On-line version ISSN 0717-7178
Investig. mar. vol.30 no.1 suppl.Symp Valparaíso Aug. 2002
Responses of Zooplankton Populations
to the 1997/98 El Niño in the Coastal
Upwelling Zone off Northern Chile:
Is El Niño that bad?
1Centro Oceanográfico del Pacífico Sur-Este,
Universidad de Concepción,
P.O. Box 160 C, Concepcion, Chile,
2Departamento de Oceanografía, Universidad de
Concepción, Casilla 160C, Concepcion, Chile,
3Instituto de Investigaciones Oceanológicas,
Universidad de Antofagasta,
P.O. Box 170, Antofagasta, Chile,
The 1997/98 El Niño (EN) has been described as one of the strongest on record and expected to have caused major changes in productivity and community structure in the upwelling region of the eastern south Pacific. However, the available information on how pelagic components responded to this event in the Humboldt Current System (HCS) is rather sparse and conclusions are uncertain. At the coastal upwelling site off Antofagasta (23° S) we assessed the abundance of zooplankton populations, their distribution, dominant species assemblages, and some demographic characteristics upon conditions prior to and during the 1997/98 EN. We aimed to: 1) evaluate how zooplankton populations and species assemblages may have been affected by EN, 2) give insights to elucidate the widely assumed view that EN has a negative impact on the pelagic ecosystem, and 3) determine and discuss whether the impact of EN may differ between offshore and inshore waters.
Off Antofagasta the EN signal was first observed in April 1997. There was a sharp deepening of the thermocline and the oxycline. These conditions did not persist however and there were alternate cooling/warming periods through the austral winter/early spring (May-October). A second and stronger Kelvin wave arrived by the end of November and this condition persisted up to the end of our study (end of January 1998) (Fig. 1). During the whole period, including conditions prior to and during EN, the phytoplankton biomass, measured as Chlorophyll-a, was not subject to drastic changes, and remained high, apparently exhibiting only the seasonal signal (Fig. 1). This was the first evidence that herbivore zooplankton were not subject to food-shortage during the warm event.
Abundance of copepod species decreased slightly during EN when comparing the same months before and during the event, and there were some species exchanges between inshore and offshore waters, although the species diversity was rather stable throughout the study (Fig. 2). One of the dominant copepods, Calanus chilensis, increased in numbers during EN, but adult females were smaller. We attributed this to an effect of elevated temperatures under enough food to sustain individual and population growth. High temperature may accelerate development rates, shortening generation times and thus giving rise to more cohorts during seasons. Euphausiids seemed to respond in a quite similar manner, and adult and larval stages also exhibited reduced sizes during the warm event, although sampling limitations may have introduced some bias when estimating their abundance, such that caution is to be exercised in the interpretation of our results. Two oceanographic surveys were carried out in December 1996 (before EN) and in July 1997 (during EN conditions). Despite the expected seasonal differences (summer vs winter), anomalous oceanographic conditions due to EN were clearly seen in July 1997. Phytoplankton biomass was still high in July 1997, although more uniformly distributed.
Fig. 2 Changes in copepod abundance and species diversity off Antofagasta on conditions previous to and during the 1997/98 EN.
Both copepods and euphausiids were subject to changes in their vertical distribution during EN, such that they appeared more dispersed in the vertical plane. Disappearances of some species were assumed to result from migration to deep (>200 m) waters. Thus, zooplankton populations apparently did well during the EN, maintaining nearly normal sizes and reproductive cycles, and one dominant species was even able to increase in abundance under warmer conditions. Other studies have considered that the inshore (shallow waters) may respond differently to EN as compared to the open ocean. It has even been proposed that EN may have positive effects on some benthic populations in the nearshore. These EN consequences have often been overlooked and considered of minor importance, perhaps because the spatial scale on which they occur can be assumed to be small. Our results may add more evidence supporting the view that inshore waters may be little affected by El Niño. However, the continental shelf in our study area is extremely narrow. The shelf break is usually located within 10 km of the shoreline. Thus it is difficult to establish limits to define inshore and offshore waters. For instance, the presence of mesopelagic larvae and some oceanic species of euphausiids and copepods in the coastal zone suggests that the nearshore area is indeed dominated by a mixture of coastal and oceanic waters. Furthermore, the coastal upwelling zone off northern Chile, considered to be among the most productive of the world ocean, is well recognized to be restricted to a narrow band within an almost absent continental shelf. Our findings may therefore represent plankton responses derived from more than just inshore waters. If this is true, then the pelagic ecosystem of the upwelling region off northern Chile should be considered as more resilient to large scale perturbations. To our knowledge this possibility has not yet been explored or documented for the coastal upwelling zone off California.