Abstract

Marine pelagic copepods must cope with a highly heterogeneous environment in coastal upwelling systems. In addition to spatial and temporal heterogeneity in temperature and food supply, advective forces may strongly influence their populations. Off northern Chile, the population of Calanus chilensis, a dominant copepod associated with upwelling centres, is subjected to large interannual variations in abundance, not explained by changing temperature regimes occurring upon cold and warm phases of the ENSO (El Nino Southern Oscillation) cycle. During two spring and two summer periods (1998-2000), we studied the abundance, distribution and growth of the copepodids of C. chilensis. The instantaneous mean weight-specific growth rate was estimated as 0.29 day-1 and was not significantly different among cruises. Weights of late stages did not differ either. However, the numerical abundance and biomass substantially differed among cruises, such that daily production of copepodids increased with greater biomasses. Chlorophyll-a was significantly greater in spring 1999 and summer 2000, as compared to spring 1998 and summer 1999, coinciding with greater abundances of copepodids and stronger upwelling. The current field in the upper 200 m indicated that strong upwelling favoured nearshore water retention, whereas weak or moderate upwelling caused more offshore advection. This finding is opposite to the usual view that strong upwelling favours offshore advection. We further concluded that neither food or temperature had much influence on growth, abundance and production of C. chilensis, but instead seasonal and interannual variation in upwelling caused by changes in local winds may be the primary cause for the observed fluctuations in population abundance.