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Abstract Stable isotopes (15,14N, 18,16O) of dissolved inorganic nitrogen (N) were measured in sediment porewaters and benthic flux chambers across the Peruvian oxygen minimum zone (OMZ) from 74 to 1000 m water depth. Sediments at all locations were net consumers of bottom water NO 3 −. In waters shallower than 400 m, this sink was largely attributed to dissimilatory nitrate reduction to ammonium (DNRA) by filamentous nitrate-storing bacteria (Marithioploca and Beggiatoa) and to denitrification by foraminifera. The apparent N isotope effect of benthic NO 3 − loss (15ε app) was 7.4 ± 0.7‰ at microbial mat sites and 2.5 ± 0.9‰ at the lower fringe of the OMZ (400 m) where foraminifera were abundant. The OMZ sediments were a source of 15N-enriched NO 2 − (28.9 to 65.5‰) and NH 4 + (19.4–20.5‰) to the bottom water. Model simulations generally support a previous hypothesis attributing the 15NH 4 + enrichment to a coupling between DNRA and anammox (termed DAX) using biologically-stored NO 3 − from Marithioploca and NH 4 + from the porewater. The model predicts that 40% of NO 3 − that is actively transported into the sediment by Marithioploca is reduced to N 2 by this pathway. DAX enhances N 2 fluxes by a factor of 2–3 and accounts for 70% of fixed N loss to N 2. Moreover, because most of the ambient porewater NH 4 + is generated by DNRA, up to two-thirds of biologically-transported NO 3 − could end up being lost to N 2. This challenges the premise that Marithioploca -dominated sediments tend to conserve fixed N. By limiting the flux of 15NH 4 + back to the ocean, DAX also tends to decrease benthic N fractionation. Tracking the fate of NH 4 + once it leaves the sediment is critical for understanding how the benthos contributes to N isotope signals in the water column. [ABSTRACT FROM AUTHOR]

The distribution of zooplankton groups, with an emphasis on fish larvae, in the Oxygen Minimum Zone off southern Mexico (December 2020) was analyzed. A hydrographic section of five sampling stations was made in the confluence of Transitional Water and Tropical Surface Waters. In each station, horizontal zooplankton trawls on three different dissolved oxygen conditions (~100, < 44 and < 4.4 μmol kg−1) were carried out by a MOCNESS net (333 μm). The 100 μmol kg−1 oxypleth (oxic condition) was ~60 m depth along the section, but the 4.4 μmol kg−1 oxypleth (suboxic) rose southward from Transitional Water (~ 150 m) to Tropical Surface Water (~ 90 m), approaching the well oxygenated layer. The distribution of the zooplankton biomass, and the most abundant zooplankton groups (e.g. copepods, chaetognaths, ostracods, euphausiids) and fish larvae showed statistically significant differences (P < 0.01) between the oxic (100 μmol kg−1) and the deeper suboxic conditions. The larvae of typically dominant fish species such as the bathypelagic Vinciguerria lucetia, Diogenichthys laternatus, Diaphus pacificus and Cubiceps pauciradiatus, were present only in the oxygenated depths in the Transitional Water, and were almost absent from all depths in the Tropical Surface Water, where the oxycline shoaled. These differences in larval fish abundance were found despite little change in chlorophyll a concentration (relative units "r.u.") along the sections, indicating that the oxycline is a limiting factor for the fish larvae. The fish larvae results contrast with previous observations from the mouth of the Gulf of California, where some species have distributions independent of water column dissolved oxygen conditions, probably as a consequence of coastal processes. Overall, our results show that even within the OMZ, variations in oxycline depth have biological implications, particularly on meroplanktonic organisms. • Transitional Water and Tropical Surface Water converge in the OMZ off southern Mexico. • The suboxic layer (< 4.4 μmol kg−1) is close to the oxycline in Tropical Surface Water. • The highest zooplankton abundance is in the oxic layer. • Most fish larvae are in the oxic layer in the Transitional Water. • The suboxic water elevation impacts the meroplanktonic organisms (fish larvae). [ABSTRACT FROM AUTHOR]