Abstract

To estimate the predation effect of the predominant ctenophore Pleurobrachia bachei on the small-copepod community in the upwelling area off Mejillones (23° S), northern Chile, a series of oceanographic cruises and predation experiments were conducted in the austral springs 2000, 2001 and 2002. The daily consumption rates and predatory effect of P. bachei on the small copepods (in terms of % of standing stock and biomass removed daily) were determined at three stations located in relation to the shelf-break (coastal, shelf-break and oceanic) reaching values up to 4.5% per day of the <1500 ?m copepod standing stock. Our results indicate that the ctenophores were most abundant at the coastal station, that small copepods dominated the copepod community (being more abundant nearshore), and that the relative frequency of ctenophores with copepods in their guts was also higher near the coast. The predatory effect of P. bachei on the small-copepod community was also higher in the coastal zone. However, the effect of this predation on the copepod biomass in terms of carbon did not decrease steadily seawards, which may be due to the larger sized copepods consumed at the offshore stations. Determinations of predatory effect on the secondary production of the more abundant small-copepod populations (i.e 26% daily in 2000) suggest that this single species of Pleurobrachia is modulating the population growth rate of the small copepods, the copepod community size structure, and maybe even the alternance of key species in the Mejillones coastal upwelling zone. © The Author 2005. Published by Oxford University Press. All rights reserved.