Revista Chilena de Historia Natural 77 (2): 305-318, 2004
RESEARCH ARTICLE
Nitrogen sediment fluxes in an upwelling system off central Chile (Concepción Bay and
adjacent shelf) during the 1997-1998 El Niño
Two stations representing coastal and shelf environments (bay of Concepción and
adjacent shelf, respectively) were sampled in order to assess benthic nitrogen fluxes. During the study period anomalously high oxygen
bottom waters conditions were observed, coinciding with the 1997-1998 El Niño event. From both sites, intact sediment cores were
collected for incubation experiments to directly estimate total ammonium fluxes. Porewater chemistry was also assessed to estimate
diffusive fluxes of nitrogen compounds (NH<sub>4</sub><sup>+</sup>, NO<sub>3</sub><sup>-
</sup> and NO<sub>2</sub><sup>-</sup>). The NH<sub>4</sub><sup>+</sup> flux from
incubation experiments ranged from 0.55 to 2.18 mmol m<sup>-2</sup> day<sup>-1</sup> in the inner bay and
from 1.84 to 2.14 mmol m<sup>-2</sup> day<sup>-1</sup> on the shelf. NO<sub>3</sub><sup>-
</sup> diffusive fluxes to the sediments were observed in the bay in March ( 17 x 10<sup>-2</sup> mmol m<sup>-
2</sup> day<sup>-1</sup>) and on the shelf in November (-2.7 x 10<sup>-2</sup> mmol m<sup>-
2</sup> day<sup>-1</sup>), associated with the lowest NH<sub>4</sub><sup>+</sup> diffusive
fluxes and coincident with a stronger gradient of Eh in the first centimeters of the sediments. Besides, total
NH<sub>4</sub><sup>+</sup> fluxes lower than in normal periods were observed, suggesting the influence of less
reducing conditions, consequence in turn of the lower fluxes of particulated organic material and higher dissolved oxygen in the bottom
waters. These conditions were probably imposed by the El Niño event. On the other hand, and relative to previous reports, an increase
of irrigation coefficients (DI) was estimated. Thus, the flux extension of other nitrogen compounds could have acquired relevance in
relation to NH<sub>4</sub><sup>+</sup> fluxes during the study period.
Key words:
ammonia, pore-water
nutrients, sediment-water exchanges, coastal upwelling, El Niño event
