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

The continental shelf region off central Chile (similar to36 degreesS), 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 Nino 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. (C) 2004 Elsevier Ltd. All rights reserved.