Abstract13 C tracer experiments were conducted at sites spanning the steep oxygen, organic matter, and biological community gradients across the Arabian Sea oxygen minimum zone, in order to quantify the role that benthic fauna play in the short-term processing of organic matter (OM) and to determine how this varies among different environments. Metazoan macrofauna and macrofauna-sized foraminiferans took up as much as 56 6 13 mg of added C m 22 (685 mg C m 22 added) over 2-5 d, and at some sites this uptake was similar in magnitude to bacterial uptake and/or total respiration. Bottom-water dissolved oxygen concentrations exerted a strong control over metazoan macrofaunal OM processing. At oxygen concentrations .7 mmol L 21 (0.16 ml L 21 ), metazoan macrofauna were able to take advantage of abundant OM and to dominate OM uptake, while OM processing at O 2 concentrations of 5.0 mmol L 21 (0.11 ml L 21 ) was dominated instead by (macrofaunal) foraminiferans. This led us to propose the hypothesis that oxygen controls the relative dominance of metazoan macrofauna and foraminifera in a threshold manner, with the threshold lying between 5 and 7 mmol L 21 (0.11 to 0.16 ml L 21 ). Large metazoan macrofaunal biomass and high natural concentrations of OM were also associated with rapid processing of fresh OM by the benthic community. Where they were present, the polychaete Linopherus sp. and the calcareous foraminiferan Uvigerina ex gr. semiornata, dominated the uptake of OM above and below, respectively, the proposed threshold concentrations of bottom-water oxygen.1 Present address: Danish Meteorological Institute, Lyngbyvej 100, 2100 Copenhagen, Denmark. AcknowledgmentsThe experimental work in this study was conducted aboard the RRS Charles Darwin. We thank Oli Peppe and Eric Breuer for running the lander and megacorer. We also thank two anonymous reviewers for their constructive feedback.

[1] The biological pump affects atmospheric CO 2 levels and fuels most heterotrophic activity in the deep ocean. The efficiency of this pump depends on the rate of carbon fixation, export out of the euphotic zone and the depth of respiration. Here we study the depth dependence of respiration patterns, hence particulate carbon flux, using a compiled data set of sediment oxygen consumption rates. We show that the depth relationship can best be described by a double exponential model. For the upper part of the ocean, our resulting equation is similar to previous fluxdepth relationships but predicted fluxes are significantly larger in deeper waters. This implies a more efficient biological pump. Total oceanic respiration below the shelf break (200 m) is estimated to be 827 Tmol O 2 yr À1.

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