Sinking marine particles in oxygen minimum zones produce ammonium, even at depths at which ammonium is undetectable in the water column

Ocean biogeochemical processes are driven both by free living bacterioplankton and those attached to particles, and evidence suggests that bacteria in these two environments have distinct yet complementary roles. We combined diel and spatial sampling of ammonium concentrations throughout the water column with measurements of ammonium production from sinking particles that were collected and incubated on particle-collector/incubators traps. We observed that while ammonium concentrations were usually evident at concentrations up to one micro-molar below the primary chlorophyll maximum in the upper oxycline above the oxygen minimum zone, they were undetectable within the core of the oxygen minimum zone throughout the incubation. Particles, however did produce ammonium throughout the oxygen minimum zone. The production of ammonium by particles but its absence throughout the oxygen minimum zone suggests that it is removed efficiently by microorganisms in the low oxygen water column, presumably either through anammox or assimilation processes. We also hypothesized that migrating swarms of zooplankton and micronekton, observed via acoustics, would correspond with diel increases in ammonium concentration, as those organisms are known to produce ammonium. We did not see such a signal, again suggesting that any ammonium produced by these organisms is quickly removed in the water column.