R. Escribano, G. Daneri, L. Farías,
V. A. Gallardo, H. González,
C. B. Lange, C. E. Morales, S. Pantoja,
O. Pizarro, R. Quiñones,
W. Schneider, O. Ulloa

Centro de Investigación Oceanográfica del Pacífico
Sur-Este (COPAS)Universidad de Concepción, P.O.
Box 160C, Concepción, Chile,
E-mail: rescriban@netscape.net

The high biological productivity of the Humboldt Current System (HCS) is sustained by wind-driven upwelling. This system, however, is subject to major interannual variability caused by the ENSO cycle. The general view supports the idea that drastic changes may occur in the community structure and biological production as a consequence of alternating El Niño/La Niña periods. Although the number of studies dealing with El Niño effects on the physics, chemistry and biology of the HCS is steadily increasing, a synthesis has not been attempted so far. In particular, the recent 1997/98 El Niño (EN) provided an opportunity to carry out multidisciplinary studies in different regions along the Chilean coastal seas. This contribution is an attempt to synthesize information on the major changes associated with this event.

Based on large-scale fishery zones, three major upwelling regions (Fig. 1) can be distinguished along the HCS off Chile: 1) northern (18 - 26° S) , 2) central Chile (26 - 32 °S), and 3) southern (32 - 37° S) upwelling regions. Recent research indicates that the differences in the 1997-98 EN effects among these regions are associated with the timing of the onset of the event and the latitude-dependent background variability, and possibly with distance from the forcing zone. Differential effects in the cross-shelf field, mostly related to oceanographic boundaries for the upwelling process, also occur. The northern region revealed remarkable changes in the pelagic environment originating from a suppressed fertilizing effect of upwelling in the productive layer. Phytoplankton were subject to changes in community structure, with small-sized species prevailing during EN. Chlorophyll concentration fluctuated between high and low values, associated with pulses and relaxation periods of EN since the onset in May 1997 and throughout the rest of 1997 and part of 1998. Zooplankton assemblages also shifted to small-sized species, although total biomass did not suffer abrupt collapses compared to previous seasons. The benthic community which, due to the narrowness of the shelf and the steep slope, mostly occupies the shallow nearshore area, did not change much except for some species which increased in abundance during the event and then decreased after the next season. The major anchoveta fishery in this region was stable during 1997 but a sharp decrease in the total catch occurred during 1998. A tendency to recover to previous landing levels was observed during 1999 and 2000. A trend toward a stock decrease was evident before the EN (1995). After 1992 the largest catch occurred in 1994 with 1.3 million tons, followed by a gradual decrease reaching a minimum in 1998 of about 200 thousand tons.

Fig.1 The upwelling regions in the Humboldt Current System of Chile

The oceanographic changes associated with the 1997/98 EN in the central and southern regions included intrusions of oceanic, low-nutrient, warmer (the positive SST anomaly lasted until the austral winter of 1998), and more oxygenated waters into the coastal areas. These changes were responsible for the following effects: a) Reduction in the extension of the productive zone (upwelling was restricted to a narrow band near the coast); b) Change in the `normal' seasonal regime of spring/summer benthic hypoxia, resulting instead in nearly one year of continued oxygenation near the bottom and relatively lower fresh carbon input to the benthic subsystem, especially in the shallower sites; c) Significant increase in macrofaunal biomass in inshore sediments dropping to low values in the following spring/summer period after the EN; d) the all but disappearance of the filamentous bacteria Thioploca; e) benthic bioturbation enhancement, more intense in the deeper areas normally subjected to hypoxia; f) alteration of nursery and feeding habitats and concomittant behaviour changes of jack mackerel as a result of modified migratory routes.

The above EN consequences in the HCS revealed that the upwelling regions responded to altered oceanographic conditions. Abnormally high temperatures prevailed for nearly one year, thermocline and oxycline deepening occurred down to 200 m at times during the event, suppressed new production and low nutrients may have caused changes in species abundances and size structures and perhaps food shortage for fish larval success affecting recruitment, in conjuction with changes in migratory behaviour of fish populations, possibly increasing their dispersion and deepening. It is revealing, however, that despite the clearly altered oceanographic conditions for one, or even more than one year, there were no collapses of major fisheries and the production capacity in all trophic levels of the upwelling regions considered seemed to have recovered rapidly for the subsequent seasons after the EN retreat. Available information from the Peru region leads to similar conclusions.