Microbial degradation on POC flux in the eastern tropical Pacific oxygen minimum zone

One of the main uncertainties regarding organic carbon fluxes to the deep ocean is why such a large fraction of organic matter reaches the deep sea in oxygen minimum zones (OMZs). Most remineralisation is carried out by bacteria and zooplankton, implying that the behaviour or metabolic rates of one or both classes of organisms must be severely perturbed in low oxygen regions. Here we present estimates of downward particulate organic carbon (POC) flux between 50 and 350 m made using Marine Snow Catchers (MSCs), in the Eastern Tropical North Pacific (ETNP), a region characterized by near zero oxygen levels below 50 m. The efficiency with which POC flux was transferred to depth was high (> 60 %), consistent with previous findings. We addressed the role of bacterial degradation in controlling this flux by measuring the respiration of sinking organic material. Slow sinking particles were degraded at least an order of magnitude faster than fast sinking particles, with both fractions having constant degradation rates with depth. Microbial degradation of POC can almost completely account for the loss of fast sinking particles in the offshore (bottom depth > 1000 m) region. In the onshore region microbial degradation only explains a small amount of the flux attenuation with depth. We suggest this occurs because fast sinking material in this region is fragmented by zooplankton into slow sinking particles, as degradation rates overestimated remineralisation on these particles. Oxygen minimum zones are likely to expand - meaning that the depth at which organic matter is respired is likely to increase, leading to an enhanced biological storage of carbon in the deep ocean.