Climate change alters plankton populations

New research using sediment cores suggests human-driven climate change is having a significant impact on the composition of the world’s marine plankton.

A German study, published in the journal Nature, reveals that modern communities of foraminifera, a type of hard-shelled plankton, differ markedly from those from the pre-industrial era, which began just 170 years ago. 

The researchers – Lukas Jonkers and Michal Kucera from the University of Bremen and Helmut Hillebrand from Carl von Ossietzky University, Oldenburg – found that the amount of differentiation correlates with the degree of temperature change. The direction of change is consistent with the global pattern of sea-surface temperature warming seen in historical times.

 “We observe community changes towards warmer or cooler compositions that are consistent with historical changes in temperature in 85% of the cases,” they write.

Marine ecosystems are known to be altering in response to climate change, but the scale is often hard to determine because scientists lack a comparative baseline from pre-industrial times. 

Sediment cores, which contain the remains of organisms going back centuries, offer a way to resolve this problem, and planktonic foraminifera make an ideal subject.

They are globally ubiquitous, with about 40 known species; their distribution is primarily controlled by temperature; and while they occur most abundantly in the surface mixed layer, some species can be found alive down to several hundreds of metres.

Most importantly, and uniquely among marine zooplankton, their calcite shells are well-preserved in marine sediments. 

“This renders them an ideal model system to investigate the influence of global change on marine zooplankton, because seafloor sediments offer the chance to obtain an accurate picture of the composition of planktonic foraminifera communities in the past,” the authors write.

To date, however, studies looking at the influence of anthropogenic climate change on planktonic foraminifera communities have largely had a regional rather than a global focus. 

Jonkers and colleagues took things up a notch, comparing foraminifera found in the pre-industrial levels of globally distributed sediment cores at more than 3500 sites with those from sediment trap samples taken between 1978 and 2013. 

Most of the trap sites are in the northern hemisphere, but they cover the global thermal gradient and include time series spanning up to 12 years.

“Warming signatures are found across the globe and in a range of environments (open ocean, coastal regions, upwelling regions, and at low and high latitudes), which indicates that species compositions have shifted worldwide,” the researchers write. 

“Some parts of the ocean are known to have been cooling during the Anthropocene. This historical cooling has also affected the composition of Anthropocene assemblages, such that the direction of change inferred from the change in the planktonic foraminifera community is — in the large majority of cases (85%) — consistent with the observed temperature change, irrespective of whether the observed historical trend has been warming or cooling.

“The single case in which the species composition indicates warming in a cooling area is in the North Pacific gyre, where the amount of temperature change has been negligible. 

“Thus, we conclude that the chance of finding false warming signatures is low and that the observed pattern in community change is robust, indicating that the communities have responded to the dynamic pattern of changes in sea-surface temperatures induced by global warming.”

The authors suggest that the described shifts in planktonic foraminifera are indicative of a more general phenomenon across marine ecosystems, in which present-day assemblages differ from historical ones in a way that reflects the change in environmental conditions since the onset of the Anthropocene. 

And the results of the study, they suggest, raise concerns over the functioning of marine ecosystems and the reliability of the services that these ecosystems deliver to society.

“These findings place emphasis on the recent discussion on how well communities are adapted to rapid environmental change,” they conclude.

“If the potential for spatial displacement and adaptation lags behind the rate of change in the environment, modern-day assemblages may always show a trait distribution with suboptimal fitness.”