Brown lakes - source or sink of CO2?

Over the past few decades, lakes have been undergoing a shift in water colour from rather clear to brown as a consequence of increased input of coloured dissolved organic carbon (DOC) from the terrestrial surroundings. This increase in DOC loadings can partly be explained by climate change. Higher temperatures yields enhanced biomass production in the catchment and together with intensified precipitation and run-off, more DOC enters the lakes.

Enhanced concentrations of coloured DOC, affects the CO₂ concentrations in lakes. CO₂ production increases when bacterioplankton consume the DOC and respire CO₂ and when the coloured DOC absorbs sunlight and photochemical processes converts it to CO₂. Sunlight absorption by DOC also shades the phytoplankton and hinders photosynthetic CO₂ uptake. Lakes with high concentrations of DOC therefore often act as net sources of CO₂ to the atmosphere.

In a new study, published this week in limnology and oceanography, we studied the CO₂ production in 70 lakes along a wide gradient in DOC concentrations. From physical, chemical and biological variables, we estimated microbial as well as photochemical CO₂ production. We were especially interested in whether the amount of photochemical CO₂ production in lakes depended on the DOC concentration.

Microbial CO₂ production was strongly dependent on DOC concentrations with more production in brown than in clear lakes. To the contrary, photochemical CO₂ production did not depend on colour but was similar in all lakes. The reason for this is that regardless of lake colour, all incoming solar radiation was absorbed in the water column. The largest difference between brown and clear lakes was at which depth the photochemical reactions took place. In the brownest lakes, close to all the photochemical CO₂ production happened at the surface, within only a couple of cm. In the clearest lakes, on the other hand, photochemical CO₂ production took place down to around five meters. This also implies that photosynthetic activity was restrained to the top few cm in brown lakes but could occur in a larger part of the clear lakes. CO₂ was mainly produced through respiration, and photochemical processes accounted for only a small fraction of lake CO₂ production. Increased sunlight absorption because of brownification therefore mainly affects lake CO₂ concentrations through inhibited photosynthesis and not through enhanced photochemical processes. 

With the ongoing climate warming, we can expect DOC input to lakes to keep increasing and by that lakes becoming browner. As they do, CO₂ production in lakes will increase, mainly because of increased respiration, and most probably photosynthetic CO₂ consumption will decrease, turning more lakes net sources of CO₂ to the atmosphere.

The paper can be found here.

Lina Allesson