Climate change may double greenhouse gas emissions from freshwater lakes: Study

Climate change may cause the levels of greenhouse gases emitted by freshwater northern lakes to increase by between 1.5 and 2.7 times, according to a study. Researchers from the University of Cambridge in the UK noted that every drop of freshwater contains thousands of different organic molecules that have previously gone unnoticed. Small shallow lakes dominate the world's freshwater area, and the sediments within them already produce at least one-quarter of all carbon-dioxide, and more than two-thirds of all methane released from lakes into our atmosphere, they said.

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'We've been measuring 'carbon' in freshwater as a proxy'

"What we've traditionally called 'carbon' in freshwater turns out to be a super-diverse mixture of different carbon-based organic molecules," said Andrew Tanentzap from Cambridge, who led the research published in the journal PNAS. "We've been measuring 'carbon' in freshwater as a proxy for everything from water quality to the productivity of freshwater ecosystems. Now we've realised that it's the diversity of this invisible world of organic molecules that's important," Tanentzap said in a statement.

As the climate warms, vegetation cover is increasing in forests of the northern latitudes, the researchers said. By simulating this effect in two lakes in Ontario, Canada, the study found an increased diversity of organic molecules -- molecules containing carbon within their structure -- entering the water in the matter shed by nearby plants and trees. Organic molecules are a food source for microbes in the lake sediments, which break them down and release carbon dioxide and methane as by-products.

Increasing levels of organic molecules can, therefore, enhance microbial activity and produce more greenhouse gases. Since the same microbes can make greenhouse gases from many different organic molecules, the diversity of organic molecules was shown to be more closely linked with levels of greenhouse gas concentrations than the diversity of the microbes. An elevated diversity of organic molecules may elevate greenhouse gas concentrations in waters because there are more molecules that can be broken down by sunlight penetrating the water.

To conduct the research, containers were filled with varying ratios of rocks and organic material -- consisting of deciduous and coniferous litter from nearby forests --and submerged in the shallow waters of the two lakes. Analysis of the samples two months later, using the techniques of ultra-high resolution mass spectrometry and DNA sequencing, showed that the diversity of organic molecules was correlated with that of microbial communities in the water. It also found that the diversity of both increased as the amount of organic matter increased, the researchers said.

Accurately predicting carbon emissions from natural systems is vital to the reliability of calculations used to understand the pace of climate change, and the effects of a warmer world, they said. "Climate change will increase forest cover and change species composition, resulting in a greater variety of leaves and plant litter falling into waterways. "We found that the resulting increase in the diversity of organic molecules in the water leads to higher greenhouse gas concentrations," said Tanentzap. He noted that understanding these connections means researchers could look at ways to reduce carbon emissions in the future, for example by changing land management practices. The researchers said that changing the vegetation around freshwater areas could change the organic molecules that end up in the water.

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