Plain Language Summary

Marine snow are microscopic particles that form in the surface of the ocean and sink into the deep ocean. Most of these particles are the remains of dead algae and feces of tiny animals (zooplankton). The deeper the particles sink into the ocean before microbes or animals consume them, the longer it takes before the carbon in those particles can return to the atmosphere. In parts of the ocean where there is no oxygen, more particles sink to greater depths, for reasons that are not well-understood. We used an underwater camera to observe marine snow particles in the ocean just west of Mexico where there is little to no oxygen at depth. We compared the observations to predictions from several computer simulations representing hypothesized mechanisms to explain why particles are consumed less in water without oxygen. Our measurements suggest that one reason that particles sink to deeper depths here is because microbes consume the particles slowly when there is no oxygen. Meanwhile, zooplankton still break large particles into smaller ones and produce fecal pellets in these low oxygen waters.