Temporal variability of nitrogen cycling in continental-shelf sediments of the upwelling ecosystem off central Chile

The continental shelf region off central Chile (∼36 °S), one of the widest and most productive areas of the eastern South Pacific, is an important site of coastal upwelling. In order to understand how seasonal and inter-annual variability in bottom-water physical and chemical conditions affect benthic nutrient regeneration and sediment characteristics in this area, ammonium (NH4+) and nitrate (NO3−) fluxes at the water–sediment interface were experimentally quantified (March 1998–April 2001), along with net NH4+ production, potential nitrification and denitrification rates (November 1998–August 2000). NH4+ fluxes to the overlying water up to 10.4 mmol m−2 d−1, occurred during the upwelling season (i.e. austral spring and summer), while NH4+ removal from the water column up to −5.7 mmol m−2 d−1 during non-favorable upwelling conditions was observed (i.e., austral winter and the 1997–1998 El Niño condition). The fate of the benthic N regenerated as NH4+ appears to be controlled by the amount of labile organic carbon (here indexed as chlorophyll-a) in the surface sediment and, indirectly, by the bottom-water oxygen concentration. The balance between net NH4+ production and potential nitrification (4.4–34.3 and 0.3–2.9 mmol m−2 d−1, respectively) does not support the observed NH4+ fluxes, suggesting the occurrence of other NH4+ dissamilative (by dissolved metal or anammox) or assimilative consuming processes. Throughout the entire study period, the sediments acted as a large sink for NO3− (−3.4±1.4 mmol m−2 d−1) and as an important denitrification site (0.6–2.9 mmol m−2 d−1) coupled with NO3− produced by nitrification (58–97%). Other processes such as NO3− ammonification or active NO3− uptake by Thioploca mats could account for NO3− uptake from the water column.