The water beneath the Antarctic sea ice was long thought to be unsuitable for life. However, a new study now suggests that phytoplankton - the food base for other marine life - grows under the thick layer of ice, at least seasonally. The authors measured the chlorophyll content and light intensity under the ice and supplemented the measurements with NASA Earth observation data and climate models. The results suggest that up to five million square kilometers of compact ice-covered Southern Ocean could provide conditions conducive to algal growth.
The densely packed, mostly snow-covered sea ice of the Southern Ocean reflects almost all light that strikes it. So far, it has been assumed that photosynthesis is hardly possible underneath and therefore no phytoplankton can grow. After all, the tiny algae that form the basis of all aquatic food webs need the sun's light to form biomass. In the Arctic, where the ice has already thinned due to climate change and reflects sunlight less than in Antarctica, phytoplankton blooms have already been detected under the ice. A research team led by Christopher Horvat from the University of Auckland in New Zealand has now searched for plankton under the Antarctic sea ice.
Search for traces with diving robots
It was already known that plankton blooms occur in Antarctic spring and summer in places that are not seasonally covered by ice. "The phytoplankton communities in the Southern Ocean respond rapidly to changes in light levels," the authors explain. "As soon as the edge of the sea ice recedes in spring, plankton blooms are often observed." However, Horvat and his team suspected that the tiny algae can already be growing under the ice. To test this thesis, the researchers sent automated diving robots, so-called Argo Floats, under the dense ice to take biogeochemical samples.
The robots swimming along the underside of the ice measured, among other things, the chlorophyll-a content in the water. This pigment is required for photosynthesis and is found in all types of phytoplankton. In addition, they recorded how much light the plankton particles found there scattered back - a measure of their size and concentration. Between 2014 and 2021, the floaters undertook a total of 2197 dives in spring and summer, which the researchers combined into 79 series of measurements.
Phytoplankton blooms before sea ice retreated
The result: "We found out that almost all series of measurements under the Antarctic sea ice show an increase in phytoplankton even before the sea ice retreated," says Horvat. "In many cases we have observed significant blooms." To classify the data, the researchers combined it with NASA Earth observation data and climate models: "We used a new data product, coming from the laser altimeter of NASA's new ICESat-2 satellite, to understand the compactness of the ice around Antarctica, and used a range of global climate models to study how much light reaches the upper ocean,” explains Horvat.
88 percent of the series of measurements that took place under the compact sea ice, which covers between 80 and 100 percent of the sea surface, showed significant phytoplankton growth even before the ice retreated. In 26 percent of the cases, the increase was so significant that the researchers interpreted it as an algal bloom under the ice. From the researchers' point of view, such a subglacial algal bloom is probably made possible by the fact that the ice cover of the Southern Ocean is dense, but not completely closed everywhere: "Since the sea ice in the Southern Ocean often consists of individual floes, small areas of open water remain free, which Letting light through and making photosynthetic life possible,” says Horvat.
Hidden ecosystems under the ice?
With the help of satellite data and climate models, the researchers generalized the results obtained by the robots. According to their estimates, three to five million square kilometers of the ice-covered Southern Ocean could provide conditions in which phytoplankton blooms can develop. This could have significant implications for Antarctic ecosystems: "Higher trophic levels migrate to where productivity is, and if it's under the ice, you might expect the food web to follow," says Horvat. It is possible that previously undiscovered ecosystems are hidden under the ice. More research is needed to discover these, if any, and to understand how they work.
Source: Christopher Horvat (University of Auckland, New Zealand) et al., Frontiers in Marine Science, doi: 10.3389/fmars.2022.942799