Colored organic matter increases CO₂ in meso‐eutrophic lake water through altered light climate and acidity

Abstract

Many 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.

Comprehensive analysis of chemical and biological problems associated with browning agents used in aquatic studies

“…In conclusion, our extensive tests of leonardite products raise multiple concerns especially on the suitability of HF as proxies for natural browning of freshwater ecosystems. Our experiments (this study, Attermeyer et al 2019; Nydahl et al 2019, Chaguaceda et al unpubl.) show that it is feasible to prepare reverse osmosis concentrates for browning of several thousand liters.…”

“…It was located in Lake Erken (59°50′09.6″N, 18°37′52.3″E), held and fixed to a floating wooden jetty close to the lake shore. Details of the experimental setup can be found in Nydahl et al (2019). In short, after filling the mesocosms with lake water, four treatments with five replicates of the following DOC concentrations (mean ± standard error) were established: Control (13.0 ± 0.05 mg C L −1 ), HF (18.4 ± 0.06 mg C L −1 ), RO (18.1 ± 0.10 mg C L −1 ), and HF + RO (23.5 ± 0.05 mg C L −1 ).…”

Potential terrestrial influence on transparent exopolymer particle concentrations in boreal freshwaters

“…The mesocosm experiment was part of a larger study that aimed to investigate the influence of light and addition of terrestrial DOM on biogeochemical processes and the food web (refer to Nydahl et al for more details). In August/September 2016 for a period of 4 weeks, 20 mesocosms were set up in Lake Erken in Sweden (meso‐eutrophic, dimictic; 59°50′09.7″N 18°37′52.7″E) with four different treatments in a 2 × 2 factorial design and with five replicates each.…”

Diverse drivers of long-term pCO2 increases across thirteen boreal lakes and streams

“…In 4 of the lakes, water color increased; hence, despite no change in TP, the darkening of the water in these 4 lakes could have reduced the primary production. As water color increases, less light is available for primary production, and thus CO 2 bio-uptake would decrease and pCO 2 subsequently increase (Jones 1992, Thrane et al 2014, Nydahl et al 2019. However, teasing apart the effect that increased DOC may have on primary production from the effect that increased DOC can have on bacterial mineralization is difficult.…”