Stable nitrogen isotopes reveal weak dependence of trophic position of planktivorous fish on individual size: A consequence of omnivorism and mobility

摘要

Relationships between trophic position and body size in pelagic organisms were investigated using stable nitrogen isotopes (δ~(15)N) in the northwestern shelf of the Iberian Peninsula. Samples included size-fractionated plankton, planktivorous fish, squid and dolphin. Values of δ~(15)N increased with the logarithm of individual body mass when considering all organisms, but the slope of the regression line (0.195) was smaller than values reported for benthic communities. In addition, there was a large overlap in the range of S N values between planktivorous consumers, and consequently trophic position estimates were not related to individual size in most species. This suggests strong competition for planktonic prey, which may be minimized by migration. As an example, large (>20 cm) sardines (Sardina pilchardus) showed significant differences in δ~(15)N between shelf zones, while small sardines did not. Such differences support a lower mobility of adults compared to juvenile sardines along the shelf.