Nitrogen stable isotopes (δ¹⁵N) are typically used to estimate trophic position, providing insight into ecological roles and broader food web structure. Ecological inferences drawn from these estimates rely on quantification of isotopic baselines, i.e., low trophic level organisms reflecting the predominant nitrogen sources that support food web biomass. When baselines vary due to environmental (e.g., temperature) or anthropogenic factors (e.g., nutrient run-off), interpretation of trophic position based on δ¹⁵N may not be ecologically sound. Here, we tested the effects of assuming stable versus spatially variable δ¹⁵N baselines used to estimate the trophic position of a cosmopolitan estuarine predator—juvenile bull sharks (Carcharhinus leucas). Sampling across the San Antonio Bay system, TX, USA revealed that baseline consumers exhibited spatially variable δ¹⁵N values, which were strongly associated with capture salinity representing the influence of anthropogenically introduced nitrogen largely from fluvial run-off. As a result, estimates of bull shark trophic position differed based on baseline assumptions—sharks exhibited an ontogenetic shift in trophic position when spatial variability of baseline δ¹⁵N was accounted for, while an uncorrected approach indicated no relationship between body size and trophic position. Diet data supported ontogenetic shifts in bull shark diets, with increased consumption of larger-bodied prey among older individuals. Evaluation of isotopic baselines in spatially dynamic ecosystems like estuaries is essential, especially for highly mobile species like sharks that traverse dynamic isoscapes. A literature review revealed that only 16% of studies leveraging stable isotopes to assess the trophic ecology of sharks have accounted for potential spatial variability of isotopic baselines. As such, greater consideration of variability in isotopic baselines is important moving forward considering the ubiquitous application of this technique by ecologists.

氮稳定同位素 (δ ¹⁵ N) 通常用于估计营养位置，提供对生态作用和更广泛的食物网结构的深入了解。从这些估计得出的生态推论依赖于同位素基线的量化，即反映支持食物网生物量的主要氮源的低营养级生物。当基线因环境（例如，温度）或人为因素（例如，养分流失）而变化时，基于 δ ¹⁵ N对营养位置的解释可能不符合生态学。在这里，我们测试了假设稳定与空间可变 δ ¹⁵ N 基线的影响，用于估计世界性河口捕食者——幼年牛鲨的营养位置。Carcharhinus leucas )。对美国德克萨斯州圣安东尼奥湾系统的抽样显示，基线消费者表现出空间可变的 δ ¹⁵ N 值，这与捕获盐度密切相关，代表人为引入的主要来自河流径流的氮的影响。因此，牛鲨营养位置的估计因基线假设而异——当基线 δ ^{15 的}空间变异性时，鲨鱼表现出营养位置的个体发生变化N 被考虑在内，而未经校正的方法表明体型和营养位置之间没有关系。饮食数据支持公牛鲨饮食的个体遗传变化，老年人对体型较大的猎物的消费增加。评估空间动态生态系统（如河口）中的同位素基线至关重要，尤其是对于穿越动态等值景观的鲨鱼等高度移动的物种而言。一项文献综述显示，只有 16% 的利用稳定同位素来评估鲨鱼营养生态的研究考虑了同位素基线的潜在空间变异性。因此，考虑到生态学家对该技术的普遍应用，更多地考虑同位素基线的可变性很重要。