A 2019 flood raised an Antarctic lake by 2 metres after water escaped from a neighbouring glacier-fed lake; scientists traced its effects on carbon cycling and microbial life | World News

In January 2019, a rare glacial flood caused an Antarctic lake to rise by two metres after a neighbouring lake released 17.5 million cubic metres of water. The event transformed Lake Untersee’s water chemistry and prompted scientists to investigate its effects on an unusual microbial ecosystem living beneath its permanent ice cover. The study, published in ResearchGate and titled ‘Glacial lake outburst floods enhance benthic microbial productivity in perennially ice-covered Lake Untersee (East Antarctica)’, found that the flood altered carbon concentrations and other chemical properties, potentially supporting photosynthetic microbial activity. Researchers also analysed microbial mats for evidence of earlier flooding, finding indications that similar events may have periodically influenced the lake’s carbon cycling over thousands of years.

How the 2019 Antarctic Lake flood changed microbial life

The flood began on 14–15 January 2019, when water was suddenly released through the ice cover of Lake Obersee. The researchers reported that the water travelled approximately 8.3 kilometres through a narrow channel along the eastern lateral moraine before entering the north-eastern corner of Lake Untersee. The event followed a decrease of 11.3 metres in Lake Obersee’s water level.According to the researchers, the flood added 17.5 million cubic metres of water to Lake Untersee, increasing its volume by three per cent. Lake Obersee experienced a volume decrease of 22.9 million cubic metres, suggesting that some of the released water may have drained towards the coast rather than entering Lake Untersee. The study estimated the flood’s average discharge at approximately nine to ten cubic metres per second over 21 days. The researchers also documented thermal erosion of the Anuchin Glacier and the adjacent moraine, along with the formation of a melt pool approximately 50 metres wide where the incoming water entered Lake Untersee.

Lake Untersee’s unique microbial ecosystem before the 2019 flood

Lake Untersee is one of the large, permanently ice-covered lakes in the Untersee Oasis of Dronning Maud Land, East Antarctica. It is approximately 6.5 kilometres long and 2.5 kilometres wide, with a maximum depth of 169 metres. Its water chemistry is unusual, with a high pH of around 10.6 and extremely low concentrations of dissolved inorganic and organic carbon. Before the 2019 flood, the lake had remained in a state of hydrological stability for several hundred years, according to the study.Despite the low availability of nutrients and the limited light penetrating its thick ice cover, Lake Untersee supports a benthic microbial ecosystem. The researchers described photosynthetic microbial mats, small pinnacles and large conical structures resembling stromatolites on the lake bed. Cyanobacteria, including Tychonema, Phormidium, Leptolyngbya and Pseudanabaena, dominate the upper layer of the mats, where photosynthesis fixes carbon. The underlying layers contain heterotrophic microorganisms, including members of the Actinobacteria, Verrucomicrobia, Proteobacteria and Bacteroidetes groups.

How the 2019 flood changed Lake Untersee’s water chemistry

The researchers found that the flood altered the chemical composition of Lake Untersee’s oxygenated water column. The lake’s pH decreased from 10.5 to 9.5, while specific conductivity fell from between 505 and 520 microsiemens per centimetre to 485 microsiemens per centimetre. Dissolved oxygen also decreased, from 150 per cent to 128 per cent saturation.The concentration of total inorganic carbon increased from 0.35 milligrams of carbon per litre to 0.5 milligrams per litre. The researchers also recorded changes in carbon isotope measurements, which helped them investigate the origin of the additional carbon. When the team returned to the lake in November 2019, a small stream was still flowing through the flood channel into its north-eastern corner. The stream had a pH of 7.7 and a total inorganic carbon concentration of 1.8 milligrams per litre, considerably different from the lake’s usual high-pH, low-carbon conditions.

How the 2019 flood affected carbon availability in Lake Untersee

The researchers explained that the lake’s photosynthetic microbial mats operate under conditions of extremely limited carbon availability. Before the flood, the uppermost layer of the mats was understood to be carbon dioxide-depleted, restricting photosynthesis and the sequestration of organic carbon.The flood may have supplied additional carbon to the lake, helping to support phototrophic activity. The researchers attributed the higher concentrations of total inorganic carbon and modern carbon recorded after the event partly to the uptake of atmospheric carbon dioxide through the melt pool that formed near the flood’s entry point. They estimated that an additional uptake of 0.11 milligrams of carbon per litre, equivalent to 21 per cent of the measured total inorganic carbon, would be required to account for both the post-flood carbon concentration and radiocarbon measurements.

How ancient floods shaped Lake Untersee’s carbon storage

The researchers also investigated whether similar floods had occurred before 2019. They analysed the grain sizes and carbon isotopes of microbial mats, finding evidence consistent with periodic glacial lake outburst floods during the Holocene. These findings offered a possible explanation for some of the complex patterns of carbon cycling and sequestration observed in Lake Untersee.Lake Untersee stores between 5.42 billion and 6.71 billion grams of carbon, with more than 95 per cent of that carbon held in its microbial mats. The researchers concluded that periodic flooding could replenish carbon in lakes where carbon dioxide availability is limited, potentially supporting microbial productivity over long periods. They suggested that similar processes might be relevant to other carbon dioxide-depleted Antarctic ecosystems, while also discussing their possible relevance to icy lakes on early Mars.The 2019 event demonstrated that a lake that had remained stable for centuries could undergo a substantial hydrological and chemical change following the partial drainage of a neighbouring lake. By combining satellite observations, water chemistry measurements and analyses of microbial mats, the researchers documented how the flood changed Lake Untersee and investigated the role that such events may play in sustaining its unusual ecosystem.