The relative importance of phytoplankton aggregates and zooplankton fecal pellets to carbon export: insights from free-drifting sediment trap deployments in naturally iron-fertilised waters near the Kerguelen Plateau

摘要

The first KErguelen Ocean and Plateau compared Study (KEOPS1), conducted inthe naturally iron-fertilised Kerguelen bloom, demonstrated that fecalmaterial was the main pathway for exporting carbon to the deep ocean duringsummer (January–February 2005), suggesting a limited role of direct exportvia phytodetrital aggregates. The KEOPS2 project reinvestigated this issueduring the spring bloom initiation (October–November 2011), when zooplanktoncommunities may exert limited grazing pressure, and further explored the linkbetween carbon flux, export efficiency and dominant sinking particlesdepending upon surface plankton community structure. Sinking particles werecollected in polyacrylamide gel-filled and standard free-drifting sedimenttraps (PPS3/3), deployed at six stations between 100 and 400 m, to examineflux composition, particle origin and their size distributions. Resultsrevealed an important contribution of phytodetrital aggregates (49 ± 10and 45 ± 22% of the total number and volume of particlesrespectively, all stations and depths averaged). This high contributiondropped when converted to carbon content (30 ± 16% of total carbon,all stations and depths averaged), with cylindrical fecal pellets thenrepresenting the dominant fraction (56 ± 19%).At 100 and 200 m depth, iron- and biomass-enriched sites exhibited thehighest carbon fluxes (maxima of 180 and 84 ±27 mg C m-2 d-1, based on gel and PPS3/3 trap collectionrespectively), especially where large fecal pellets dominated overphytodetrital aggregates. Below these depths, carbon fluxes decreased (48 ±21% decrease on average between 200 and 400 m), and mixed aggregatescomposed of phytodetritus and fecal matter dominated, suggesting an importantrole played by physical aggregation in deep carbon export.Export efficiencies determined from gels, PPS3/3 traps and 234Thdisequilibria (200 m carbon fluxet primary productivity) were negativelycorrelated to net primary productivity with observed decreases from~ 0.2 at low-iron sites to ~ 0.02 at high-iron sites. Varyingphytoplankton communities and grazing pressure appear to explain thisnegative relationship. Our work emphasises the need to consider detailedplankton communities to accurately identify the controls on carbon exportefficiency, which appear to include small spatio-temporal variations inecosystem structure.