Data from: 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 δ13C 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 δ13C 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 δ13C to environmental variables. Remarkably, a single variable model based on bottom temperature has good predictive power and predicts scallop δ13C with mean error of only 0.6 ‰ (3%). 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 (101–104 km) of the δ13C of two fish species (dab Limanda limanda and whiting Merlangus merlangius) and 44% of the δ13C 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 δ13C cannot be directly measured, a calculated baseline value can eliminate a large proportion of the unexplained variation in δ13C at higher trophic levels.

碳稳定同位素（carbon stable isotopes）可用于示踪支撑食物链的能量来源，并量化不同食物源对消费者饮食的贡献占比。然而，若缺乏合适的同位素基线（isotopic baseline），仅依靠消费者的δ¹³C特征值无法推断其食物来源：我们无法判断消费者δ¹³C的差异究竟反映了食物来源的变化，还是同位素基线本身的波动。在大空间尺度下应用稳定同位素技术，或是针对开放海洋环境中相互关联的食物链开展研究时，构建同位素基线是一项极具挑战性的工作。现有一类研究思路是利用滤食性消费者（filter feeding consumers）整合浮游植物（phytoplankton）同位素特征的高频、小尺度变异，以此作为替代基线（surrogate baseline）；但由于部分滤食性物种较为稀有或分布不连续，在大空间尺度下开展单一物种种群采样往往难度较大。本研究以东北大西洋广泛分布的滤食性生物——女王扇贝（queen scallop, *Aequipecten opercularis*）的同位素特征为基础，构建了将基底δ¹³C与环境变量相关联的预测模型。值得注意的是，仅基于底层水温（bottom temperature）的单变量模型便具备优异的预测性能，其对扇贝δ¹³C的预测平均误差仅为0.6‰（相对误差3%）。当利用该模型对研究区域内未均匀采样扇贝的区域进行同位素基线预测时，该模型可解释两种目标鱼类——鲽（*Limanda limanda*, dab）和牙鳕（*Merlangius merlangius*, whiting）δ¹³C在大空间尺度（101–104 km）下76%与79%的空间变异，同时可解释混合鱼类群落（mixed fish community）中44%的δ¹³C变异。研究结果表明，若在更大尺度的食物网研究中仅采用单一同位素基线，食物来源溯源工作将出现显著偏差。此外，当无法直接测定基线δ¹³C时，通过计算得到的基线值可大幅消除更高营养级（trophic levels）生物δ¹³C中未被解释的变异。