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vol.30 número1  suppl.SympIs the Reduction in Abundance of Haliotis fulgens and H. corrugata (Gastropoda: Haliotidae) at Islas San Benito (Baja California, Mexico) a Combined Effect of Overfishing and Climatic forcing?Effect of El Niño 1997/98 on a Population of the Southern Sea Lion (Otaria flavescens Shaw) from Punta Patache/Punta Negra (Iquique, Chile) 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/S0717-71782002030100058 

Long-Term Effects of "El Niño"
on The Structure of Soft-Bottom
Macrozoobenthos Communities
in Northern Chile

Manuel E. Rojo, Marcelo E. Oliva, Cecilia Villalobos

Inst. Invest. Oceanol. Universidad de Antofagasta,
P.O. Box 170 Antofagasta, Chile,
E-mail: meoliva@uantof.cl

Currently, it is well known that the El Niño (EN) effects are of global dimension, but the oceanographic ano-malies derived from EN may affect the Pacific coast of South America, mainly that of Peru and Chile, more strongly. Because of the importance of marine resources (mainly small pelagic fishes) for the economy of Peru and Chile, studies of the biological effects of EN have mostly been focused on the impact on population parameters (fecundity, reproduction, mortality) of commercially exploited marine resources, such as fish (anchovy, sardine, jack-mackerel), molluscs ("loco", mussels, scallops, octopus) and macroalgae (Lessonia sp.). Although the impact of EN can be adressed as "positive" or "negative", with a few exceptions, emphasis on "negative impacts" has been the rule for studies dealing with EN at the population level. At the community level, the studies on benthic communities are sparce, at least for the coast of northern Chile. There are only a few reports, based more on qualitative rather than quantitative observations, and there are no studies (at least for benthic communities) based on long-term time scales. Since 1995 we have been monitoring a soft sediment community in the bay of Mejillones (approx. 23° S) (Fig.1) on a bi-annual basis (winter and summer). Samples (8 in winter, 10 in summer) were taken with a van Veen grab (0.1 m2) operated from a boat, at 3 depths: 5, 10 and 20 m. Samples were washed on a screen of 1 mm. Material retained by the mesh was transferred to plastic jars and fixed in 10% formalin in seawater. Samples were stained with Rose Bengal in order to facilite the sorting and counting of the macrofauna. In the laboratory, the fauna were identified to the lowest taxonomic level. Polychaeta were identified to family level.


Fig. 1 Mejillones del Sur bay. Arrows shows the sampling area.

For the analyses of classification and ordination and for each depth level, a similarity matrix (Bray-Curtis) was constructed, after applying a log (n+1) transformation. Non-metric multidimensional scaling (NMDS) was used for ordination and the unweighted pair-group method, using the arithmetic average (UPGMA), was used for classification. Temperature, salinity and dissolved oxygen were measured with a CTD. The results of classification and ordination techniques showed differences along the depth gradient studied. For the 5 m level, NMDS was unable to generate a discrimination of ENSO 1997-1998, but at 10m winter 1997 was well discriminated, and at 20 m winter and summer 1997 and winter 1998 were also well discriminated (Fig. 2).


Fig. 2 Non-metric multidensional scaling ordination based on the similarity matrix of abundance data for m.

At functional group level, there is evidence of changes. At the 5 m level, deposit- feeders increased sharply in the number of individuals (8.7% in summer 1997 to 97% in winter 1997), whereas carnivores dropped from 80% to less than 2% in the same period. At 10 m deposit feeders increased from 14.2% to 41.2% and suspensivores decreased from 60.4% to 30.4%. Finally, at 20 m, carnivores dropped from 91.8% to 35.4% and suspensivores increased from 2.4% to 57%. The species richness increased only for 20 m depth.

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