We determined microbial decomposition of dissolved organic carbon (DOC) over 3.7 year long dark bioassays of six Swedish lake waters. The overall lost DOC fraction was similar in clearwater lakes ...(34.8 ± 2.4%) and in brownwater lakes (37.8 ± 1.9%). Reactivity continuum modeling revealed that the most labile DOC fraction, degrading at rates >0.01 d−1, was larger in the clearwater lakes (11.1 ± 1.2%) than in the brownwater lakes (0.8 ± 0.1%). The initial apparent first‐order decay coefficientk was fivefold larger in the clearwater lakes (0.0043 ± 0.0012 d−1) than in the brownwater lakes (0.0009 ± 0.0003 d−1). Over time, k decreased more steeply in the clearwater lakes than in the brownwater lakes, reaching the k of the brownwater lakes within 5 months. Finally, k averaged 0.0001 d−1 in both lake categories. In the brownwater lakes, colored dissolved organic matter (CDOM) absorption decayed with an initial k twice as large (0.0018 ± 0.0008 d−1) as that of DOC. The initial kwas inversely correlated with initial specific UV absorption and CDOM absorption and positively correlated with initial tryptophan‐like fluorescence as proxy for autochthonous DOC. Exposure to simulated sunlight at the end of the incubations caused loss of color in the clearwater lakes and loss of DOC in the brownwater lakes, where subsequent mineralization was also stimulated. The DOC lost in the absence of photochemical processes fell below previously reported watershed‐scale losses in Sweden by 25% at most. This suggests that a major part of the in situ DOC loss could potentially be attributed to dark reactions alone.
Key Points
Allochthonous and autochthonous DOC were similarly bioavailable in the long term
Optical properties predicted initial differences in bioavailability
Substantial mineralization of allochthonous DOC without solar irradiation
We quantified sedimentation of organic carbon in 12 Swedish small boreal lakes (<0.48 km²), which ranged in dissolved organic carbon (DOC) from 4.4 to 21.4 mg C l⁻¹. Stable isotope analysis suggests ...that most of the settling organic matter is of allochthonous origin. Annual sedimentation of allochthonous matter per m² lake area was correlated to DOC concentration in the water (R ² = 0.41), and the relationship was improved when sedimentation data were normalized to water depth (R ² = 0.58). The net efflux of C as CO₂ from the water to the atmosphere was likewise correlated to DOC concentration (R ² = 0.52). The losses of organic carbon from the water column via mineralization to CO₂ and via sedimentation were approximately of equal importance throughout the year. Our results imply that DOC is a precursor of the settling matter, resulting in an important pathway in the carbon cycle of boreal lakes. Thus, flocculation of DOC of terrestrial origin and subsequent sedimentation could lead to carbon sequestration by burial in lake sediments.
The biogeochemical processing of dissolved organic matter (DOM) in inland waters is inherently related to its molecular structure and ecological function. Controlled bioassays are a valuable tool to ...analyze these relationships, but are seldom conducted and compared at temporal scales that typically prevail in natural inland waters. Here we incubated water from six boreal lakes in the dark and examined changes to the initial fluorescence and absorbance after 3.5years. We identified five fluorescence components with parallel factor (PARAFAC) analysis (CC, CM, CA, CX and CT) and found a consistent change in the relative intensity of two dominant PARAFAC components (increase in CA:CC, corresponding to Peak A:Peak C), commonly found in lake water, that represent terrestrially-derived DOM. Surprisingly, we only found minor changes to specific absorbance (SUVA), and did not find any changes to other spectral indexes including the fluorescence index, humification index and freshness index. By incorporating lakes spanning a wide range of initial total organic carbon concentrations (3.7 to 32.5mgL−1), water residence times, and spectral characteristics (e.g. SUVA 1.13 to 3.77L·mgC−1·m−1), we found that the relative intensities of two humic-like peaks were the most revealing of changes to DOM structure during dark incubations. We also verified that inner filter effects were adequately corrected within the concentration range of incubated samples. Thus, the processing of DOM under dark conditions, including microbial decomposition and flocculation, may have a greater influence on the humic-like peaks, particularly CC (Peak C), with negligible changes to more commonly used spectral indexes.
► One of the two common DOC fluorescence peaks degrades faster in the dark. ► Dark degradation processes within lakes could alter DOC structure. ► Previous spectral indexes should be interpreted with care for dark incubations.
The molecular composition and origin has recently been demonstrated to play a critical role in the persistence of organic matter in lake water, but it is unclear to what degree chemical attributes ...and sources may also control settling and burial of organic matter in lake sediments. Here we compared the annual contribution of allochthonous and autochthonous sources to the organic matter settling in the water column and present in the sediments of 12 boreal lakes. We used the fluorescence properties and elemental composition of the organic matter to trace its origin and found a consistent pattern of increasing contribution of terrestrial compounds in the sediments as compared to the settling matter, with an annual average allochthony of ~87% and ~57%, respectively. Seasonal data revealed a predominance of in‐lake‐produced compounds sinking in the water column in summer. Yet only a slight concurrent decrease in the contribution of terrestrial C to lake sediments was observed during the same period, and sediment allochthony increased again to high levels in autumn. Our results reveal a preferential preservation of allochthonous matter in the sediments and highlight the role of lakes as sequesters of organic carbon primarily originating from the surrounding landscape.
Key Points
High contribution of terrestrially derived organic matter to lake sediments
Dominance of autochthonous C in settling matter in summer but not in sediments
Lakes as sequesters of terrestrially derived C in the boreal landscape
We explore the role of lakes in carbon cycling and global climate, examine the mechanisms influencing carbon pools and transformations in lakes, and discuss how the metabolism of carbon in the inland ...waters is likely to change in response to climate. Furthermore, we project changes as global climate change in the abundance and spatial distribution of lakes in the biosphere, and we revise the estimate for the global extent of carbon transformation in inland waters. This synthesis demonstrates that the global annual emissions of carbon dioxide from inland waters to the atmosphere are similar in magnitude to the carbon dioxide uptake by the oceans and that the global burial of organic carbon in inland water sediments exceeds organic carbon sequestration on the ocean floor. The role of inland waters in global carbon cycling and climate forcing may be changed by human activities, including construction of impoundments, which accumulate large amounts of carbon in sediments and emit large amounts of methane to the atmosphere. Methane emissions are also expected from lakes on melting permafrost. The synthesis presented here indicates that (1) inland waters constitute a significant component of the global carbon cycle, (2) their contribution to this cycle has significantly changed as a result of human activities, and (3) they will continue to change in response to future climate change causing decreased as well as increased abundance of lakes as well as increases in the number of aquatic impoundments.
The flocculation of allochthonous dissolved organic carbon (DOC) in lakes was investigated in a series of laboratory experiments. Flocculation was enhanced by increased temperatures, and addition of ...glucose increased the flocculation further. A change in pH within the range of 3.3 to 7.3 resulted in decreased flocculation with increasing deviation from the original pH. Flocculation was similar under oxic and anoxic conditions. For all treatments (i.e., temperature and glucose, pH, and O₂ regime), flocculation was positively correlated to bacterial respiration. However, bacterial biomass made up a negligible fraction of the formed floes, suggesting that the formation of detrital particles was a result of bacterial activity, but there was no significant contribution of bacteria to the mass of the particles formed. In all experiments, both the concentration of DOC and concentration of colored dissolved organic matter (CDOM) decreased concomitantly with flocculation, suggesting that CDOM is the precursor of the floes. Bacteria mediate a translocation of DOC in the water column into particles prone to gravitational settling.
We measured flocculation of dissolved organic carbon (DOC) in the water from a humic lake (DOC = 14.9 mg C L⁻¹) and from an adjacent mire (DOC = 25.7 mg C L⁻¹), in in situ enclosure experiments with ...different light regimes. Light stimulated the formation of organic particles in both waters, and organic particle formation was observed at all incubation depths, even in the dark controls. Production of phytoplankton biomass was negligible, and allochthonous DOC was the most important precursor of the sinking particles. 8-22% and 25-60% of the loss of DOC in lake and mire water, respectively, could be accounted for by flocculation. Depthintegrated flocculation based on the enclosure experiments was 14.7 mg C m⁻² d⁻¹. Lake-water DOC concentration and water color has been increasing during the last decade, and sediment trap studies show that gross sedimentation of organic carbon also increased. Thus flocculation of allochthonous DOC, stimulated by light, constitutes a pathway for the sequestration of carbon in lake sediments.
The membrane lipid ergosterol is found almost exclusively in fungi, and is frequently used by environmental microbiologists as an indicator of living fungal biomass, based on the assumption that ...ergosterol is labile, and therefore rapidly degraded after the death of fungal hyphae. We studied the degradation of ergosterol in environmental samples without living fungi. Under the conditions used in this study, ergosterol was very stable both when added as a pure compound and when associated with dead fungi. The decrease of ergosterol was at most 34% during 2 months when protected from sunlight. Presence of a natural bacterial assemblage did not enhance degradation over this time period, as compared to sterile controls. However, photochemical degradation was significant, and led to a 43% decrease of in ergosterol content during 24 h. These results suggest that ergosterol should be used cautiously as a biomarker for living fungi.
The biogeochemical processing of dissolved organic matter (DOM) in inland waters is inherently related to its molecular structure and ecological function. Controlled bioassays are a valuable tool to ...analyze these relationships, but are seldom conducted and compared at temporal scales that typically prevail in natural inland waters. Here we incubated water from six boreal lakes in the dark and examined changes to the initial fluorescence and absorbance after 3.5 years. We identified five fluorescence components with parallel factor (PARAFAC) analysis (C C, C M , C A , C X and C T ) and found a consistent change in the relative intensity of two dominant PARAFAC components (increase in C A :C C , corresponding to Peak A:Peak C), commonly found in lake water, that represent terrestrially-derived DOM. Surprisingly, we only found minor changes to specific absorbance (SUVA), and did not find any changes to other spectral indexes including the fluorescence index, humification index and freshness index. By incorporating lakes spanning a wide range of initial total organic carbon concentrations (3.7 to 32.5 mg L − 1 ), water residence times, and spectral characteristics (e.g. SUVA 1.13 to 3.77 L·mg C − 1 ·m − 1 ), we found that the relative intensities of two humic-like peaks were the most revealing of changes to DOM structure during dark incubations. We also verified that inner filter effects were adequately corrected within the concentration range of incubated samples. Thus, the processing of DOM under dark conditions, including microbial decomposition and flocculation, may have a greater influence on the humic-like peaks, particularly C C (Peak C), with negligible changes to more commonly used spectral indexes.
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