Tuna carbon isotopes suggest global shift in phytoplankton communities
Research by the CLIOTOP Marine Predator Isotopes Task Team was recently published in Global Change Biology. The study compiled more than 5000 stable isotope values from 2000 to 2015 of three tuna species sampled globally. Their analyses, unprecedented in scope and spatial range, found a decline in tuna δ13C values of up to 2.5‰ which was up to five-fold larger than expected from the Suess effect alone, i.e. fossil fuel-derived and isotopically-light carbon incorporated into marine food webs. The team used time-series analysis and Bayesian modelling to relate the observed temporal trend to various processes known to influence ocean carbon cycling in the global oceans. The study concluded that tuna isotope signatures are not only indicating changes in fossil-fuel derived carbon emissions but also a substantial shift in phytoplankton communities (from a likely dominance of larger diatoms to smaller coccolithophorids, flagellates or cyanobacteria).
More than 90% of the heat associated with global warming, and more than 30% of the fossil-fuel carbon emissions have been absorbed by the oceans. While such processes are predicted to impact marine biota through changes in ocean stratification and ocean acidification, current estimates of trends are based on localized ocean time series or satellite observations with significant uncertainties.
The findings from this CLIOTOP study could have broad ramifications for marine food webs. Such shifts in phytoplankton could decrease the availability of energy and some essential macro and micro nutrients to fish and human consumers. The study represents a heretofore unrecognized application of stable isotope analyses to reveal decadal changes in the ocean carbon cycle. Such empirical data will be invaluable in calibrating and validating global earth system models used by the IPCC to project the effects of climate change on oceanic productivity.