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vol.30 número1  suppl.SympA Comparative Analysis of the Community Structure of the Central Chile Marine Ecosystem During an El Niño (1992) and La Niña (1998) ConditionsThe CHOC (Chile-Peru Oxygen Minimum and Circulation) Program 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/S0717-71782002030100067 

Effects of El Niño 1997-98 on Particle
Fluxes from two Coastal Upwelling
Areas: Northern Chile and
Southern California

O. Romero1, C. Lange2, D. Hebbeln1

1Department of Geosciences, Bremen University,
POBox 33 04 40, 28334 Bremen, Germany,
E-mail: oromero@allgeo.uni-bremen.de
2Departamento de Oceanografía, Universidad de
Concepción, Casilla 160 C, Concepción 3, Chile

Objectives

We present results from two sediment trap time series (1993-1998), one off Coquimbo in northern Chile (CH, ~30°S, 73°W) and a second one from the Santa Barbara Basin, California (SBB, ~34°N, 120°W). We have chosen to compare these two sites because they are both located in upwelling areas, and are influenced by strong seasonal and interannual changes. This allows evaluation of the effect of physical forcing on particle fluxes, especially for El Niño events. Results from sediment trap studies, in conjunction with measurements of oceanographic conditions, are of relevance to paleoceanographers who need to identify and calibrate climate-ocean proxies which can be applied to the sedimentary record.

Results

Bulk components

Our analyses of fluxes of siliceous microorganisms, organic carbon, calcium carbonate, biogenic silica, and lithogenic particles at the SBB site show that overall flux variations at depth are closely coupled with oceanographic conditions at the surface. Lithogenic material is the single largest contributor to the total mass flux, typically accounting for 50-80 % of the total. Terrigenous input, as measured by the lithogenic flux, was much higher during the 1997-98 El Niño period (up to 8 g m-2 d-1 vs. 1-2 g m-2 d-1 for the previous non-El Niño period). Both biogenic opal and organic carbon fluxes mirror the normal productivity cycle in that fluxes are usually high during the boreal spring-summer upwelling period and low during boreal fall and winter. Opaline silica is by far the dominant biogenic sediment produced in SBB, periodically accounting for up to 35 % of the total flux. Organic carbon fluxes almost always account for less than 5 % of the total flux. However, for the winter of 1998 organic carbon fluxes were unusually high for that time of the year (up to 0.15 g m-2 d-1 compared to values of 0.05 g m-2 d-1 for the previous winters), and coincided with a February peak in the carbonate flux and high abundances of tintinnids of the genus Stenosemella.

Off northern Chile, the flux of particulates of the 1993-94 period showed a clear seasonal trend, with almost 40% of the annual total flux between mid-Aug and mid-Oct 1993 (late austral winter-early spring), while 54% of the total biogenic opal was trapped in the same period. The major flux event (6.6 g m-2 d-1) occurred in late austral winter (low SSTs) and was characterized by the highest contribution of biogenic opal (ca. 1.7 g m-2 d-1). Lithogenic material, organic carbon, and CaCO3 fluxes also peaked at this time. Some secondary maxima were observed in the total flux until late austral spring, dominated by calcium carbonate. From mid austral summer to late fall 1994, total flux diminished and calcium carbonate contributed the most to the particle flux. A significant decrease in fluxes was observed for the 1997-98 period; however, the well-defined seasonal pattern was retained, and the highest export event also occurred in late austral winter. At that time (mid-Aug and mid-Oct 1997), 31% of the annual total mass and 50% of the total biogenic opal was trapped.

Siliceous microplankton

At both trap locations, SBB and off Chile, diatoms were the main contributors to the biogenic opal fraction. In general, fluxes of diatoms, radiolarians and silicoflagellates in the SBB exhibit a distinct seasonal pattern, with marked production maxima at different times of the year reflecting a succession of these microplankton groups: radiolarians in late summer and fall, silicoflagellates in winter, and diatoms in spring. This "normal" scenario was altered during the 1997-98 El Niño event. In the nearly 5 years of sediment trap data, from August 1993 to April 1998, the lowest diatom fluxes ever measured (2.6 and 4.5 x 103 valves m-2 d-1) occurred at the height of the El Niño in February 1998.

Off Chile, highest daily contributions of diatoms (ca. 70 x 105 valves m-2 d-1) and silicoflagellates (ca. 42 x 104 skeletons m-2 d-1) occurred in late austral winter 1993. Radiolarians also peaked between late austral winter and mid spring, but their maximum preceded that of diatoms and silicoflagellates by ca. 10 days. Siliceous plankton flux during 1997 showed the same general pattern as during 1993-94: a strong winter maximum dominated by diatoms, with silicoflagellate and radiolarian fluxes one and two orders of magnitude lower, respectively.

Discussion

The California coastal waters were strongly influenced by El Niño conditions beginning in boreal summer of 1997. Terrigenous input to our sampling site, as measured by the lithogenic flux, was significantly higher during the El Niño period presumably reflecting higher rainfall and runoff into the basin. Opal fluxes decreased in winter 1998, and major changes in the contribution of siliceous microplankton assemblages were observed during El Niño conditions: (1) Diatom fluxes were an order of magnitude lower and species richness was higher than in the 1996 non-El Niño period. (2) The flux of radiolarians was 20 % lower in late 1997- early 1998 when compared to the 1993-1996 period. (3) The fall-winter peak in silicoflagellate fluxes, seen annually from 1993 to 1996, was missing in 1997. In addition, major changes in species composition were recorded, including a significant increase in the proportion of warm water flora and fauna, and a decrease in the relative contribution of the siliceous microorganisms indicative of boreal spring upwelling conditions in the SBB. For the SBB, our findings point to a reduction in biogenic opal export production in the region and an increase in siliceous microplankton diversity consistent with the prevailing El Niño conditions. This reduction in export production may have been a consequence of reduced intensity of coastal upwelling and lowered biogenic opal productivity in the upper waters.

Off Chile, the differences in siliceous plankton flux between 1993-94 and 1997-98 were also related to oceanographic disturbances derived from the 1997-98 El Niño event. The decline in the 1997 fluxes was probably due to poorly mixed subsurface waters and reduced intensity of coastal upwelling. The decrease during the El Niño 1997-98 seems to affect different groups of siliceous organisms to different degrees: on an annual basis, diatom and silicoflagellate export is approximately 75% lower under El Niño conditions than in a "normal" year, while radiolarian flux is reduced by 40%. However, the seasonal timing of particle export from the surface waters to the deep ocean off northern Chile varies little under both "normal" and El Niño conditions. Biogenic opal and siliceous plankton fluxes depict almost unimodal patterns of downward transport, attaining their annual maximum within 3-5 weeks in winter, in coincidence with the seasonal pattern of highest pigment concentrations. This shows that the export of siliceous microorganisms is primarily controlled by the seasonal cycle of production in the photic zone.

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