Summary
: The KErguelen : Ocean and Plateau compared Study (KEOPS) examined the origin of elevated phytoplankton biomass over the Kerguelen plateau in the Southern Ocean in mid-summer (Jan. - Feb. 2005). Here we report sinking particle characteristics determined from image analysis of thousands of individual particles caught in viscous polyacrylamide gels placed in free-drifting sediment traps (at depths of 100, 200, 330, 430m) at two sites during KEOPS - the highest biomass site over the central plateau (A3), and a moderate biomass site at its periphery (C5). Based on morphology using low-power stereo-microscopy and image analysis, the particles were divided into three types i) marine 'snow' aggregates, ii) cylindrical faecal pellets , and iii) oval faecal pellets. We estimate that 2 to 11% the sinking flux occurs in the form of discrete faecal pellets (median imaged areas ranging from .01 to .05 mm2 across all depths) at C5, and 21 to 34% at A3, with the remainder in the form of large (.04 to .09 mm2) 'marine snow' aggregates. Because these aggregates are themselves predominantly formed from faecal pellets, the overall flux is controlled by grazing, rather than by phyto-detrital aggregation. This observation that the vast majority of the particle flux is processed through the heterotrophic foodweb, in this region of persistent iron inputs, contrasts with relatively small contributions from zooplankton to the control of carbon export as observed in short-term artificial iron fertilization experiments. The KEOPS results offer more appropriate scaling of the response of production and export to persistent iron fertilization than do short-term experiments.