There has been an increase in iron stress in the phytoplankton of the Southern Ocean over a period of 26 years and this raises concerns that it may lead to a decline in the productivity of these microscopic marine plants, which would have a negative impact on the future of Earth’s climate and ecosystem energy supplies, research undertaken by a climate science group at the Council for Scientific and Industrial Research (CSIR) has found.

The study suggests that there may be a reduction in the supply of iron to the Southern Ocean’s surface sunlit waters, explained study co-lead authors CSIR senior researcher Dr Thomas Ryan-Keogh and CSIR research group leader for the Southern Ocean carbon-climate observatory Dr Sandy Thomalla. The study was published in the journal Science.

“Iron is an important building block for phytoplankton photosynthesis. Therefore, changes in iron availability in the southernmost waters of the world ocean directly affect photosynthesis and limit the growth of phytoplankton in this vast oceanic region,” said Ryan-Keogh.

“Non-anthropogenic carbon dioxide in the atmosphere is regulated by plants on land and phytoplankton in the ocean, as part of the natural carbon cycle, but changes in this cycle can influence the ability of the ocean to absorb anthropogenic carbon dioxide,” he added.

The Southern Ocean is a globally important region that helps to mitigate the impacts of climate change by absorbing, mainly through physical processes, a significant proportion of the man-made carbon dioxide that is emitted through the burning of fossil fuels.

The researchers developed a new method that looks at a physiological response of phytoplankton to iron availability to determine whether or not it is limiting. They applied the method retrospectively to 26 years of in situ data collected from the Southern Ocean by both research ships and underwater robotic platforms between 1996 and 2022.

“Across this time period, we found that Southern Ocean phytoplankton are becoming increasingly iron stressed, which suggests there may be a reduction in the supply of iron to the surface sunlit waters. This increase in iron stress impacts phytoplankton growth negatively, with the large majority of the Southern Ocean becoming less productive over time.

“This has important implications for the role of marine plants in driving the natural carbon cycle and ecosystem energy supplies,” Thomalla explained.

“Interestingly, these trends are the opposite of what is currently being predicted by Earth system models, which instead propose that the Southern Ocean will become more productive over the next century,” she highlighted.

Further, the researchers believe this contradiction suggests that Earth system models may be underestimating ongoing change in the natural carbon cycle in this globally important region, which may impact the uncertainties of climate projections.

They plan to collaborate with their international counterparts to further understand why the Earth system models are not able to reproduce the current trends.

Meanwhile, the researchers sourced and investigated almost three decades’ worth of data. This was made possible by the supercomputing capabilities of the Centre for High Performance Computing.

What emerged was the understanding that physical aspects of climate change, and lesser known biological and chemical aspects, are all affecting the primary productivity of the Southern Ocean, the researchers pointed out.