“The change we observed in tuna, which are near the top of the marine food web, reflects profound changes in physiology or species composition occurring at the bottom of the food web,” said Cassar, professor of biogeochemistry.
By analyzing nearly 4,500 samples of muscle tissue from three common species of tuna caught in the Atlantic, Pacific and Indian oceans between 2000 and 2015, Cassar and his colleagues discovered that the fishes’ carbon stable isotope composition values (δ13C) declined by between 0.08% and .25% during the study period.
About a quarter of the decline in δ13C values is attributable to the increased availability of isotopically light fossil fuel-derived carbon in marine ecosystems, Cassar said. This was a change the researchers expected to see.
What’s driving the rest of the decline isn’t yet clear, he said, but one possibility is that much of it may be linked to increased ocean stratification.
Over the last 50 years, 90% of the heat and 30% of the carbon emissions associated with global warming have been absorbed by the oceans. This has promoted conditions in which water masses with different densities, temperatures or levels of saltiness can stratify in layers beneath the surface rather than mix together, creating barriers that inhibit nutrients from rising up from deeper waters to provide fuel for phytoplankton near the surface.
While other contributing factors can’t yet be ruled out, analysis conducted as part of the new study supports the hypothesis that as nutrients become more limited at the surface because of stratification, large δ13C-rich phytoplankton such as diatoms may be outflanked by smaller phytoplankton cells, which have a natural competitive advantage in lean times.
The magnitude of the decline observed in tuna δ13C values over such a short time span suggests “this is an issue that merits close scrutiny and additional study,” Cassar said. “Because tuna travel long distances, they integrate changes in the food web structure over very large spatial scales.”
Scientists from France, the United States, Germany, Australia, New Zealand, Spain, the Seychelles and the French territory of New Caledonia contributed to the research. They published their findings in the journal Global Change Biology.