Low delta C-13 signatures in pelagic seabirds: lipid ingestion as a potential source of C-13-depleted carbon in the Procellariiformes

Stable-isotope ratios of carbon (delta C-13) were determined in liver samples from a number of procellariiform seabirds from New Zealand. Generally, delta C-13 values were low (depleted in C-13) and there was a significant degree of intra- and inter-specific variation. We suggest that the pelagic versus inshore/benthic foraging model for delta C-13 values in marine consumers is insufficient to explain the intra- and inter-specific variation. Nor can observed delta C-13 values of procellariforms be linked to variation in foraging distances. We propose that carbon relatively depleted in C-13 derived from dietary Lipids is incorporated into proteins. Support for this hypothesis is provided by depleted delta C-13 signatures we measured in lipids extracted from liver tissue, which were always lower than delta C-13 signatures in Liver tissue (by 4.2 to 6.8 parts per thousand, depending on species). Additionally, delta C-13 values were determined in a small number of stomach-oil samples; these too were relatively depleted and lower than delta C-13 values measured in Liver tissue. Incorporation of dietary lipids, relatively depleted in C-13, into protein could explain both intra- and inter-specific variation in delta C-13 signatures in procellariiforms and may represent an additional explanation for relatively low delta C-13 values in pelagic consumers.