Ecosystem controls on carbon export efficiency from the naturally iron-fertilised phytoplankton bloom over the Kerguelen Plateau

In the ocean, the perpetual 'snowfall' of biogenic marine particles exports organic carbon from the well-lit surface layer to the deep sediments, promoting its sequestration. The efficiency of this 'biological carbon pump' (BCP), presents strong spatio-temporal variations that are not yet fully explained. Changes in surface plankton communities and trophic interactions appear important because they lead to modifications of sinking particle characteristics (e.g. composition, structure, sinking velocity). These controls are explored here via the characterization of sinking particles originating from varying planktonic community structures and evaluation of their ability to export carbon. During the second KErguelen Ocean and Plateau compared Study (KEOPS2) conducted in Oct.-Nov. 2011, six sites were sampled over and downstream of the Kerguelen Plateau (Southern Ocean), where a mosaic of phytoplankton blooms of changing communities forms in response to natural iron fertilisation. Sinking particles were collected with free-drifting sediment traps at four mesopelagic depths to examine their form and composition, including optical characterization using polyacrylamide gel filled traps. Concurrently, aggregates were formed in roller tanks from surface water phytoplankton assemblages, to explore the intricate influences of particle size, structure and composition on the sinking velocity. At each site, carbon export efficiencies were calculated as the ratio of carbon flux to net primary productivity (e-ratio). High productivity was associated with the lowest carbon export efficiency (e-ratio ~0.02) while maximum export efficiency (e-ratio ~0.2) was found at low-productivity sites. Two explanations were identified. Firstly, at high-biomass sites, strong zooplankton grazing generated large fecal pellets sustaining high carbon fluxes at 100-200 m (180 mg C m⁻² d⁻¹). This export pathway represented a 'dead end' due to rapid attenuation of the fecal pellet flux at 200-400 m, releasing most of the carbon (48±21 % carbon flux decrease). Secondly, based on the roller tank results, the morphology of dominant diatom species appeared to be an important control on aggregate sinking velocities, possibly via species-specific coagulation efficiency affecting particle structure and density. At high-biomass sites, dominant small spine-forming species formed loose slow-sinking aggregates (~10 m d⁻¹), whereas chain-forming diatoms without spines, at low-productivity sites, produced compact fast-sinking aggregates (~250 m d⁻¹). The similarity of aggregate morphology and structure from roller tank and gel traps (2-D fractal number = 1.8 and 1.9 respectively), increased confidence in applying these results to in situ conditions. A generic 3-D physical-biogeochemical (BGC) model was modified to investigate conceptually the influence of planktonic community variations on carbon export efficiency through their control on detritus composition and sinking velocity. This was achieved by introducing in the model a variable detritus sinking velocity based on phytoplankton and zooplankton detrital fractions (using experimental results). Changes from a constant (100 m d⁻¹) to a variable sinking velocity induced a significant increase of the annual integrated carbon flux at 100 m (45±23 %) highlighting the importance of sinking velocity parameterisation in BGC models. Simulations indicate that export efficiency could depend upon subtle trophic interactions between phyto- and zooplankton communities influencing the relative rates of productivity and associated carbon flux and determining the conditions of biomass retention or export. The insights and tools developed here improve our understanding of how a climate-mediated shift of surface plankton communities can alter the efficiency of the BCP.