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Investigaciones marinas

On-line version ISSN 0717-7178

Investig. mar. vol.30 no.1 suppl.Symp Valparaíso Aug. 2002

http://dx.doi.org/10.4067/S0717-71782002030100060 

Biological Background of "El Niño"
and "La Niña" in Intertidal and
Subtidal Communities of the Northern
Zone of Chile

Raúl Soto M.1, Sergio Muñoz M.1,
Rodrigo A. Moreno2

1Departamento de Ciencias del Mar, Universidad
Arturo Prat, Casilla 121, Iquique, Chile
2 Departamento de Zoología, Facultad de Ciencias
Naturales y Oceanográficas, Universidad de
Concepción, Casilla 160-C, Chile,
E-mail: rodmoren@udec.cl

"El Niño" and "La Niña" events occurring during the last decades of the 20th century and their effects on the intertidal as well as subtidal benthic communities of the northern zone of Chile are presented and discussed.

The northern zone of Chile is affected in a non-periodic way by the warm "El Niño" type events characterized by, among other aspects, the entrance of warm waters, deepening of the thermocline and increase of the oxygen levels on the bottom and at the surface by positive thermal anomalies of varying magnitude. Likewise, the presence of cold waters is associated in an irregular way with "La Niña" event.

The first records on the effects of "El Niño" events date from the 1970s, and they were only qualitative observations, with information about the presence of foreign species - mainly molluscs, crustaceans, fish and birds - being prominent. Since the "El Niño" 1982-83, which has been most documented in the northern zone due to its importance in Chile, several effects were observed in coastal communities of hard and soft bottoms. Among others a significant alteration was observed in the intertidal and subtidal superficial communities in regions I and II. A quantization applied to the intertidal communities of soft and hard bottoms has allowed us to discriminate in some way between the positive and negative aspects of the different "El Niño" and "La Niña" events.

The principal results obtained in the intertidal communities, associated with the "El Niño" events, are as follows:

Sandy beaches:

Alterations in the community structure, with the decrease of the bivalve Mesodesma donacium (macha) being more prominent in the beaches of region I, along with an increase in numbers of another bivalve - Donax sp. (machilla), whose presence was recorded at high densities later in the 1982-1983 and 1997-98 events, reaching biomasses of 5 kg and 1,5 to 2 kg per m2 respectively. Other populations suffered alterations in density, e.g. Emerita analoga and Excirolana brazilienzis, which also suffered the indirect effect of the so-called altiplano winter in the coastal zone of Arica (Playa Chinchorro) causing floods of continental waters which deposit, on the sandy intertidal, a sediment of terrigenous origin, very fine in comparison with the size of the sand particles in Playa Chinchorro. In these sediments, both populations showed very low densities in comparison with neighbouring zones. The crustaceous Callichirus garthi (nape), observed since the 1982-83 event, has remained with few population alterations and is even used and sold as bait by shore fishermen mainly in the zone of Arica.

Rocky beaches:

Flora: The main observation of qualitative and quantitative type refer to the presence of Lessonia nigrescens (chascón), an alga that, in the northern zone, is considered as a structural agent of the intertidal communities of hard bottoms and as the alga with the most biomass and cover, besides the great importance offered by its adhesive disk as a shelter for about 70 taxa of invertebrate macrofauna. "El Niño" events of major intensity directly affect the belt of Lessonia, causing a loosening of this alga and its posterior going aground.

Fauna: The main observations of qualitative and quantitative type have been performed in mitilid molluscs, mainly Perumytilus purpuratus and Semimytilus algosus, populations whose densities vary greatly during the intense events, and crustaceans. Observations of some species of the Grapsidae family are prominent; observations of the 1970s indicated that the major population densities in decreasing order were: Cyclograpus cinereus, Leptograpus variegatus, Geograpsus lividus and Grapsus grapsus. Observations of qualitative and quantitative type after the 1982-83 event, up until the year 2001, indicate that the densities are completely different, Grapsus grapsus being the most representative population in the rocky intertidal at present,with a variation of from 1 to 3 organisms per 100 linear meters of coast (1970) and 50 to 70 organisms per 100 m of rocky coast.

With respect to the probable effects of "La Niña" event in the rocky intertidal, results from a monitoring zone in Punta Cavancha, Iquique, lead us to infer that some changes occurred in the community structure after the 1997-98 "El Niño" event, for example, the populations of Semimytilus algosus and Perumytilus purpuratus still remained with a low population density. Most evident has been the presence of the balanid Austromegabalanus psittacus in platforms of the rocky intertidal. These species have been observed in non-habitual places, being recorded at an average of between 7 and 12 organisms per 25 * 25 cm quadrant. The majority of these balanids were preyed upon by birds during low tide and these empty shells have been used as a shelter by different species characteristic of the rocky intertidal, with mainly ovigerous females of the crustaceans Acanthocyclus gayi, A. hassleri and Pilumnoides perlatus being prominent. Another species not characteristic of the rocky intertidal was the macroalga Lessonia trabeculata, but this alga has also been observed in the intertidal in "El Niño" periods, so this can not be attributed as a direct effect of "La Niña".

Results observed in the subtidal communities refer to the abundance and biomass of the macrofauna. In relation to abundance during the 1999-2000 "La Niña" event, the groups Polychaeta, Mollusca and Crustacea were prominent, groups that were also dominant during "El Niño" events, but with different percentages of abundance. With respect to biomass, the most evident change was in the Crustacea group, which, in percentage, was more important during the 1997-98 "El Niño" event. During the 1999-2000 "La Niña" event the Mollusca group was the most important while the Crustacea decreased to fourth place of importance in percentage. Other observations performed sporadically in the intertidal communities during the different "El Niño" events have certainly been the presence of foreign species identified as fishers, molluscs and crustaceans. Some foreign species were already common, depending on the intensity of "El Niño" events, others were recorded for the first time, such as the presence of peneid shrimps observed during the 1997-98 event.

The intertidal and subtidal communities of the northern zone of Chile have historically been affected by several "El Niño" and "La Niña" events whose effects have presented a direct relation with their intensity and extent. Since the warm and cold events are aperiodic, recurrent and have unexpected consequences for the communities, permanent multidisciplinary studies are required in order to improve knowledge of the functioning of natural systems and the response mechanisms of the populations and communities against the evident and probable environmental changes. This knowledge certainly will allow us to make better decisions in exploiting the living resources.

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