Data from: Marine ecosystem connectivity mediated by migrant–resident interactions and the concomitant cross-system flux of lipids

Accumulating research argues that migrants influence the functioning and productivity of local habitats and ecosystems along migration routes and potentially drive cross-system energy fluxes of considerable magnitude, yet empirical documentation of local ecological effects and descriptions of the underlying mechanisms are surprisingly rare. In this study, we discovered migrant–resident interactions and substantial cross-system lipid transportation in the transition zone between the Baltic Sea and the North Sea where a resident cod population (predators) was found to interact with a herring population (prey) on a seasonal basis. We traced the lipids, using fatty acid trophic markers (FATM), from the herring feeding grounds in the North Sea to the cod livers in the Western Baltic Sea. Time series analysis of population dynamics indicated that population-level production of cod is positively affected by the herring subsidies. However, the underlying mechanisms were more complicated than anticipated. During the herring season, large cod received most of its dietary lipids from the herring, whereas smaller cod were prevented from accessing the lipid pool due to a mismatch in predator–prey size ratio. Furthermore, while the herring were extremely rich in bulk energy, they were surprisingly poor in a specific functional fatty acid. Hence, our study was the first to illustrate how the magnitude cross-system fluxes of subsidies in migrant–resident systems are potentially constrained by the size structure of the resident predator population and the nutritional quality of the migrants.

不断累积的研究证实，洄游生物会影响其迁徙沿线的本地生境与生态系统的功能与生产力，并可能驱动规模可观的跨系统能量流动。然而，目前针对这类本地生态效应的实证记录，以及对其背后作用机制的阐释却极为匮乏。
本研究在波罗的海与北海的过渡带中，发现了洄游生物与定居生物的相互作用，以及显著的跨系统脂质运输现象：该区域内的定居鳕鱼种群（捕食者）会与鲱鱼种群（猎物）发生季节性互动。
研究团队利用脂肪酸营养标记（fatty acid trophic markers, FATM）追踪了脂质的迁移路径，从北海的鲱鱼摄食场一直延伸至西波罗的海的鳕鱼肝脏中。
对种群动态的时间序列分析表明，鳕鱼的种群级生产力会受到鲱鱼带来的营养补贴的正向影响。然而，其背后的作用机制比预期更为复杂。
在鲱鱼洄游季，大型鳕鱼的大部分膳食脂质均来自鲱鱼，而小型鳕鱼却因捕食者与猎物的体型比不匹配，无法获取该脂质库。此外，尽管鲱鱼的整体能量含量极高，但其却缺乏一种特定的功能性脂肪酸。
因此，本研究首次阐明了洄游-定居系统中，外源补贴的跨系统流动规模，可能会受到定居捕食者种群的体型结构，以及洄游生物的营养品质的制约。