Abstract

While oxygen minimum zones (OMZ) are frequently referred to as ‘dead zones’ from a biological standpoint, coastal OMZ can actually be highly dynamic in regard to nutrient cycling. Under the low-oxygen conditions (≤ 10 µM O2) that define OMZ , nutrients are released from the sediment to the water and can contribute to increased primary production in surface water. However, considerable nitrogen (N) loss to the atmosphere, one of the key processes limiting primary productivity, can also occur. Diapycnal mixing due to internal waves, coastal upwelling and lateral advective and diffusive transport can significantly enhance these processes by accelerating nutrient supply to the near-surface layers.

There are conflicting opinions on which biochemical processes (e.g., denitrification, dissimilatory nitrate reduction to ammonium, and/or anammox) control N cycling within OMZ . Furthermore, while it is known that internal waves can significantly affect nutrient fluxes, hydrodynamics in coastal OMZ can be highly variable and effects on the nutrient distribution are not currently understood. We thus performed a process study focusing on the Peruvian OMZ in the Eastern Tropical South Pacific boundary current system, a region which is characterized by both (1) an extreme N deficit due to N loss and (2) elevated non-linear internal waves. Paired microstructure and CTD data were used to assess water-column turbulence and nutrient concentrations, respectively, and benthic chambers were used to evaluate nutrient fluxes from the sediment. Velocity and hydrographic time series from a mooring array allowed for investigation of advective and eddy nutrient transport. With these data, a nutrient budget was determined by quantifying water-column flux divergences of N species at the upper continental slope and shelf regions. Together with N fluxes from the sediment, we established an overall N balance which we then used to evaluate which biochemical processes were likely contributors to N cycling in this critical region.