Trends in tuna carbon isotopes suggest global changes in pelagic phytoplankton communities. (11th November 2019) - Lorrain, Anne
Abstract: 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 δ 13 C 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 13 C‐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 δ 13 C 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. Abstract : Anthropogenic climate change is expected to alter open‐ocean marine ecosystems from phytoplankton to top predators. Our study revealed that changes in the marine carbon cycle can be traced in the tissues of marine top predators. We detected a worldwide decrease in tuna δ 13 C values over the 2000–2015 period. Our analysis suggests a widespread shift in marine phytoplankton community structure or a change in phytoplankton physiology, while this does not exclude other factors that may act in synergy (Suess effect, productivity, trophic fractionation factor).