FOOD-WEB STRUCTURE AND THE FRACTIONATION OF CARBON ISOTOPES IN THE BERING SEA

13C undergoes modest biomagnification in the food web, apparently as a result of being respired at a slower specific rate than 12C. The degree of 13C enrichment at each trophic level is related to the fraction of assimilated C which is respired. Qualitative and semi-quantitative aspects of ecosystem C cycling can be deduced from 13C enrichments, provided that isotope enrichments arising from other causes are accounted for. Most important among these is lipid storage, which enriches animals in 12C. This complication can be handled in various ways and was done here by normalizing 13C: 12C ratios to a constant lipid content. Remaining variations in 13C:12C ratio presumably result mainly from respiratory isotope fractionation. The Bering Sea ecosystem provided a test for this procedure. Certain aspects of food web structure are illustrated and various functional aspects are discussed.