Degradation potentials of dissolved organic carbon (DOC) from thawed permafrost peat

Global warming can substantially affect the export of dissolved organic carbon (DOC) from peat-permafrost to aquatic systems. The direct degradability of such peat-derived DOC, however, is poorly constrained because previous permafrost thaw studies have mainly addressed mineral soil catchments or DOC pools that have already been processed in surface waters. We incubated peat cores from a palsa mire to compare an active layer and an experimentally thawed permafrost layer with regard to DOC composition and degradation potentials of pore water DOC. Our results show that DOC from the thawed permafrost layer had high initial degradation potentials compared with DOC from the active layer. In fact, the DOC that showed the highest bio- and photo-degradability, respectively, originated in the thawed permafrost layer. Our study sheds new light on the DOC composition of peat-permafrost directly upon thaw and suggests that past estimates of carbon-dioxide emissions from thawed peat permafrost may be biased as they have overlooked the initial mineralization potential of the exported DOC

Reactive dissolved organic carbon dynamics in a changing environment : Experimental evidence from soil and water

Dissolved organic carbon (DOC) is the major form of organic carbon in aquatic ecosystems. Biological and photochemical degradation of DOC are major causes of greenhouse gas emissions from aquatic ecosystems. In response to current changes in climate, studies at different northern-hemisphere locations have shown both increases and decreases in total DOC export from land to water. However, there is lack of knowledge on the effect of changing environmental conditions on bio- and photo-degradation potentials. Therefore, this thesis aimed to combine empirical and experimental methods to determine the dynamics of DOC reactivity in response to key environmental parameters related to soil frost conditions and hydrology. I collected samples from the boreal region to carry out laboratory degradation experiments and to analyze the DOC degradation potentials in relation to environmental variables and intrinsic properties of the DOC.In a boreal forest region that has seasonal soil frost, the DOC bio-reactivity decreased with experimental reductions in the extent and duration of soil frost, while photo-reactivity remained constant. However, DOC from experimentally thawed permafrost peat in the subarctic north showed higher bio- and photo-degradation potentials than the DOC from active peat layers above the permafrost. The nature of the soil and intrinsic DOC properties explained these patterns in soil DOC reactivity. In the surface water network, bio-degradation was the dominant DOC degradation mechanism in brow-water lakes, while photo-degradation played a relatively larger role in clearer waters. The surface water photo-reactivity per unit of light absorption was high at both acidic and at alkaline conditions, while lower at intermediate pH, probably due to effects of the protonation state of DOC on photo-reactivity. Therefore, pH change along the aquatic network caused extrinsic control on the photo-reactivity at the landscape scale.In conclusion the results show that climate warming decreases the export of bio-reactive DOC in a boreal forest, but it likely increases the export of bio- and photo-reactive DOC in a permafrost peatland. In a wetter climate with shorter residence times, clear waters may experience browning and relative shift from photo-degradation to bio-degradation, while already brown lakes will remain brown and dominated by bio-degradation. Furthermore, the loss of photo reactive DOC during transport in aquatic network may be counter balanced by the changes in extrinsic variables such as pH

Influence of weather variables on methane and carbon dioxide flux from a shallow pond

Freshwaters are important sources of the greenhouse gases methane (CH4) and carbon dioxide (CO2) to the atmosphere. Knowledge about temporal variability in these fluxes is very limited, yet critical for proper study design and evaluating flux data. Further, to understand the reasons for the variability and allow predictive modeling, the temporal variability has to be related to relevant environmental variables. Here we analyzed the effect of weather variables on CH4 and CO2 flux from a small shallow pond during a period of 4 months. Mean CH4 flux and surface water CH4 concentration were 8.0 [3.3-15.1] +/- A 3.1 mmol m(-2) day(-1) (mean [range] +/- A 1 SD) and 1.3 [0.3-3.5] +/- A 0.9 A mu M respectively. Mean CO2 flux was 1.1 [-9.8 to 16.0] +/- A 6.9 mmol m(-2) day(-1). Substantial diel changes in CO2 flux and surface water CH4 concentration were observed during detailed measurements over a 24 h cycle. Thus diel patterns need to be accounted for in future measurements. Significant positive correlations of CH4 emissions with temperature were found and could include both direct temperature effects as well as indirect effects (e.g. related to the growth season and macrophyte primary productivity providing organic substrates). CO2 flux on the other hand was negatively correlated to temperature and solar radiation, presumably because CO2 consumption by plants was higher relative to CO2 production by respiration during warm sunny days. Interestingly, CH4 fluxes were comparable to ponds with similar morphometry and macrophyte abundance in the tropics. We therefore hypothesize that CH4 and CO2 summer emissions from ponds could be more related to the morphometry and dominating primary producers rather than latitude per se. Data indicate that CH4 emissions, given the system characteristic frameworks, is positively affected by increased temperatures or prolonged growth seasons

Photo-reactivity (apparent quantum yield) of dissolved organic carbon during the freshwater transit from land to sea

In spite of substantial greenhouse gas emissions from photo-degradation of dissolved organic carbon (DOC), little is known about the variability in DOC photo-reactivity. It has been suggested that photo-reactivity depends on the degree of DOC pigmentation. However, recent evidence suggests that also non-colored fractions can be photo-reactive. Furthermore photo-degradation processes interact with extrinsic factors such as pH and Fe concentrations. Thus, the overall changes in photo-reactivity in the aquatic network are poorly known. We measured apparent quantum yield for CO2 production normalized to 350 nm (AQY350) in water from streams, lakes and rivers in boreal Sweden. Additionally, samples were incubated for 365 days to follow AQY350 over the long-term course of microbial degradation. We found no trends of decreasing AQY350 with overall losses in DOC color across space and time. Interestingly, there was a quadratic (U-shaped) relationship between AQY and pH. Overall, in situ AQY350 increased with reductions in specific DOC absorbance and with rising autochthonous DOC contributions. We suggest that partly due to extrinsic factors, photo-reactivity of DOC is sustained during the DOC residence time in the aquatic network

Bioavailability of stream dissolved organic carbon (DOC) during spring flood and base flow in high-latitude streams

An important component of the carbon cycle is the lateral flow of dissolved organic carbon (DOC) from terrestrial ecosystems to streams and rivers. The fate of this carbon depends very much on the bioavailability of DOC (BDOC), which may determine whether DOC is returned to the atmosphere as CO2 or deposited in sediments. This study focuses on the linkages between stream DOC composition, optical characteristics and bioavailability along vegetation gradients in subarctic Sweden. We sampled streams from tundra, birch forest, and boreal forest ecoregions, which encompass large differences in C:N ratios (6.4-30.1) and spectroscopic characteristics, all related to variation in landscape properties. The DOC bioavailability was determined through laboratory bioassays carried out twice during the year (spring-flood and base flow). During spring flood, DOC concentration varied between 0.5 - 6.7 mg L-1 and the BDOC ranged between 3- 24 %, with the highest BDOC from birch forest/tundra and tundra streams. Results suggest that broad-scale transitions in vegetation structure across sub-arctic landscapes have important implications for the quantity and quality of DOC delivered to aquatic ecosystems

Quality transformation of dissolved organic carbon during water transit through lakes : contrasting controls by photochemical and biological processes

Dissolved organic carbon (DOC) may be removed, transformed, or added during water transit through lakes, resulting in changes in DOC composition and pigmentation (color). However, the process-based understanding of these changes is incomplete, especially for headwater lakes. We hypothesized that because heterotrophic bacteria preferentially consume noncolored DOC, while photochemical processing removes colored fractions, the overall changes in DOC color upon water passage through a lake depend on the relative importance of these two processes, accordingly. To test this hypothesis we combined laboratory experiments with field studies in nine boreal lakes, assessing both the relative importance of different DOC decay processes (biological or photochemical) and the loss of color during water transit time (WTT) through the lakes. We found that influence from photo-decay dominated changes in DOC quality in the epilimnia of relatively clear headwater lakes, resulting in systematic and selective net losses of colored DOC. However, in highly pigmented brown-water lakes (absorbance at 420 nm > 7 m(-1)) biological processes dominated, and there was no systematic relationship between color loss and WTT. Moreover, in situ data and dark experiments supported our hypothesis on the selective microbial removal of nonpigmented DOC, mainly of low molecular weight, leading to persistent water color in these highly colored lakes. Our study shows that brown headwater lakes may not conform to the commonly reported pattern of the selective removal of colored constituents in freshwaters, as DOC can show a sustained degree of pigmentation upon transit through these lakes