Abstract
AbstractMany surface waters across the boreal region are browning due to increased concentrations of colored allochthonous dissolved organic carbon (DOC). Browning may stimulate heterotrophic metabolism, may have a shading effect constraining primary production, and may acidify the water leading to decreased pH with a subsequent shift in the carbonate system. All these effects are expected to result in increased lake water carbon dioxide (CO2) concentrations. We tested here these expectations by assessing the effects of both altered allochthonous DOC input and light conditions through shading on lake water CO2 concentrations. We used two mesocosm experiments with water from the meso‐eutrophic Lake Erken, Sweden, to determine the relative importance of bacterial activities, primary production, and shifts in the carbonate system on CO2 concentrations. We found that DOC addition and shading resulted in a significant increase in partial pressure of CO2 (pCO2) in all mesocosms. Surprisingly, there was no relationship between bacterial activities and pCO2. Instead the experimental reduction of light by DOC and/or shading decreased the photosynthesis to respiration ratio leading to increased pCO2. Another driving force behind the observed pCO2 increase was a significant decrease in pH, caused by a decline in photosynthesis and the input of acidic DOC. Considering that colored allochthonous DOC may increase in a warmer and wetter climate, our results could also apply for whole lake ecosystems and pCO2 may increase in many lakes through a reduction in the rate of photosynthesis and decreased pH.
Highlights
Studies have shown that bacterial respiration of allochthonous dissolved organic carbon (DOC) is one of the key drivers of net heterotrophy in high-DOC lakes (Tranvik 1992; del Giorgio and Peters 1994)
We propose that increased allochthonous DOC input will lead to enhanced CO2 concentrations, which, in mesoeutrophic lakes, will result in a reversal from net uptake to net release of CO2 (Fig. 1)
Changes in light climate In Experiment A, additions of DOC significantly reduced the light throughout the water column in the mesocosms, with the DOC from HuminFeed having a stronger effect on light attenuation than DOC from the reverse osmosis concentrate (Table 2)
Summary
Allochthonous DOC generally contains large proportions of humic-like components with high amounts of aromatic structures, which give water a brownish color. These chromophoric aromatic structures are effective at absorbing photosynthetically active radiation (PAR); allochthonous DOC may have a strong positive effect on light attenuation (Jones 1992; Pace and Cole 2002). This increased light attenuation can constrain primary production as a large fraction of PAR is absorbed by the DOC rather than by the photoautotrophs (Jones 1992). Decreasing pH trends were observed in clear water lakes in northern Wisconsin during winter, when primary production was low, and it was suggested that this was due to build-up of under ice pCO2 levels (Kratz et al 1987)
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