Stable isotope composition of zooplankton, fishes and sea birds from the northern Humboldt Current System during Maria S. Merian cruise MSM80

Stable isotopes of carbon and nitrogen (δ13C, δ15N) were analyzed using a mass spectrometer with IAEA-VPDB (IAEA-C1) and atmospheric air (IAEA-N1) as standards, respectively. Two different approaches were used to determine a δ15N baseline for the food web of the northern Humboldt Current System. i) Particulate organic matter (POM) samples representing trophic position 1 and ii) Filter-feeding salps (Iasis cylindrica) as primary consumers representing trophic position 2. In general, low isotope ratios at the surface occurred in Thaliacea (I. cylindrica and Pyrosoma sp.). δ15N values of copepods inhabiting waters 200 m depth) within the oxygen minimum zone had about 4 to 8‰ higher δ15N values (~11-15‰) than surface living ones. For the diel vertical migrating euphausiids, mean δ15N and δ13C' values were in a similar range as for copepods from the upper water layers. The squat lobster Pleuroncodes monodon had extraordinarily high δ13C'. Among the key fish species, the eastern Pacific bonito Sarda chilensis had a slightly higher δ15N value compared to the anchovy Engraulis ringens. In some species a strong increase of the δ15N ratio of up to ~5‰ was determined from North (8.5°S) to South (16°S). These pronounced differences in d15N values highlight the importance of adequate baseline values to avoid misinterpretations of a species' regional feeding behaviour. In order to compensate for the reginal shift, using a filter-feeding salp as reference for herbivorous consumers seemed advantageous. In fact, calculated trophic positions for C. chilensis and other species did not differ between regions, when shifts in the baseline were taken into account