Climate change often manifests as dramatic, extreme events − collapsing glaciers, flooded valleys, burnt forests. But invisible changes are under way, too, with equally profound consequences for life as we know it.
A study based on water samples from the Southern Ocean over several decades shows the composition of phytoplankton – the microscopic single-celled algae that sustain the ocean food web – is shifting. Researchers warn we may be witnessing a fundamental reorganisation of life that could, over time, cascade through the marine food system and affect the ocean’s capacity to take up carbon.
Phytoplankton, known as the “grass of the ocean”, are complex, with thousands of species and ever-changing connections between them. As the Southern Ocean changes − with rising surface temperatures, less mixing between water layers and vanishing sea ice − phytoplankton communities adjust. But even subtle changes in the food web’s foundation can reverberate through the entire system, from microscopic grazers to fish, seabed corals and top predators, including seals, whales and penguins.
“We’re seeing the Southern Ocean phytoplankton responding to climate change,” says Matt Pinkerton, a mathematical ecologist at Earth Sciences New Zealand. “This shift in phytoplankton composition changes how energy flows into the rest of the food web and the efficiency with which phytoplankton take carbon out of the atmosphere and pump it down into the deep ocean.”
Pinkerton and colleague Alex Hayward led a team of researchers from New Zealand, Australia, Spain, the US and Denmark who combined satellite data which tracks the concentration of phytoplankton by monitoring their green pigment, chlorophyll, with analysis of multiple pigments in water samples collected on research voyages going back decades.
The study reveals total phytoplankton increased slightly in the Southern Ocean, but there’s a concerning trend of decline in the proportion of diatoms, a major group of algae, across large areas, where they are outcompeted by smaller, less-nutritious phytoplankton species.
This is worrying, because diatoms are the preferred prey for Antarctic krill, a keystone species in the Antarctic marine ecosystem and a crucial food source for whales and penguins. Krill feed less efficiently on smaller phytoplankton species, and as the proportion of diatoms declines, they may be replaced by gelatinous salps. The latter are less fussy and can feed on many different phytoplankton groups. A decline in diatoms may favour salps at the expense of krill, Pinkerton says, shifting zooplankton populations to salp-dominated ecosystems, with flow-on effects for every creature in the food web.
The other impact of diminishing diatoms could be disruption to the ocean’s ability to lock away carbon. Diatoms are larger and heavier than other phytoplankton species and, wrapped in dense silica shells, they sink faster when they die, taking any carbon they’ve sequestered through photosynthesis (in the same way other plants do on land) to the ocean floor, where it can remain stashed for centuries. This is known as the ocean’s biological carbon pump, distinct from the physical and chemical processes that also draw carbon from the atmosphere and make the Southern Ocean one of Earth’s most important carbon sinks.
Smaller phytoplankton species are also part of this biological pump, but are more likely to be decomposed by microbes before they sink to the bottom, releasing carbon back into the atmosphere. “Phytoplankton are our friends,” Pinkerton says. “They’re sucking the carbon out of the atmosphere and pushing it deep down into the ocean where it’s tied up for 100 years or more, doing us a great favour. But some are better at it than others.”
Another recent study confirmed the Southern Ocean is seeing “abrupt changes” triggered by a warming climate. This is most obvious in the rapid loss of sea ice, but Pinkerton says monitoring the less-visible and complex rearrangements in phytoplankton will be critical.
