Controls on the nitrogen isotopic composition of fish otolith organic matter: Lessons from a controlled diet switch experiment

Research data used in our article "Controls on the nitrogen isotopic composition of fish otolith organic matter: Lessons from a controlled diet switch experiment". Abstract The nitrogen isotopes (δ15N) in the organic fraction of accretionary hard part structures, such as fish otoliths, may provide life histories of dietary change. We performed controlled experiments to validate the dynamics of isotope incorporation into biominerals following dietary shifts and also compared whole-otolith and serial sampling approaches for diet reconstruction. Laboratory-reared Atlantic croaker (Micropogonias undulatus) were switched from a high quality, high δ15N diet (δ15N = 10.7‰) to one of two lower quality, lower δ15N diets (δ15N = 5.9 or 3.7‰). Using the oxidation-denitrifier method, including cleaning protocols required for fossil otoliths, we measured both otolith-bound δ15N (δ15Noto) of whole-otolith subsamples (δ15Nwhole) and sequentially micromilled otolith powders (δ15Nmicro) and compared these results to white muscle tissue (WMT) and liver δ15N obtained through traditional techniques. Both δ15Nwhole and δ15Nmicro recorded the diet switch, although sampling limitations muted the signal in δ15Nmicro, especially in the slowly growing otoliths of the fish on the lowest quality diet. The timescales at which otoliths and tissues approached the new δ15N after the diet switch varied, but the slowest was for WMT and the fastest was for liver. For δ15Nwhole, there were two factors: (1) the turnover time of the N provided to the otolith and (2) that the otolith is accreting and thus integrating over the entire life history of the fish. Using a model to account for the accretionary growth, turnover time for N supplying otolith growth ranged from 12 to 16 days. In both soft tissues and otoliths, the trophic discrimination factor (TDF, the δ15N elevation relative to diet) appears to have been lower in the larval fish, prior to the onset of the diet switches. This raises questions about the interpretation of the δ15Noto of otolith cores; however, the core is a small portion of the total otolith. We conclude that (1) δ15Nwhole is useful for tracking diet despite the whole-life integration of the δ15N signal and (2) δ15Nmicro also records diet, and higher resolution tracking may be possible with further optimization of δ15Nmicro sampling.本研究数据源自我们发表的论文《鱼类耳石有机质氮同位素组成的调控机制：受控饮食转换实验的启示》。 摘要 增生硬质结构（如鱼耳石）的有机质组分中的氮同位素（δ¹⁵N）可用于反演生物饮食变化的生命历程。我们开展受控实验，以验证饮食转换后同位素在生物矿物中的掺入动力学过程，并对比了全耳石与连续采样两种方法用于饮食重建的效果。将实验室饲养的大西洋异鳍石首鱼（*Micropogonias undulatus*）从高品质、高δ¹⁵N饮食（δ¹⁵N=10.7‰）转换为两种低品质、低δ¹⁵N饮食之一（δ¹⁵N=5.9‰或3.7‰）。采用氧化-反硝化法（oxidation-denitrifier method，含化石耳石所需的清洗流程），我们分别测定了全耳石子样本的耳石结合态δ¹⁵N（δ¹⁵N_oto，即δ¹⁵N_whole）以及连续微研磨耳石粉末的δ¹⁵N（δ¹⁵N_micro），并将结果与通过传统方法测得的白肌组织（WMT）和肝脏的δ¹⁵N进行对比。 δ¹⁵N_whole与δ¹⁵N_micro均能记录饮食转换事件，但采样限制削弱了δ¹⁵N_micro的信号，尤其在喂食最低品质饮食的鱼类生长缓慢的耳石中更为明显。饮食转换后，耳石与组织达到新δ¹⁵N水平的时间尺度存在差异，其中白肌组织的响应最慢，肝脏最快。对于δ¹⁵N_whole，存在两个关键影响因素：一是供给耳石的氮的周转时间，二是耳石持续增生，因此整合了鱼类整个生命历程的同位素信号。通过构建考虑增生生长的模型，我们计算得出供给耳石生长的氮的周转时间为12~16天。 在软组织与耳石中，饮食转换开始前的幼鱼阶段的营养判别因子（TDF，即相对于饮食的δ¹⁵N富集量）似乎更低。这一结果对耳石核心的δ¹⁵N_oto解释提出了疑问，但耳石核心仅占整个耳石的极小部分。我们得出如下结论：（1）尽管δ¹⁵N信号整合了鱼类全生命周期的信息，但δ¹⁵N_whole仍可用于追踪饮食变化；（2）δ¹⁵N_micro同样能够记录饮食信息，若进一步优化δ¹⁵N_micro的采样流程，有望实现更高分辨率的饮食追踪。