13C/12C and 15N/14N isotope ratios in the Southern Benguela Ecosystem: indicators of food web relationships among different size-classes of plankton and pelagic fish; differences between fish muscle and bone collagen tissues

Abstract

Measurements of the stable isotope ratios of carbon and nitrogen were obtalned for 3 sizefractions of plankton and 2 species of pelagic fish, Engraulis capensis Gilchrist and Etrurneus whiteheadi Wongratana, from the Benguela current system on the southwest coast of Africa There was a trend towards enrichment in 13c and I5N with increasing plankton size. The data are consistent with the hypothesis that larger plankton feed further up the food web than smaller plankton. Fish muscle and bone collagen tissues tended to be more enriched than the plankton. There was depletion of the heavier isotopes with increasing fish length, in contrast to the opposite tendency with increasing plankton size and towards the trophic position of fish. The fractionation of I3C/l2C and 1 5 ~/ ' " ratios during the formation of muscle tissue was different from that for bone collagen. Bone collagen was richer in I3c than muscle tissue, whereas muscle was richer than bone collagen in IsN. For both I3C and 1 5 ~, the less enriched tissue displayed the stronger negative correlation with fish length.