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Abstract The eastern Indian Ocean is substantially under sampled with respect to the biological carbon pump – the suite of processes that transport the carbon fixed by phytoplankton into the deeper ocean. Using sediment traps and other ecosystem measurements, we quantified sinking organic matter flux and investigated the characteristics of sinking particles in waters overlying the Argo Abyssal Plain directly downstream of the Indonesian Throughflow off northwest Australia. Carbon export from the euphotic zone averaged 7.0 mmol C m^-2d^-1, which equated to an average export efficiency (export / net primary production) of 0.17. Sinking particle flux within the euphotic zone (beneath the mixed layer, but above the deep chlorophyll maximum) averaged slightly higher than flux at the base of the euphotic zone, suggesting that the deep euphotic zone was a depth stratum of net particle remineralization. Carbon flux attenuation continued into the twilight zone with a transfer efficiency (export at euphotic depth + 100m / export at euphotic depth) of 0.62 and an average Martin’sb-value of 1.1. Within the euphotic zone, fresh phytoplankton (chlorophyll associated with sinking particles, possibly contained within appendicularian houses) were an important component of sinking particles, but beneath the euphotic zone the fecal pellets of herbivorous zooplankton (phaeopigments) were more important. Changes in carbon and nitrogen isotopic composition with depth further reflected remineralization processes occurring as particles sank. We show similarities with biological carbon pump functioning in a similar semi-enclosed oligotrophic marginal sea, the Gulf of Mexico, including net remineralization across the deep chlorophyll maximum. Submitted to: Deep-sea Research II HighlightsDespite low productivity, export efficiency was 17% of primary productionFlux attenuation beneath the euphotic zone (EZ) was low for a tropical regionSinking particle flux from the upper to lower EZ exceeded export from lower EZThe deep EZ was a stratum of net particle remineralization (and net heterotrophy) 

Abstract The last two decades have shown the importance of Rhizaria in the biogeochemical cycles of carbon and silicon in modern oceans. This eukaryotic supergroup, which includes Radiolaria and Phaeodaria, represents an important part of zooplanktonic carbon biomass and contributes to carbon and silica export. Still, accurate estimations of their carbon biomass are hindered by poor knowledge of their elemental composition, contrasting with well‐established allometric carbon‐to‐volume relationships for smaller protists such as phytoplankton. Here, we directly measured carbon, nitrogen, and biogenic silica content as well as silicon uptake rates of planktonic Rhizaria. We highlight that size can be used as a predictor of elemental content for a broad variety of planktonic Rhizaria ranging from 200 μm to several mm, whereas size is weakly correlated with silicon uptake rates. Our results indicate that the scaling exponent of the carbon‐to‐volume allometry is significantly lower than those for smaller protists, underlining the low carbon strategy of these organisms. Still, we show that carbon and nitrogen densities span over four orders of magnitude, possibly accounting for the differences in depth ranges, nutritional modes and colonial or solitary forms. We estimate Rhizaria sinking speeds by combining carbon, nitrogen, and silica content data and show that great variability exists among the different taxa. Besides giving a better understanding of rhizarian ecology and biogeochemistry, these analyses, at the individual scale, are a first step to subsequent biomass and flux estimations at larger scales.