New Delhi, May 26 (PTI) Climate change could weaken a lake’s natural purification process during which microorganisms convert nitrogen compounds into nitrogen gas that is then released to the atmosphere
and effectively removed from the biosphere, a new study has found.
About 20 per cent of natural nitrogen removal in inland waters is attributable to the process of ‘denitrification’, researchers, led by those from the University of Basel in Switzerland and Eawag, Swiss Federal Institute of Aquatic Science and Technology, said.
For the study, published in the journal Nature Microbiology, the team collected samples from Lake Baldegg in the Lucerne Lake District in Switzerland.
The 5.3-square-kilometre lake is typical of many lakes in our latitudes, where the water completely mixes once a year, the researchers said.
They showed that denitrification activity is closely linked to seasonal mixing. In winter, the three layers of water in Lake Baldegg mix completely — the warm, oxygen-rich surface water, the transition zone, and the cold, oxygen-poor deep water.
During mixing, denitrification is nearly 50 per cent more active than during summer stratification (separation), the researchers said.
“The ability of lakes to remove nitrogen from the water is highly dependent on the season. And this is being altered by climate change,” lead author Cameron Callbeck, postdoctoral researcher at the University of Basel, said.
In a severe warming scenario, the winter mixing phase could shorten by about 27 days, and therefore, nitrogen removal from the lake would decrease accordingly.
“However, we don’t yet know why denitrification occurs particularly in winter,” Callbeck said.
Lakes play a crucial filtering role in the global nitrogen cycle — if they do not retain and break down nitrogen, it flows into the ocean via rivers, with potential severe consequences downstream, including algal blooms in coastal regions, “dead zones” with oxygen depletion, and stress on sensitive ecosystems, the researchers said.
“The study shows that even relatively small shifts in the seasonal mixing rhythm of lakes can measurably affect nitrogen cycling at the lake scale and, in aggregate, the global nitrogen cycle,” last author Moritz Lehmann, professor at the University of Basel, said.
The researchers measured denitrification in the lake through two methods — first, they spiked sediment samples with nitrogen molecules containing the rare isotope 15N.
The isotope allowed the team to use special measurement techniques to determine how much of the labelled nitrogen was converted into nitrogen gas — a measure of denitrification activity, they said.
The team then created a model for the entire Lake Baldegg to estimate how much nitrogen the lake breaks down in total.
“The isotope measurements and the calculated total balance matched between the observations and the model. This allowed us to make reliable predictions of lake nitrogen removal, and revealed that the winter period is indeed a hotspot of denitrification,” Callbeck said.
The authors wrote, “A worst-case climate scenario may shorten the mixing period by (nearly) 27 days and reduce denitrification by 8-13 per cent, increasing nitrogen export to downstream ecosystems.” They said, “We conclude that lake microbial denitrification, and its associated denitrifying consortium, will be weakened by climate change.” PTI KRS KRS MAH MAH
