Abstract

A multi-disciplinary research program was conducted in the northern Humboldt Current System in a 80 x 80 nautical mile area off Antofagasta, northern Chile (23 degrees S) on 3 occasions: before the 1997/98 El Nino event (January 1997), and during the onset and maximum periods of the event (July 1997 and January 1998, respectively). As a part of this program, the trophodynamic role of pelagic tunicates (salps and appendicularians), as well as the crustacean zooplankton, divided into 3 size classes (large- [euphausiids], medium- [large calanoid copepods] and small-size animals [small calanoid and cyclopoid copepods]) were assessed. One of the most conspicuous physical processes was the deepening of the thermocline during the El Nino 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 organic carbon (POC) values were higher near the coast, and coastal/oceanic gradients were greater in the case of chi a. High chi a (100 to 160 mg m(-2)) and POC (11 to 17 g m(-2)) concentrations were usually found associated with bays and/or capes within 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 comparison 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 significant grazing impact, removing from 10 to 34% of primary production (PP). Salps of the species Salpa fusiformis were present in dense aggregates that could have removed up to 60% of PP during January 1997. On average, the grazing impact of this species 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 (non-El Nino) and January 1998 (El Nino), with average biomasses of 24.6 and 21.4 mg dry wt m(-3), respectively. There was, however, a shift in the size spectrum of crustacean zooplankton towards small-sized copepods between January 1997 (60 mg C m(-2)) and January 1998 (186 mg C m(-2)).