Sediments under the coastal upwelling area off central Chile (36º S): Source or sink for nitrogen ?
Michelle Graco ( mgraco@udec.cl )

The upwelling area off central Chile (UACCH, 36º S) is one of the most productive coastal systems in the world (~ 4-20 g C m^-2 d^-1), characterized by middlewaters with high nitrate and low oxygen conditions- HNLO- (NO3- 15-25 microM, O2 <45 microM). The continental shelf sediments underlying by this HNLO and high Particulate Organic Matter (POM) input (>1 g C m^-2 d^-1), are considerer in a global scale as an important sink of nitrogen (N), by both, particulate organic N burial and intense gaseous N production by denitrification (Middelburg et al., 1996; Codispoti et al., 2001). However, the denitrification estimations are few and often based in indirect measurements and budgets (e.g., NO3- fluxes). The goal of this work was study the temporal variability (1999 and 2000 years) and environmental regulator factors of various “key” microbial processes involved in the recycling of nitrogen (ammonification, nitrification, denitrification and dissimilatory nitrate reduction to ammonium), as well as the exchange of inorganic dissolved nitrogen (IDN: ammonium and nitrate) across the sediment-water interface in the UACCH continental shelf sediments. The UACCH sediments, under a seasonal high organic input (>600 mg C m^-2 d^-1) and dominant HNLO conditions, appear to annually recycle 2-7 mol N m^-2 as ammonium to the water column, whereas ca. 0.5 mol N m^-2 as gaseous N, representing 30-40% and 1-10%, respectively, of the total N input to the sediment. An ammonium dominance in the benthic N cycle in the UACCH differ from what has been reported for other coastal sediments under HNLO (e.g. upwelling regimes), as well as from the predictions of current biogeochemical models, which assume that most of the remineralized N is channeled through gaseous forms and ultimately lost from the system (>75%). While in the UACCH sediments important denitrification rates appear (1 to 3 mmol m^-2 d^-1), also exist a large ammonium release, suggest a poor coupling between ammonification-bacterial mats DNRA and denitrification. In consequence, sediments under HNLO and high input of organic matter conditions and giant bacterial mats, such as the coastal upwelling regimes as Benguela, Perú, and the Arabian Sea, or eutrophic areas, could also present a similar nitrogen dynamics with an important ammonium release to the water column. If this ammonium increase the lost of N in the water column or the productivity appear essential to understand the N cycling in the ocean.