Spatial distribution of Calanus chilensis off Mejillones Peninsula (northern Chile): ecological consequences upon coastal upwelling

Abstract

The Mejillones Peninsula upwelling system (northern Chile) appears to be a natural laboratory, suitable to test hypotheses concerning variability of copepod growth in nature.In this zone, in January 2000, we collected zooplankton samples inside and outside of an upwelling plume to assess differential responses of copepodid Calanus chilensis to spatial distribution of temperature, chlorophyll a (chl a), current field and dissolved oxygen (DO).We studied variability of prosome length (PL), dry weight (DW), oil sac volume (OSV), ovary development (GI) and a condition index (CI) of females, and PL and OSV of Copepodid Stage C5.The sea temperature ranged between 14 and 20°C in the upper 10 m, whereas chl a varied between 3 and 14 mg m -3 .We distinguished 2 zones according to temperature and the current field: a cold retention area coinciding with the upwelling plume, and a warmer highly advective zone, outside the plume.Abundance of most copepodids was associated with cold and chl a-rich waters, and positively correlated to current speeds.DW and CI of females and PL of Stage C5 were negatively correlated to temperature, but not to chl a. Within the cold retention zone, copepodids were more abundant, females and C5 copepodids were larger and heavier, and females had more developed ovaries, but there were no differences in lipid content or CI.We concluded that resulting circulation may act as an efficient mechanism to maintain copepodids within the chl a-rich upwelling center, but this also implies spatial heterogeneity in temperature and food, giving rise to variability in growth and development of copepods, which is then reflected in different-sized individuals in the late stages of development.