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vol.30 número1  suppl.SympShelf Fish Larval Abundance Along the West Coast of Baja California During a Period with two El Niño Events 1983-1987Primary Productivity and Biomass of Size-Fractionated Phytoplankton in an Upwelling Area, Bahía de Mejillones (23ºS, Chile), during and after the 1997-1998 El Niño 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/S0717-71782002030100026 

Effects of the 1997-98 El Niño on the
Oceanographic Conditions and
Zooplankton Community Structure in
the Coastal Upwelling System Off
Northern Chile*

H. E. González1, F. Pagés2, M. Sobarzo3,
R. Escribano4

1Universidad Austral de Chile, Instituto de Biología
Marina, P.O. Box 567, Valdivia, Chile,
E-mail: hgonzale@uach.cl
2Institut de Ciències del Mar (CSIC),
Paseo Marítimo 37-49, Barcelona, España
3Centro EULA, Universidad de Concepción, Chile
4Centro Oceanográfico del Pacífico Sureste,
Universidad de Concepción, Chile.

Changes in oceanographic conditions and zooplankton community structure were investigated during a five year period, which included the 1997-98 El Niño (EN), in the coastal region off Antofagasta (23ºS), northern Chile. We aimed at examining the changes in abundance and size composition of zooplankton, as well as the main oceanographic changes associated with the onset and evolution of the EN.

During January 1997, upwelling events were recorded in the coast (indicated by the rise of the 13ºC isotherm). The SSW (Subtropical water) and SAAW (Subantarctic water) occupied a narrow layer near the surface (0-20 and 20-40 m depths, respectively), while the ESSW (equatorial subsurface water) dominated below 40 m depth and reached the surface during the upwelling events. During July 1997 and January 1998 a conspicuous deepening of the isotherms was recorded in the coastal zone, predominating over the SSW in the upper 150 m of the water column (Table 1, Fig. 1).

Table 1. Average depth (m) of the 13ºC and 1 ml O2 l-1 isolines
in coastal waters off Mejillones.

Date

Average depth
(m) of the 13º
Cisoline.

Average depth (m)
of the 1 ml O2 l-1isoline.

11-16 January 1997

40

50

0

22-26 January 1997

35

40

0

01-04 July 1997

230

80

1997-98

27-29 January 1998

167

125

El Niño

17-22 October 2000

16

35

0

06-11 February 2001

38

42

0

01-07 October 2001

51

51

0


Fig. 1 Percentage (%) of water masses in coastal waters off Mejillones during cruises conducted between 1997 and 2001.

During the study period, the integrated (upper 100 m of the water column) zooplankton biomass (mgC m-2, Fig. 2a), showed high values (ranging between 21 and 28 mgC m-2), with the exception of July 1997, when a remarkable decrease was noted (4 mgC m-2). We suggest that the onset of the EN event combined with other factors, such as suppression of the nutrient fertilization of the productive layer during the winter period, might be partially responsible for such a reduction in biomass. However, during the most intense stage of EN (January 1998), the zooplankton biomass attained "average" values for the study area.


The size-spectrum distribution of the zooplankton (separated into euphausiids, large calanoid copepods, small calanoid copepods, cyclopoid copepods and appendicularians) showed a distinct shift towards small sized species during the EN event. Euphausiids contributed 73% of the total zooplankton carbon before the EN event (January 1997), decreasing to 27% during the maximum peak of EN (January 1998) and increasing their contribution up to 56% after three years with no EN. In contrast, the small-size groups of the zooplankton (small calanoids and cyclopoid copepods, and appendicularians) increased their contribution to the total zooplankton carbon significantly during EN (Fig. 2b). This, together with a decrease in the concentration of chlorophyll-a and POC, may have a direct impact on the carbon flux through the pelagic food web suggesting that during EN conditions a larger amount of the PP generated was channelled through ingestion by small-size animals than during non-EN conditions.



* This study was supported by FONDECYT Grant 1000419,
5960002-1996 and PCCI CONICYT/CSIC 2000-2-083.

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