Keywords: macrozoobenthos ◊ community analyses ◊ feeding modes ◊ Mejillones Bay ◊ El NiñoThe macrozoobenthos of Mejillones Bay (23ºS; Humboldt Current) was quantitatively investigated over a seven years period from austral summer 1995/96 to winter 2002. 78 single van Veen grab samples taken at six stations (5, 10, 20 m depth) provided the basis for the analysis of the distribution of 60 species and 28 families as well as their abundance and biomass. Mean abundance (2119 ind. m-2) was consistent with previous estimates by Zúñiga et al (1983) (2762 ind. m-2). Mean biomass was 966 g formalin wet mass m-2 and ranged between 72 g m-2 (5 m) and 1521 g m-2, (10m). Thus, the mean value exceeds prior estimations (396 g alcohol wet mass m-2) mainly due to the dominance of the bivalve Aulacomya ater, which reached a maximum abundance of up to 2400 ind. m-2 at the 10 m stations in summer 2000 and occurred along the whole depth range sampled. This bivalve was not a dominant faunistic element in earlier investigations (e.g., Zúñiga et al 1983). A possible explanation is that the regime shift to warmer conditions in the 80ies and 90ies (Arntz et al. 2006) created more favourable conditions for this bivalve off northern Chile. 43% of the taxa sampled inhabited the complete depth range. Mean taxonomic Shannon diversity (HŽ, Log e) was 1.54 ± 0.58 with a maximum at 20 m (1.95 ± 0.33), evenness increased with depth (Fig. 1). The fauna was numerically dominated by carnivorous gastropods, polychaetes and crustaceans (48%). Echinoderms occurred only in low numbers, 15% of the species were suspensivorous, 13% sedimentivorous, 11% detritivorous, 7% omnivorous and 6% herbivorous. Cluster analyses showed a significant difference between the shallow and the deeper stations. Gammarids and the polychaete family Nephtyidae characterized the 5-m-zone, the molluscs Aulacomya ater, Mitrella unifasciata and gammarids the intermittent zone, while the gastropod Nassarius gayi, Nassarius gayi and Mitrella unifasciata and the polychaete family Nereidae were most prominent at the deeper stations.The communities of the three depths did not appear to be limited by hypoxia during non-El Niño conditions. From Figure 1 it is obvious that although oxygen concentrations rose at all depth the signal was by far not as clear for the fauna of Mejillones Bay as off Peru and in observations of deeper living communities of Concepción Bay (Chile). Therefore, no typical change in community structure occurred during El Niño 1997/98. This suggests that EN-induced enhanced oxygen levels significantly increase diversity only in those communities, where abundances and species richness are normally limited due to oxygen stress and thus, unfavourable settling conditions for larvae (Arntz et al. 2006). Oxygen limitation explains the low diversity during non-EN years at deeper bottoms throughout the Humboldt Current Ecosystem. When physiological tolerance limits are no longer exceeded due to improved oxygen concentrations in the bottom water (Arntz et al. 2006) previously excluded species may extend their distribution to lower and greater depths or proliferate strongly (opportunists). This is in line with observations of Tarazona et al. (1996) who state that very strong or very prolonged EN events may produce favourable conditions only in hypoxic study areas, reflected in considerable community changes but never reaching the species numbers of comparable bays favoured generally by better oxygenation. Although the 1997-98 EN might be one of the strongest on record, according to some physical indices, the biological impact off northern Chile was not as catastrophic as generally expected. This does not only hold true for the analysed soft-bottom community, but also for the pelagic ecosystem of the same area. Differences in the impact compared with previous events may furthermore be attributed to the distinct seasonal onset (Arntz et al. 2006): while EN 1982-83 started in austral spring, just before the reproductive season, EN 1987-98 commenced in late austral autumn (and thus in the northern hemisphere reproductive season).