Vast blooms of phytoplankton may be lurking beneath the Antarctic ice

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Until now, it was commonly believed that the compacted sea ice of the Southern Ocean blocked all light from reaching the sea below, preventing phytoplankton, tiny algae that form the basis of aquatic food webs, from growing there. The less light is available, the less phytoplankton can photosynthesize and therefore the less phytoplankton there will be, severely limiting life under the ice.

However, research inspired by increased phytoplankton blooms under the ice in the Arctic has shown that Antarctic waters also have unexpected inhabitants, indicating that there is underreported ecological variability under the ice.

The blooms are often spotted as the sea ice begins its seasonal retreat, supported by plenty of light and fresh water with a high iron content. Yet a team led by Dr. Christopher Horvat of Brown University and the University of Auckland suspected there might already be potential phytoplankton blooms waiting. Write inside Frontiers in marine sciencedescribed using sampling from independent BGC-Argo floats and climate model output to estimate the availability of light under the ice to test this hypothesis.

“We found that nearly all examples of buoyancy profiles under Antarctic sea ice record increases in phytoplankton before the sea ice retreats,” Horvat said. “In many cases, we have observed significant blooms.” Horvat also pointed out that the floats sampled only a very small fraction of the millions of square kilometers of sea ice that could harbor these under-ice blooms, so there could be many more hidden phytoplankton blooms with the potential to support other life out there.

Floating laboratories

Floats are jettisoned from research vessels and left to take biogeochemical samples on their own — they can even detect freezing conditions near the water’s surface and submerge themselves to avoid being damaged by the ice. The key measurements here were levels of chlorophyll-a, a pigment shared by all phytoplankton, and particulate matter backscatter, which can be converted into an estimate of phytoplankton carbon because phytoplankton scatter light in proportion to their size. and concentration. Ultimately, the team used data from 51 floaters who made 2,197 ice dives from 2014 to 2021, which they compiled into 79 measurement runs.

“We used a new data product from a new NASA satellite, the ICESat-2 laser altimeter, to understand the firmness of the ice around Antarctica, and with a series of global climate models we looked at how much light reached the upper ocean,” Horvat said. .

“We found that 50% or more of Antarctica under the ice could support under-ice blooms, because sea ice in the Southern Ocean is composed of discrete ice floes and small areas of open water allow for light and therefore photosynthetic life.” .

Hidden ecosystems?

Measuring under compacted sea ice with complete or near complete coverage of underlying water, the scientists found that 88% of the measurement sequences recorded an increase in phytoplankton before sea ice retreat, and 26% met the requirements. criteria for a flowering under the ice .

However, the authors cautioned that the floats could sample under the ice but could not return data from these locations, meaning the coordinates for the sample locations aren’t completely accurate. “It is possible that some of the high-productivity events could be recorded in regions with low sea ice cover,” Horvat said.

‘Because the time we see these blooms is close to when the sea ice is retreating, it is also possible that some of the phytoplankton comes from processes occurring outside the sea ice zone, although we consider this unlikely given the number of high-throughput measurements that we found.”

The implications for Antarctic ecosystems could be significant. “Higher trophic levels migrate to where the productivity is, and if it’s under the ice, the food web might be expected to follow,” Horvat pointed out. More research will be needed to understand how these hidden ecosystems work and whether phytoplankton blooms attract predators and prey beneath the ice.

More information:
Evidence for phytoplankton bloom under Antarctic sea ice, Frontiers in marine science (2022). DOI: 10.3389/fmars.2022.942799

Citation: Vast phytoplankton blooms may be lurking under Antarctic ice (2022, Nov 17) Retrieved Nov 17, 2022 from

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