Trends in tuna carbon isotopes suggest global changes in pelagic phytoplankton communities

Anne Lorrain1, Heidi Pethybridge2, Nicolas Cassar3, Aurore Receveur4, Valérie Allain4, Nathalie Bodin5, Laurent Bopp6, Anela Choy7, Leanne Duffy8, B Fry9, Nicolas Goñi10, Brittany S Graham11, Alistair J Hobday12, John A Logan13, Frédéric Ménard14, Christophe Menkes15, Robert J. Olson16, Dan Pagendam17, David Point18, Andrew T Revill19, Christopher J Somes20 and Jock Young21, (1)Institute of Research for Development, Brest, France, (2)CSIRO Hobart, Hobart, TAS, Australia, (3)Duke University, Nicholas School of the Environment, Durham, NC, United States, (4)SPC, Nouméa, New Caledonia, (5)IRD, Seychelles, (6)LSCE Laboratoire des Sciences du Climat et de l'Environnement, Gif-Sur-Yvette Cedex, France, (7)Scripps Institution of Oceanography UCSD, Oceanography, La Jolla, CA, United States, (8)IATTC, CA, United States, (9)Australian River Institute, QLD, Australia, (10)AZTI, Spain, (11)NIWA, New Zealand, (12)CSIRO Environment, Hobart, TAS, Australia, (13)Massachusetts Division of Marine Fisheries, MA, United States, (14)IRD, France, (15)IRD/LOCEAN, Nouméa, New Caledonia, (16)Woods Hole Oceanographic Institution, Woods Hole, MA, United States, (17)CSIRO, Data 61, Brisbane, QLD, Australia, (18)Géosciences Environnement Toulouse (GET), UMR CNRS / IRD / Université Paul Sabatier, Toulouse, France, (19)Commonwealth Scientific and Industrial Research Organisation (CSIRO), Hobart, TAS, TAS, Australia, (20)GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, Germany, (21)CSIRO, TAS, Australia
Considerable uncertainty remains into 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 to 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. Tunas not only recorded the Suess effect, i.e. fossil fuel-derived and isotopically-light carbon being incorporated into marine ecosystems, but also profound changes at the base of marine food webs. We suggest a global shift in phytoplankton community structure, e.g. a reduction of 13C-rich phytoplankton such as diatoms, and/or a change in phytoplankton physiology during this period, while this does not prevent 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.