, particulate organic carbon), and phytoplankton contents were measured individually on 66 aggregates to identify controls on sinking velocities. Equivalent spherical diameters (ESD) ranged from 1 to 12 mm, and the particle aspect ratios, Corey shape factors, and fractal dimensions (DF1 = 1.5, DF2 = 1.8) were close to those of smaller natural aggregates (0.2 to 1.5 mm) collected in poly- acrylamide gel-filled sediment traps (DF1 = 1.2, DF2 = 1.9). Sinking velocities ranged between 13 and 260 m d �1 , and were correlated with aggregate size only when considering individually the experiments conducted at each station, suggesting that a site-dependent control prevailed over the general influence of size. Variation in dominant diatom morphologies among the sites (classi- fied as small spine-forming or chain without spines) appeared to be a determinant parameter influencing the sinking velocity (SV (m d �1 ) = 168 � 1.48 × (% small spine-forming cells), r 2 = 0.98), possibly via a control on species-specific coagulation efficiency affecting particle structure and excess density. Our results emphasize the importance of ecological considerations over that of simple compositional perspectives in the control of particle formation, and in accurate parameter- izations of marine snow sinking velocities that are essential to predictions of biological carbon sequestration.
Highlights
In the ocean, the rain of large biogenic particles is a major pathway for the export of carbon from the surface to the interior (Suess 1980, Fowler & Knauer 1986, Asper et al 1992)
No significant statistical difference was found between aggregates made from water sampled in surface layers and deeper layers (1 > p > 0.45) for a selected sample of the measured properties, and no distinction is made between these 2 aggregate groups in the rest of the study
Theoretical excess densities were calculated assuming 3 major solid components in fixed volume fractions (TEP:0.3, Biogenic silica (BSi):0.2, particulate organic matter (POM):0.5). These proportions are based on measured values of BSi and particulate organic carbon (POC) presenting the best scaling with size and using a transparent exopolymer particles (TEP):POC mass ratio of 0.7 (Alldredge et al 1998, Martin et al 2011)
Summary
The rain of large biogenic particles is a major pathway for the export of carbon from the surface to the interior (Suess 1980, Fowler & Knauer 1986, Asper et al 1992). In the Southern Ocean, a region known to play a leading role in global climate and carbon systems (Sigman & Boyle 2000), primary productivity is low due to phytoplankton growth restriction by a combination of iron and light limitation (Martin 1990, de Baar et al 1995). These waters are described as ‘high nutrient, low chlorophyll’ (HNLC), and contain a large amount of unused surface macronutrients capable of fuelling the production of high quantities of organic carbon and potentially its sequestration. Like oases in deserts, some areas in the Southern Ocean are fertilized by a natural supply of iron and display remarkable seasonal biomass increases (Sullivan et al 1993), offering a great opportunity to investigate the controls on carbon export efficiencies in the context of iron fertilization
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