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.