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vol.30 número1  suppl.SympUpwelling is the Disturbance, not El Niño: Insights from Modelling Community Organization and Flow StructureSeasonal and Interannual Variability of the Particle Flux in the Humboldt Current off Chile índice de autoresíndice de materiabúsqueda de artículos
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Investigaciones marinas

versión On-line ISSN 0717-7178

Investig. mar. v.30 n.1 supl.Symp Valparaíso ago. 2002

http://dx.doi.org/10.4067/S0717-71782002030100064 

Nitrogen Cycling in Sediments from
Central Chile (36ºS): Seasonal and
Interannual Variability

Laura Farías1,2, Michelle Graco1,3,
Verónica Molina1,3 , Dimitri Gutierrez4,
Victor A. Gallardo2, Osvaldo Ulloa1,2,3

1 Programa Regional de Oceanografía Física y Clima,
Universidad de Concepción, Casilla 160-C,
Concepción 3, Chile, E-mail: lfarias@profc.udec.cl
2 Centro de Investigación Oceanográfica en el
Pacífico Sur-Oriental, (COPAS)
3 Departamento de Oceanografía. Universidad de
Concepción, Casilla 160-C, Concepción 3, Chile
4 Instituto del Mar del Perú, P. O. Box 22, Callao, Perú

Introduction & Objective

The nearshore region off central Chile is an important site of coastal upwelling subject to significant seasonal variations. The high biological productivity in the area depends basically on the wind strength that brings subsurface waters rich in nutrient and poor in oxygen (ESSW) into the euphotic zone. This oceanographic condition could be modified on different time scales by remote forcing such as coastal trapped waves and the ENSO cycle. In order to gain a better understanding of how seasonal and interannual variability in the physical, chemical, and biological conditions in the bottom waters affect the benthic N recycling, different studies have been conducted since 1992. These studies comprised measurements of physical and chemical parameters in the bottom water (Tº, S, O2 and nutrients) and in the sediment column (i.e., carbon , chlorophyll-a, redox, NH4+ pool), NH4+ and NO3- fluxes across the water-sediment interface and some oxidation processes (i.e. nitrification, denitrification, sulfate reduction). All measurements and estimates were carried out at two stations, one located inside of the Bay of Concepcion (BC)(~28 m depth), and the other in the continental shelf at ~36º S (~88 m depth) between 1992 and 1994 (in the BC) and from 1997 to 2001 (at both stations).

Results & Discussion

The Bay of Concepción showed conspicuous seasonal geochemical and hydrographical differences in the bottom water oxygenation and the bottom fresh organic matter loading (carbon and chlorophyll-a) during the whole study period. The NH4+ flux showed a strong seasonal variation, being extremely high (up to 36 mmol m-2 d-1) during the upwelling periods and low or even negative during the winter. NO3- fluxes were positive and negative, varying by ca. + 2.5 mmol m-2 d-1. These flux patterns were associated with the inverse fluctuation between sulfate reduction and nitrification-denitrification rates in the area, being high for sulfate reduction and low for nitrification-denitrification during the upwelling period.

The continental shelf, however, showed more stable hydrographic and geochemical characteristics, maintaining sub-oxic levels throughout the study period, except during 1997-98 when unusually high oxygen concentrations were measured . The organic content of the sediments also varied on the seasonal scale, but at an order of magnitude lower than in the BC sediments. The NH4+ flux was highly variable, fluctuating between -22.4 and 10.7 mmol m-2 d-1 and NO3- was consumed at an average rate of -4.05 ± 0.90 mmol m-2 d-1. NO3- uptake from the water column was not totally accounted for by the measured denitrification rates, which ranged from 0.51 to 3.12 mmol m-2 d-1 and could be ascribed to active uptake by bacterial mats (Beggiatoa and Thioploca).

The sediments off central Chile acted as a large nitrogen sink for NO3-, and sometimes for NH4+ (possibly associated with bacterial NH4+ assimilation) and appeared to be a major site for denitrification. The results indicate that the fate of nitrogen regenerated from the sediments was mainly controlled by the amount of labile organic carbon (measured as chlorophyll-a) in the surface sediments, and indirectly by the bottom water oxygen concentration. The lower values of chlorophyll-a in sediments and the higher levels of oxygen during 1997 and the first half of 1998 compared with the same period in 1999-2001 were probably due to the influence of the 1997-98 El Niño event. In spite of the warm period reported during 1992-94 based on the equatorial Pacific index, in central Chile an extreme anoxia and high organic matter content, NH4+ flux and bacterial mats of the sediments were observed during upwelling periods. These strong differences in the organic content in the sediments and NH4+ fluxes during the study period suggest that the classic concept of Ekman transport, induced by local forcing, could be substantially modified by other processes such as El Niño events, as was observed on the continental shelf off central Chile during 1997-98. Moreover, the fertilization of surface waters not only explains the high primary production in the region, but also a combined effect of advective, turbulent, and biological processes determining that each upwelling event is singular in terms of productivity and sinking of organic matter towards the sediments. The consequences of different chemical qualities of the upwelling events also determine differences in the fate of the organic matter in the sediments and of the N recycling into the water column. The results, in terms of mass balance among periods defined as upwelling, non upwelling and upwelling-El Niño, are discussed in relation to the benthic-pelagic coupling.

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