Zooplankton time series in the coastal zone off Chile: Variation in upwelling and responses of the copepod community

Two zooplankton time series from the coastal upwelling region off Chile were studied in terms of copepod biomass and abundance in relation with upwelling variation. The time series from Antofagasta (23°S) was collected between 1988 and 2008 and based on bi-annual sampling, whereas the time series from Concepción (36°S) was based on monthly samples between 2002 and 2008. In both sites, Ekman transport was derived from local wind obtained at coastal stations as an upwelling index. At both sites Ekman transport exhibited a seasonal patterns characterized by stronger upwelling during the spring–summer, although Ekman transport was stronger in northern Chile. Ekman transport also revealed three distinct periods of upwelling intensity between 1988 and 2008 in northern Chile. Copepod biomass varied in association with these periods, as follows: (1) 1988–1997 with a positive linear trend, (2) an abrupt decline in 1998–2000 after the 1997–1998 El Niño, and (3) a negative trend associated with increased upwelling from 2001 to 2008. A negative trend of zooplankton biomass and copepod abundance also occurred at Concepción during the period 2002–2008. Seasonal peaks in copepod biomass also coincided with the upwelling season (spring–summer). The correlation between variation in upwelling and copepod biomasses in northern Chile suggested that population dynamics of copepods may depend on intensity of upwelling. Although the underlying mechanisms are unclear, and several hypotheses dealing with bottom-up or top down factors can be put forth, our findings suggest that upwelling intensity operates through an optimal window for copepod abundance and biomass and increasing upwelling in the last decade (2000–2008) may has been unfavorable for copepod populations.