Abstract. To a certain degree, Eastern Boundary Current (EBC) ecosystems are similar: Cold bottom water from moderate depths, rich in nutrients, is transported to the euphotic zone by a combination of trade winds, Coriolis force and Ekman transport. The resultant high primary production fuels a rich secondary production in the upper pelagic and nearshore zones, but where O 2 exchange is restricted, it creates oxygen minimum zones (OMZs) at shelf and upper slope (Humboldt and Benguela Current) or slope depths (California Current). These hypoxic zones host a specifically adapted, small macro-and meiofauna together with giant sulphur bacteria that use nitrate to oxydise H 2 S. In all EBC, small polychaetes, large nematodes and other opportunistic benthic species have adapted to the hypoxic conditions and co-exist with sulphur bacteria, which seem to be particularly dominant off Peru and Chile. However, a massive reduction of macrobenthos occurs in the core of the OMZ. In the Humboldt Current area the OMZ ranges between <100 and about 600 m, with decreasing thickness in a poleward direction. The OMZ merges into better oxygenated zones towards the deep sea, where large cold-water mega-and macrofauna occupy a dominant role as in the nearshore strip. The Benguela Current OMZ has a similar upper limit but remains shallower. It also hosts giant sulphur bacteria but little is known about the benthic fauna. However, sulphur eruptions and intense hypoxia might preclude the coexistence of significant mega-und macrobenthos. Conversely, off North America the upper limit of the OMZ is considerably deeper (e.g., 500-

Many infaunal sandy-bottom communities in shallow waters of the Peruvian upwelling system are inhabited by large coexisting populations of the surf clams Mesodesma donacium and Donax peruvianus as well as by the anomuran mole crab Emerila ana/oga. Under normal conditions, equilibrium states are possible with any one of these species dominating. A Mesodesma community south of Lima in Peru was investigated over 2,5 years, covering periods prior to, during and after the EI Nino (EN) of 1982-83. It was revisited several times later. Growth, recruitment and mortality and, therefore. production of Mesodesma and Donaxvaried to some extent before EN. However, during the event Mesodesma became locally extinct and had not recoloniz.ed the area by July 1986, three years after the return of normal temperatures. Donax, which took over immediately after EN, never reached the densities of the former dominant Mesodesma. Emerila remained a rare species as well, whereas spionid polychaetes increased in importance. The medium-term effect of the exceptionally strong EN of 1982-83 appears to have been very marked on Peruvian sandy beaches. A comparison is made with other shallow-water communities of the upwelling system, and the importance of EN in terms of oscillations of clam stocks off Peru and Chile is discussed.

Besides a well oxygenated shallow water area characterized by an extremely rich benthic life, most of the Peruvian and Chilean continental shelf and upper slope lies within an oxygen minimum zone (omz), the poor macrobenthic colonization of which is in striking contrast to the abundant pelagic life in the overlying waters. However, the eukaryotic benthos in this zone shares the seafloor with an important prokaryotic component of filamentous (‘sphagetti’) bacteria belonging mainly to the genus Thioploca . Under ‘normal’, non-Niño conditions, near-seafloor oxygen saturation values in this area rarely exceed 0.5 ml 1 −1 , and in some cases the bottoms are even anoxic. During strong El Niños, saturation periodically increases to >3.5 ml l −1 , and higher oxygenation may persist for more than a year after the end of the event proper (which is characterized by increased water temperatures). In the shallow parts of the omz, the changes induced by El Niño cause drastic increases of macrobenthic density, biomass and diversity; immigration of benthos and fish species from (sub)tropical equatorial and oceanic areas; changes in trophic structure; and reduction of ‘spaghetti’ bacteria. In the case of the particularly strong 1982–83 El Niño, the communities of the shallower part of the omz had shifted back into their pre-Niño state about 13 months after the end of the warming event. In the deeper part of the omz, despite a similarly strong increase of oxygen saturation, a clearly positive development of macrobenthos during El Niño could not be observed. The reasons that may be responsible for this difference are discussed.

Data are presented on macrobenthic (;;:: 1 mm) biomass and species composition, sulfur bacteria (Thioploca), demersal fish catches, organic content of sediment and dissolved oxygen at 65 stations (35-360 m) along the upwelling area off north Peru in 1980-1981. Oxygen concentration close to the bottom was high only down to about 20 m depth and at 20-700 m it was generally < 0.8 mil-I. Organic content of sediment increased significantly with water depth.Macrofauna were found at all stations with a general dominance of small polychaetes. Macrofaunal biomass showed a significant positive correlation with oxygen concentration; below 0.6 ml 1-1 of oxygen biomass was impoverished. No correlation was found between biomass and depth. A mean macrofaunal biomass of 5.9 g 0.1 m-l was recorded at depths < 100 m and 3.1 g 0.1 m-ll at 100-200 m. Biomass was higher in the north compared with the south and showed a significant positive correlation with demersal fish catches. In contrast, Thioploca biomass showed a significant negative correlation both with macrobenthic biomass and demersal fish catches. From this study and previous work, we conclude that oxygen concentration was the dominant ecological factor determining macrobenthic biomass and species composition in the upwelling area off Peru and northern Chile. The benthic fauna living in low oxygen concentrations have probably developed this tolerance through evolutionary adaptation. Based on oxygen concentration and exposure, five zones in this upwelling area are characterized and their dominant benthic macrofauna documented.

Abstract. Monthly changes in the community structure of hypoxic soft‐bottom macrobenthos have been studied at a station at 34 m depth in Ancón Bay (Peru) before and during two El Niño (EN) events. Of these events, 1982‐83 is considered the strongest, and 1991–93 one of the most prolonged in the 20th century. On the oceanographic scale, EN 1982–83 ranges as “very strong”, whereas EN 1991–93 ranges as “moderate”. The thermal anomalies at the station during EN 1982–83 (+ 7.8 °C) were almost twice those of EN 1991–93 (+ 4.1 °C). However, the community changes were not in all cases proportional to the extent of warming. Species numbers increased in a similar way (up to 24 species from near zero in each of the two events), but maximum faunal density was five times higher, and mean biomass was twice as high in 1982–83 compared with the event a decade later. Species diversity was slightly higher during EN 1982–83, whereas successional and trophic changes occurred on a broader scale during EN 1991–93. On the whole, the impact of the long‐lasting event on the small soft‐bottom macrofauna was not much weaker than that of the exceptionally strong event. The authors discuss the mechanisms which may be responsible for the differences and similarities encountered in the benthic community dynamics during these two events. Both in the unusually strong and the unusually long EN, the community revealed a pattern of early biological response several months ahead of the onset of local warming which marks the official begin of EN. The question is addressed to what extent the increase of certain parameters in the benthic community could be used for predictive purposes.