Arctic lakes have two ways of emitting carbon dioxide | Polarjournal
(Photo: Karl Heuchel / Umeå University)

Arctic lakes are amongst the most common type of freshwater body. Therefore, knowing what happens to the organic matter that winds up in them is crucial for understanding whether they are helping to remove carbon dioxide from the atmosphere or contributing more to it, thus hastening the pace of global warming, and, more importantly, what will happen in a warmer climate. Previous research has failed to give a clear answer. The latest attempt underscores that the final tally is the sum of multiple factors.

As part of research for his thesis, Dirk Verheijen, of Umeå University, looked at 43 lakes in northern Sweden. One of the results was that Arctic lakes emit carbon dioxide either by letting off the carbon dioxide that has been produced elsewhere (what he calls serving as a “chimney”, or, more burdensome for the climate’s balance sheet, by converting the organic carbon they receive from the surrounding landscape into carbon dioxide (making them what he calls a “reactor”).

In most cases, the carbon dioxide a lake emits comes mostly from one process or the other, and precisely which type of process dominates is determined by the features of the lake itself and the surrounding area. Deep lakes in forested areas, for example, tend to release large amounts of carbon dioxide that was produced in the water. This is worth noting, according to Mr Verheijen, since it suggests that with tree lines moving North more lakes can be expected become producers of carbon dioxide.

This, however, is just one factor that must be taken into account when considering the impact of of Arctic lakes on carbon dioxide levels. Another is the seasonal variation in emissions. By conducting his research over a full year, rather than, as earlier studies tend to have done, Mr Verheijen found that, on average, lakes emit 55% of their carbon dioxide during ice-melt; lakes with low levels of carbon emitted nearly all of their carbon dioxide during this period. Not taking this into account, he reckons, could lead to lakes being classified as carbon sinks, when they are, in fact, adding more carbon dioxide to the air to the atmosphere than they are taking out.

On the other hand, Mr Verheijen’s work suggests that a warmer climate may, contrary to expectations, make lakes less productive through increased nutrient competition and changes to species composition. As a result, warmer lakes may produce less carbon dioxide, and may actually remove more from the atmosphere than they release.

“In a broader perspective, the thesis contributes to our knowledge of how Arctic lakes … relate to regional carbon cycles, and what lake and landscape drivers lead to them acting as ‘chimneys’ or ‘reactors’ in the landscape,” Mr Verheijen said.

Kevin McGwin, PolarJournal

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