Trophic position of twelve dominant pelagic copepods in the eastern tropical Pacific Ocean

In this study, the trophic structure of the pelagic copepod community was analyzed in six geographical zones in the eastern tropical Pacific Ocean. Zooplankton samples were collected on two oceanographic cruises from August to December 2003 using a bongo cone net (333 mu m mesh light). The isotopic signatures were measured as delta N-15 and delta C-13 in twelve selected species based on their dominance and their feeding types. We observed a significant latitudinal gradient in delta N-15 values, generally increasing northwards. The values of delta C-13 isotopes did not show a significant longitudinal gradient, but geographical differences occurred in some species. Pleuromamma robusta was recognized as the species with the highest trophic position in the copepod community. In general, there was a positive relationship between average body size and trophic position, except for the herbivorous species Eucalanus inermis, which was detected in the lowest trophic position. The isotopic niche was similar for each of the 12 species of copepods studied, but in most cases, the niche overlap between each pair of species was low and not higher than 50%. While the differences in isotopic signatures can be attributed mainly to the dominant nutrient sources in each zone, the low trophic niche overlap may be explained by the differential spatial distribution of species, reducing competition for food resources.