Temporal dynamics of nitrogen loss in the coastal upwelling ecosystem off central Chile: Evidence of autotrophic denitrification through sulfide oxidation

Abstract

Coastal upwelling areas are highly productive marine systems in which the development of oxygen‐depleted conditions and the availability of diverse electron donors (e.g., organic matter, , H2S) favor the processes involved in nitrogen (N) loss. We characterize the temporal and vertical variability of anammox and denitrification over the continental shelf off central Chile (36.5°S), through 15N and 13C tracer experiments, including amendments with H2S and along with measurements of 15N2 and 15N2O production and oceanographic variables during a year (2009 to 2010). Restricted to the bottom waters, both anammox and denitrification contributed similarly (∼ 500 nmol N2 L−1 d−1) to N loss during spring, while a marked decrease in the activity of these processes occurred in summer (103 and 14 nmol N2 L−1 d−1 for anammox and denitrification, respectively). During fall, denitrification was the only contributor to the observed nitrogen deficit (894 nmol N2 L−1 d−1). Interestingly, a substantial increase in the rates of denitrification (∼ 1200 nmol N2 L−1 d−1) and dark 13C assimilation were observed after the addition of H2S, indicating an autotrophic contribution to denitrification, which could be fueled in situ by H2S emitted from sediments or produced in the water column. The observed patterns seem to be controlled (stimulated or inhibited) by the availability of oxygen, organic matter, and H2S. This study establishes the magnitude and co‐occurrence of the different processes responsible for N removal in the coastal upwelling system of central Chile. This linkage of the nitrogen, carbon, and sulfur cycles is relevant to a global climate change scenario.