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Investigaciones marinas

versión On-line ISSN 0717-7178

Investig. mar. v.30 n.1 supl.Symp Valparaíso ago. 2002 

Impacts of El Niño Events on the
Coastal Benthos of the Eastern Pacific
Upwelling Systems*

Juan Tarazona1, Wolf E. Arntz2,
Sonia Valle3

1 Facultad de Ciencias Biológicas, Universidad
Nacional Mayor de San Marcos, Lima, Perú,
2 Alfred Wegener Institute for Polar and Marine
Research, Section for Comparative Ecosystem
Postfach 12 01 61, 27515 Bremerhaven, Germany,
3Centro de Investigación Científica y Educación
Superior de Ensenada (CICESE), Baja California,


Several papers have compared the impacts of El Niño (EN) events on the benthos of the California and Peruvian coastal upwelling ecosystems. However, most studies are hardly comparable, owing to differences of methodology and scale. Therefore, a comparative analysis of oceanographic changes generated by EN events in both upwelling systems, their impacts on benthic populations and communities of shallow areas, and mechanisms and processes which control the changes has been undertaken.


Oceanographic changes

The ENSO cycle constitutes the major intra-decadal variability in the California (~ 23 - 42° LN) and Peruvian (~ 4 - 40° LS) coastal upwelling systems. This variability includes: an increment of sea temperature, nearshore sea level rise, lower trade wind stress which normally drives upwelling, alterations in water circulation, increase in dissolved oxygen concentration, deepening of the thermocline, more frequent Kelvin waves towards and along the coast, agitated seas, changes of salinity and decrease in nutrient transport to the surface (Arntz & Tarazona, 1990). These oceanographic changes are responsible for a decrease in primary productivity of up to nearly 50%, as well as for a series of other positive and negative changes in benthos distribution and abundance.

The intensity of the oceanographic changes induced by EN is apparently stronger in the Humboldt current upwelling ecosystem. Thus, during the latest EN events, temperature anomalies of up to 10°C were recorded in the Peruvian coastal current, whereas the anomalies in the California current were mostly up to 3°C and in extreme cases up to 6°C (Felonov & Tereshchenko, 2000; Hayward, 2000; Tarazona & Arntz, 2001). On the other hand, the beginning and duration of EN events tends to be simultaneous in both systems (Huyer & Smith, 1985).

El Niño impacts on benthic populations and communities

In both systems, significant changes in latitudinal distribution of some benthic species associated with EN events have been reported. In the latest EN events, southward displacements of over 10° of latitude in the Humboldt coastal current area have been recorded (Arntz & Fahrbach, 1991; Paredes et al., 1998; Tarazona & Arntz, 2001). A poleward increase in distribution of many species is observed in both systems, and it is believed that the subsurface current which flows polewards, and is intensified during EN, plays an important part in this process. Changes in the vertical distribution of subtidal and continental shelf benthos during EN are mainly associated with the increase in dissolved oxygen concentration. One of the most characteristic manifestations of the positive impact of EN is the proliferation of some benthic species populations with commercial value, such as the scallop Argopecten purpuratus and the stalked barnacle Pollicipes elegan (Kameya, 1986; Wolff, 1987; Arntz et al., 1987 and 1988). The negative impact of EN is drastic in the subtidal areas of both systems, for example in the kelp forests Macrocystis pyrifera and the belts of the commercial mussel Aulacomya ater and the intertidal mussel Semimytilus algosus (Soenens, 1985; Tegner & Dayton, 1987; Tarazona et al, 1988; Fernández et al., 1999; Tegner, 2001). In most cases, the process of post-EN recovery of populations and communities is completed during the cold phase, including La Nina, before the next EN event. In general, intertidal species show a faster recovery than subtidal species.

The benthos of sandy beaches of both ecosystems develops extraordinarily high densities of some key species. EN events induce drastic changes in the zonation of the benthic communities of these beaches, which change their structure and dynamics, and induces fluctuations of dominant species, for example in the bivalves Mesodesma donacium and Donax marincovichi and the crustaceans Emerita analoga and Excirolana braziliensis (Efford, 1965; Penchaszadeh, 1971; Tarazona et al., 1988; Arntz et al., 1987). Soft bottom hypoxia increases with depth. As a consequence, benthic communities decrease in diversity and show a dominance of opportunistic species which are tolerant to oxygen deficiency. EN events, by increasing dissolved oxygen at the sea floor, induce an ecological succession to more mature stages. During this process, immigrants which increase competition and predation are very important.


Mechanisms and processes which control the biological impact of El Niño events

The mechanisms inducing positive or negative effects during the ENSO cycle are not yet completely understood, however, they seem to be associated with tolerance of environmental factors, such as temperature changes or oxygen deficiency, life strategies, species interactions, capacity for metabolic adaptation and genetic predisposition. For the first three of these facets, three hypotheses considered essential for understanding mechanisms and processes associated with EN biological impacts on the shallow benthos are discussed. These hypothese may serve to establish a conceptual model generalized for both upwelling systems. The first hypothesis considers that the additional production of particles in coastal upwelling systems constitutes a stimulus for the explosive development of some benthic suspension feeders. The second points out that changes in circulation and stratification of waters during EN has an important control on larval transport and settlement, determining the success or failure of recruitment events. The third one assumes that there is a hierarchy of factors which control the development of communities: the first hierarchical level would be the influence of the intensity of ENSO-induced disturbances (e.g. mechanical, temperature or hypoxic stress); the second level, the particularities of life history of species in the communities; and the third level, the biological interactions which are established during EN events. The two other facets mentioned above have received insufficient attention and require further study.

* Keynote presentation.

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