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vol.30 número1  suppl.SympCommunity Structure of Euphausiids in the Southern Part of the California Current during October 1997 (El Niño) and October 1999 (La Niña)Revisiting in Situ Chlorophyll-a Data Along the Coast in North-Central Chile Considering Multiscale Environmental Variability 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 

El Niño 1997-98 by Means of
Planktonic Foraminifera off
Coquimbo, Chile *

M. Marchant1, C. Coloma2,
D. Hebbeln3

1 Department of Zoology, University of Concepcion,
Casilla 160-C, Concepcion. Chile,
2 Magister of Zoology, University of Concepcion,
Casilla 160-C, Concepcion, Chile.
3 Faculty of Geology, Bremen University,
Postfach 330440, D-28334 Bremen, Germany

The Peru-Chile Current (PCC), extending to about 40° latitude and associated with continuous coastal upwelling, is the most productive region in the world ocean within the Eastern Boundary Currents (EBC) (>200 gC m-2 y-1, Berger et al., 1989).

Generally, the PCC extends to a few degrees south of the equator before turning westward to become the South Equatorial Current (SEC). Influenced by the PCC and intense coastal upwelling, relatively cold water (e.g. sea surface temperature (SST) of 14°C in winter and 19°C in summer at 30°S; 73°W, Marchant et al., 1998) is found over an extensive region off the coasts of Chile and Peru during most of the year. The highest temperatures extend furthest south during the southern summer (Strub et al., 1998). The high phytoplankton concentrations induced by upwelling along the South American coast support one of the world's richest fisheries (Wallace et al., 1988).

In Chile (30ºS), the prevailing winds are upwelling-favorable throughout the year (Strub et al.,1998). Along the coast of northern Chile (approximately 18° to 30°S), a region of relatively low pigment concentrations (often <0,3 mgm-3) persists throughout the year, with slightly elevated concentrations in austral winter (July-September, 1 mg m-3; Thomas et al., 1994).

At irregular intervals of a few years, the high temperatures extend 3 to 5 degrees of latitude further south than usual (Pickard and Emery, 1982) (e. g. 1991/92 SST of 15°C in winter and 21,5°C in summer at 30°S; 73°W, Marchant et al., 1998) and the thermocline deepens by 100 m or more (Ramage, 1986). This is the oceanographic condition known as El Niño (EN).

The mechanistic explanation for El Niño is that weaker-than-normal south-east trade winds in the tropical Pacific - the Southern Oscillation- cause an eastward return-flow from the warm water pool of the western Pacific. This strengthens the eastward flowing North Equatorial Countercurrent (NECC) (Pickard and Emery, 1982). When this strengthened current reaches the South American coastline, the thermocline deepens and the water that upwells is now warm; upwelling decreases, further reducing the supply of the nutrients to the food web. This anomaly in the temperature persists for a year or more (Ramage, 1986).

The terms EN describe the warming of surface waters along the South American coast, while the term Southern Oscillation (SO) stands for mainly atmospheric perturbations in the tropical Pacific. The negative phase of the SO occurs during El Niño events.

The 1980's and 1990's have featured a very active ENSO cycle, with 5 El Niño episodes (1982/83, 1986/87, 1991-1993, 1994/95, and 1997/98). The period also featured two of the strongest El Niño episodes of the century (1982/83 and 1997/98), as well as two consecutive periods of El Niño conditions during 1991 - 1995 without an intervening cold episode (Climate Prediction Center, 2000). However, the 1990-1995 event was mainly restricted to the tropical Pacific (ENSO), and only occasionally resulted in a remarkable sea surface warming during 1991/92 along the South American coasts (Marchant et al., 1998).

Little is known about the seasonal variability in upwelling and accompanying productivity in the PCC. However, such variability can be traced by studying the seasonal succession of planktonic foraminifera species and by studying the structure of the water column as recorded in the foraminiferal stable isotope signal. Comparing e.g. stable isotope data of planktonic foraminifera species prefering different habitats in terms of water depth allows the detection of variations in e.g. thermocline depth and upwelling intensity (Ravelo and Fairbanks, 1992; Steens et al., 1992, Marchant et al., 1998).

The 45 species of planktonic foraminifera are grouped into five zoogeographic zones i.e polar, subpolar, transition, subtropical and tropical zone (Bé and Tolderlund, 1971).

The investigation area off the Chilean port of Coquimbo is situated in a boundary zone between subantarctic and tropical conditions, characterized by a mixed planktonic foraminiferal fauna. This fauna consists of cold water species such as Globigerina bulloides and Globigerinita uvula, and warm water species such Neogloboquadrina dutertrei and Globigerinoides ruber.

Based on the variations in coiling direction of N. pachyderma, tentatively traced the limit between the subantarctic zone and the transitional zone at 33°S (Boltovskoy 1976).

Within the frame work of the international Chilean JGOFS, FONDAP and FONDECYT projects, sediment traps have been deployed in the Peru-Chile Current at 2300 m water depth and 100 nm off the Chilean coast near Coquimbo (30°; 73°W).

Therefore, the principal aims of this project are to estimate the impact of El Niño on the planktonic foraminifera fauna by means of the analysis of flux (or concentration), faunal composition and from stable isotope studies on the shells of the planktonic foraminifera, abd also, to compare the intensity of El Niño events during 1991/92 and 1997/98.

The sediment traps have been retrieved approximately every six months. As they are each equipped with 20 sample bottles, the temporal resolution is 6 to 13 days per sample. An almost continuous record of particle flux, and thus also of the planktonic foraminiferal flux, exists since 1991 to the present day.

The seasonal cycle under Normal Conditions (1993/94) is marked by high planktonic foraminiferal fluxes (daily average 4,000 shells m-2 d-1, >150 µm) between August and January (southern winter to summer) and low fluxes during the rest of the year (southern summer to winter). High foraminiferal fluxes are coincident with increased coastal upwelling as indicated by stable oxygen isotope data of the planktonic foraminifera species Globigerina bulloides and Neogloboquadrina pachyderma (dex.). The isotope data also point to a stratified water column without upwelling in early 1994 concomittant with a low flux of foraminifera (Marchant et al., 1998).

A comparison of the daily fluxes averaged over several months or years demostrates that the foraminiferal fluxes in the PCC are among the highest, comparable only with station Papa in the Northeast Pacific (3,200 shells m-2 d-1, >150 µm, 1 year average; Reynolds and Thunell, 1989). In other upwelling regions in the Eastern Pacific, such as the California Current and the Panama Basin (Thunell and Reynolds 1984, 1992; Ortiz and Mix, 1992), foraminiferal fluxes are considerably lower.

However, a three year record from station Papa shows strong interannual variability with fluxes between 1700 shells m-2 d-1 (>125 µm), during the El Niño 1982/83 and 3,200 shells m-2 d-1 (125 µm) during the post-El Niño period (Reynolds and Thunell, 1989). Comparable interannual variability is also seen in subsequent investigations in the PCC, which shows a considerable decrease in the annual carbonate flux after June 1994. Nevertheless, even under ENC (1991/92, 1997/98) (3,000 shells m-2 d-1, >150 µm, 6 month average) the PCC appears to be a region marked by very high foraminifera production.

* Fondecyt Project N°1010912.

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