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Aim:We examined potential environmental drivers of broad-scale spatial patterns in the trophic structure of marine ecosystems as represented by nitroge-n stable isotopes in globally distributed marine predators. Additionally, we assessed the effects of spatial scale on the predictive capabilities of environmental variables.Location: Global oceans. Time period: 2000 to 2015.Major taxa studied: Tunas: Thunnus albacares, Thunnus obesus, Thunnus alalunga. Methods:We undertook a global compilation and meta-analysis of the bulk nitrogen stable isotope ratios (δ 15 N values) of three tuna species (n = 4,281). After adjusting for regional variations in baseline δ 15 N values using a global ocean biogeochemistry model, generalized additive mixed models were employed to infer global-scale oceanographic controls of trophic structure, using cosmopolitan tuna species as a model. Results: For the three tuna species, variation in trophic position estimated using bulk δ 15 N values was largely explained by geographical location and the corresponding oxygen minimum layer depth. Tuna trophic positions declined in areas with reduced oxygen at depth. Food-chain length, as captured by maximum trophic position, was longer in areas of the western Pacific Ocean and shorter in the northern Atlantic and eastern Pacific Oceans. Trophic adaptability of the tuna predators, as indicated by intraspecific variability, was highest in the western and central Pacific Ocean and lowest in the northern AtlanticOcean. Our analysis demonstrated that while tunas share similar functional trophic roles, deeper-foraging tuna species had higher trophic positions globally. The predictive capacity of environmental variables decreased at finer (regional) spatial scales. Main conclusions:Our work suggests that habitat compression resulting from the predicted global expansion of oxygen minimum zones with ocean warming will impact the

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Considerable uncertainty remains over how increasing atmospheric CO2 and anthropogenic climate changes are affecting open‐ocean marine ecosystems from phytoplankton to top predators. Biological time series data are thus urgently needed for the world's oceans. Here, we use the carbon stable isotope composition of tuna to provide a first insight into the existence of global trends in complex ecosystem dynamics and changes in the oceanic carbon cycle. From 2000 to 2015, considerable declines in δ13C values of 0.8‰–2.5‰ were observed across three tuna species sampled globally, with more substantial changes in the Pacific Ocean compared to the Atlantic and Indian Oceans. Tuna recorded not only the Suess effect, that is, fossil fuel‐derived and isotopically light carbon being incorporated into marine ecosystems, but also recorded profound changes at the base of marine food webs. We suggest a global shift in phytoplankton community structure, for example, a reduction in 13C‐rich phytoplankton such as diatoms, and/or a change in phytoplankton physiology during this period, although this does not rule out other concomitant changes at higher levels in the food webs. Our study establishes tuna δ13C values as a candidate essential ocean variable to assess complex ecosystem responses to climate change at regional to global scales and over decadal timescales. Finally, this time series will be invaluable in calibrating and validating global earth system models to project changes in marine biota.