IMPACT OF 2 EL-NINO EVENTS OF DIFFERENT INTENSITY ON THE HYPOXIC SOFT-BOTTOM MACROBENTHOS OFF THE CENTRAL PERUVIAN COAST

Monthly changes in the community structure of hypoxic soft-bottom macr obenthos have been studied at a station at 34 m depth in Ancon Bay (Pe ru) before and during two El Nino (EN) events. Of these events, 1982-8 3 is considered the strongest, and 1991-93 one of the most prolonged i n 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 degrees C) were alm ost twice those of EN 1991-93 (+ 4.1 degrees C). However, the communit y changes were not in all cases proportional to the extent of warming. Species numbers increased in a similar way (up to 24 species from nea r 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 d uring EN 1982-83, whereas successional and trophic changes occurred on a broader scale during EN 1991-93. On the whole, the impact of the lo ng-lasting event on the small soft-bottom macrofauna was not much weak er than that of the exceptionally strong event. The authors discuss th e mechanisms which may be responsible for the differences and similari ties encountered in the benthic community dynamics during these two ev ents. Both in the unusually strong and the unusually long EN, the comm unity revealed a pattern of early biological response several months a head of the onset of local warming which marks the official begin of E N. The question is addressed to what extent the increase of certain pa rameters in the benthic community could be used for predictive purpose s.