Marine pelagic copepods must cope with a highly heterogeneous environment in coastal upwelling systems. In addition tospatial and temporal heterogeneity in
temperature and food supply, advective forces may strongly influence theirpopulations. 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 uponcold and warm phases of the ENSO (El Nin˜o Southern Oscillation) cycle. During two spring and two summer periods(1998 Á 2000), we studied the abundance, distribution and growth of the copepodids of C. chilensis. The instantaneous meanweight-specific growth rate was estimated as 0.29 day (1 and was not significantly different among cruises. Weights of latestages did not differ either. However, the numerical abundance and biomass substantially differed among cruises, such thatdaily production of copepodids increased with greater biomasses. Chlorophyll-a was significantly greater in spring 1999 andsummer 2000, as compared to spring 1998 and summer 1999, coinciding with greater abundances of copepodids andstronger upwelling. The current field in the upper 200 m indicated that strong
upwelling favoured nearshore waterretention, whereas weak or moderate upwelling caused more offshore advection. This finding is opposite to the usual viewthat strong upwelling favours offshore advection. We further concluded that neither food or temperature had muchinfluence on growth, abundance and production of C. chilensis, but instead seasonal and interannual variation in upwellingcaused by changes in local winds may be the primary cause for the observed fluctuations in population abundance.
