Data from: Environmental correlates of large-scale spatial variation in the δ13C of marine animals (and related published studies of carbon and nitrogen isotopic baselines)

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，并估算不同来源对消费者膳食的贡献占比。然而，若缺乏合适的同位素基线（isotopic baseline），仅依靠消费者的δ¹³C特征值不足以推断其食物来源，因为无法确定消费者δ¹³C的差异究竟反映了食物来源的变化还是基线本身的波动。在大空间尺度下应用稳定同位素技术，或研究开放海洋环境中相互关联的食物链时，构建同位素基线是一项极具挑战性的工作。一种可行方案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波动，以此作为替代基线（surrogate baseline）；但由于目标消费者物种较为稀有或分布不连续，在大空间尺度下对其进行采样往往困难重重。本研究以东北大西洋（northeastern Atlantic）分布广泛的滤食性生物——女王扇贝（*Aequipecten opercularis*）的同位素特征为基础，构建了将基础δ¹³C与环境变量相关联的模型。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能力，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系统采集扇贝样本的区域进行同位素基线预测时，该模型可解释两种鱼类——欧洲菱鲆（*Limanda limanda*）和牙鳕（*Merlangus merlangius*）——的δ¹³C在大空间尺度（101–104 km）下76%和79%的空间变异，同时可解释混合鱼类群落δ¹³C 44%的变异量。研究结果表明，若在更大尺度的食物网中统一使用单一同位素基线，食物来源相关研究将产生显著偏差。此外，若无法直接测定同位素基线δ¹³C，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。