Distribución de eufáusidos en el sistema de surgencia frente a la península de Mejillones (23°S) asociada a condiciones previas y durante El Niño 1997-98

Abstract

Euphausiids are abundant, although little studied components of the zooplancton in the Humboldt Current ecosystem. For instance, the effect that large scale ocean variability may exert on their population dynamics, abundance and distribution is unknown. In this work, based on 2-year time series data and two spatial surveys on December 1996 and July 1997, the distribution of euphausiids in coastal zone off Mejillones Peninsula was assessed. 10 species were identified, of which the most abundant were Euphausia mucronata, E. distinguenda and a non-identified species of the genus Euphausia. In July 1997 (El Nino conditions) fewer species were observed as compared to December 1996 (prior to the El Nino). In both situations (prior and during the El Nino) the abundance of various species was associated with surface dissolved oxygen and water density. There was no correlation between euphausiids abundance and phytoplankton biomass. Previous to the El Nino 1997-98 a Diel Vertical Migration (DVM) pattern was observed for most species, with greater abundance in the upper 0-50 m layer at night. Under the El Nino conditions DVM was apparently altered by intrusion of the warm water mass. E. mucronata, E. distinguenda, Euphausia sp. and Nematoscelis megalops, were found during the time series study. The abundance of adults of these species was significantly reduced during the warm conditions of the El Nino. By contrast, juveniles and larvae (furcilia and calyptopis), not identified to the species level, were significantly more abundant during the El Nino phase, suggesting that reproduction was continuous throughout the study and even incremented during the El Nino. During the warm conditions of the El Nino we also observed a decrease in body length of adults, juveniles and larvae. When comparing equal months in both periods (El Nino and non-El Nino), the body lengths of juveniles, larvae and at least for one species in adults, were significantly smaller during the El Nino period.