Science Daily –
The ratio of carbon isotopes in three common species of tuna has changed substantially since 2000, suggesting major shifts are taking place in phytoplankton populations that form the base of the ocean’s food web, a new international study finds.
“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 Nicolas Cassar, professor of biogeochemistry at Duke University’s Nicholas School of the Environment.
Specifically, the phytoplankton changes reflect more fossil fuel carbon capture by the oceans and possible stratification of ocean layers, which is driven by warming.
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 (the ratio of Carbon 13 to Carbon 12, called delta C-13) declined by between 0.08% and .25% during the study period.
To conduct their study, they analyzed carbon stable isotope ratios and nitrogen stable isotope ratios in yellowfin, bigeye and albacore tuna caught in tropical, subtropical and temperate waters. Carbon stable isotope ratios are widely used to track the flow of nutrients up marine food webs and identify a species’ primary food source.