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A multi-disciplinary research program was conducted in the northern Humboldt Current System in a 80 X 80 nautical n7ile area off Antofagasta, northern Chile (23" S) on 3 occasions: before the 1997/98 El Nirio event (January 1997), and dunng the onset and maximum penods of the event (July 1997 and January 1998, respectively). As a part of this program, the trophodynamic role of p e l a g~c tunlcates (salps and appendiculanans), as well as the crustacean zooplankton, divided into 3 size classes (large-[euphauslids], medium-[large calanoid copepods] and small-size animals [small calanoid and cyclopoid copepods]) were assessed One of the most conspicuous physical processes was the deepenlng of the thermocline during the El Nirio event, which in turn led to suppression of nutrient enrichment by upwelling events and low chlorophyll a (chl a ) concentrations in the study area. Chl a and particulate organlc carbon (POC) values were hlgher near the coast, and coastaWoceanic gradients were greater m the case of chl a. High chl a (100 to 160 mg m-') and POC ( l 1 to 17 g m-2) concentrations were usually found associated with bays and/or capes w~thin 40 nautical miles of the coast. In contrast, oceanic areas showed low concentrations throughout the study. Small calanoid and cyclopoid copepods represented only a minor fraction of the total crustacean zooplankton carbon (4 to 20%), although they often accounted for a relatively large portion of the total ingestion (26 to 68%) of the crustacean zooplankton community. Euphausiids represented a large fraction of total crustacean zooplankton carbon (28 to 73 %), and accounted for between 5 and 61 % of total crustacean zooplankton grazing. Ingestion rate as a percentage of body carbon was negatively correlated with animal size, varying between 4 % in euphausiids (17 mm) and 123% in small calanoid copepods (0.8 to 1.7 mm). A companson of estimated daily respiration rates and daily carbon ingestion rates indicated that ingestion of phytoplankton would have exceeded respiratory demands for both large and small calanoid copepods during all 3 sampling periods. In contrast, ingestion of phytoplankton failed to account for the respiratory needs of the euphausiids. This, together with microscopical observations, led to the conclusion that euphausiids utilised other sources of carbon such as detritus, micro-and mesozooplankton in addition to phytoplankton. Crustacean zooplankton showed a s~gnificant grazing impact, removing from 10 to 34 % of primary production (PP). Salps of the species Salpa fuslformis were present in dense aggregates that could have removed up to 60% of PP during January 1997. On average, the grazing impact of this specles was approximately one-half (16%) that of the whole crustacean zooplankton community combined (34 % ) . We did not find any significant difference in zooplankton biomass between January 1997 (nonEl Nino) and January 1998 (El Nino), with average biomasses of 24.6 and 21.4 mg dry wt m-" respectively. There was, however, a shift in the size spec...