versão On-line ISSN 0717-7178
Investig. mar. v.30 n.1 supl.Symp Valparaíso ago. 2002
SESSION 4: BENTHIC
Comparing the Effects of El Niño/La
Niña Phases of ENSO on the
Continental Margin Benthos of the
California and the Humboldt Current
1Centro de Investigaciones Oceanográficas en el
Pacifico Sur-Oriental, Universidad de Concepción,
Casilla 160-C, Concepción 3, Chile,
2Integrative Oceanography Division, Scripps Institution
of Oceanography, La Jolla, CA 92093-0218, USA,
3Instituto del Mar del Perú, P.O. Box 22, Callao, Perú,
E-mail: firstname.lastname@example.org, email@example.com
Introduction & Objectives
The California and the Humboldt Current systems have similar oceanographic settings, but their shelf and upper slope benthic ecosystems differ widely. Both systems are subjected to strong seasonal upwelling and high production in surface waters. However, the positive effect of this production on the benthos is restricted on the Peruvian and Chile margin by a lack of oxygen. Although high production generates a distinct midwater oxygen minimum layer in both the California and Humboldt systems, there are important differences between the systems in the intensity of the oxygen minimum, the degree to which it impinges on the shelf and upper slope, and moreover, the way it responds to the warm (El Niño) and the cold (La Niña) phases of the ENSO cycle. While the central and southern Peruvian shelf is subjected to permanent hypoxia, the Chilean shelf experiences seasonally varying, somewhat less intense hypoxia. Off Peru and Chile oxygen deficiency above the shelf bottoms is all but eliminated and a deepening of the oxycline is observed during strong El Niño events. In contrast, the oxygen minimum layer off California (with values down to 0.3 ml L-1) is centered at 500-600 m, does not impinge upon the shelf, and thus is much less affected by El Niño events. Our main objective is to compare the benthic responses to El Niño in the continental margin of the California and Humboldt marine ecosystems.
Results & Discussion
Benthic responses to ENSO cycles off central Chile and Perú are driven largely by changes in oxygenation of shelf bottom waters and in organic matter flux to the sediments. These changes are dramatic and involve shifts in community composition, community trophic structure, and bioturbation. After prolonged cold, productive, La Niña phases, bacterial mats of the giant, sulfide-oxidizing, nitrate-reducing Thioploca spp. dominate the OMZ-related shelf benthic community composition, while the warm El Niño phase quickly and effectively destroys them. Off central Chile, another response to El Niño is the decrease of the `freshness' of the biopolymeric fraction of the sedimentary organic matter, especially in shallower, semi-enclosed areas. During the 1997-98 El Niño, the macrofaunal assemblages changed from a predominance of tube-dwelling interface-feeders to dominance by subsurface-deposit feeders and endofaunal omnivores/predators. In addition, several observations indicate an enhancement of bioturbation in the margin sediments off central Chile and Perú.
ENSO-driven changes in the California shelf benthos are more subtle, and are limited largely to individual species and to shifts in mobile megafauna such as Sicyonia ingentis and Pleuroncodes planipes. New species appear and others disappear on the California shelf during El Niño events, probably caused by migration of warm water masses from the south. The most dramatic responses are seen among megafauna at upper slope depths (300-500 m). Mats of sulfur bacteria (Beggiatoa spp.) are present in hypoxic southern California borderland basins (580 m), but do not appear to respond to ENSO cycles. This presentation will explore hydrographic, faunal, and ecosystem changes in the benthos associated with El Niño and La Niña off Callao, Peru (12°S ), Concepción, Chile (36°S), and Southern California (32-33° N ). We will discuss in particular the influence of ENSO-driven oxygen minimum zone variation in creating shifts in food web structure and the role of chemosynthetic production.