Environmental correlates of large-scale spatial variation in the δ13C of marine animals

Carbon stable isotopes can be used to trace the sources of energy supporting food chains and to estimate the contribution of different sources to a consumer's diet. However, the delta C-13 signature of a consumer is not sufficient to infer source without an appropriate isotopic baseline, because there is no way to determine if differences in consumer delta C-13 reflect source changes or baseline variation. Describing isotopic baselines is a considerable challenge when applying stable isotope techniques at large spatial scales and/or to interconnected food chains in open marine environments. One approach is to use filter-feeding consumers to integrate the high frequency and small-scale variation in the isotopic signature of phytoplankton and provide a surrogate baseline, but it can be difficult to sample a single consumer species at large spatial scales owing to rarity and/or discontinuous distribution. Here, we use the isotopic signature of a widely distributed filter-feeder (the queen scallop Aequipecten opercularis) in the northeastern Atlantic to develop a model linking base delta C-13 to environmental variables. Remarkably, a single variable model based on bottom temperature has good predictive power and predicts Scallop delta C-13 with mean error of only 0.6 parts per thousand (31%). When the model was used to predict an isotopic baseline in parts of the overall study region where scallop were not consistently sampled, the model accounted for 76% and 79% of the large-scale spatial variability (10(1)-10(4) km) of the delta C-13 of two fish species (dab Limanda limanda and whiting Merlangus merlangius) and 44% of the delta C-13 variability in a mixed fish community. The results show that source studies would be significantly biased if a single baseline were applied to food webs at larger scales. Further, when baseline delta C-13 cannot be directly measured, a calculated baseline value can eliminate a large proportion of the unexplained variation in delta C-13 at higher trophic levels. (c) 2008 Elsevier Ltd. All rights reserved.