The Arctic is a water-rich region, with freshwater systems covering about 16 % of the northern permafrost landscape. Permafrost thaw creates new freshwater ecosystems, while at the same time ...modifying the existing lakes, streams, and rivers that are impacted by thaw. Here, we describe the current state of knowledge regarding how permafrost thaw affects lentic (still) and lotic (moving) systems, exploring the effects of both thermokarst (thawing and collapse of ice-rich permafrost) and deepening of the active layer (the surface soil layer that thaws and refreezes each year). Within thermokarst, we further differentiate between the effects of thermokarst in lowland areas vs. that on hillslopes. For almost all of the processes that we explore, the effects of thaw vary regionally, and between lake and stream systems. Much of this regional variation is caused by differences in ground ice content, topography, soil type, and permafrost coverage. Together, these modifying factors determine (i) the degree to which permafrost thaw manifests as thermokarst, (ii) whether thermokarst leads to slumping or the formation of thermokarst lakes, and (iii) the manner in which constituent delivery to freshwater systems is altered by thaw. Differences in thaw-enabled constituent delivery can be considerable, with these modifying factors determining, for example, the balance between delivery of particulate vs. dissolved constituents, and inorganic vs. organic materials. Changes in the composition of thaw-impacted waters, coupled with changes in lake morphology, can strongly affect the physical and optical properties of thermokarst lakes. The ecology of thaw-impacted lakes and streams is also likely to change; these systems have unique microbiological communities, and show differences in respiration, primary production, and food web structure that are largely driven by differences in sediment, dissolved organic matter, and nutrient delivery. The degree to which thaw enables the delivery of dissolved vs. particulate organic matter, coupled with the composition of that organic matter and the morphology and stratification characteristics of recipient systems will play an important role in determining the balance between the release of organic matter as greenhouse gases (CO2 and CH4), its burial in sediments, and its loss downstream. The magnitude of thaw impacts on northern aquatic ecosystems is increasing, as is the prevalence of thaw-impacted lakes and streams. There is therefore an urgent need to quantify how permafrost thaw is affecting aquatic ecosystems across diverse Arctic landscapes, and the implications of this change for further climate warming.
As Arctic regions warm and frozen soils thaw, the large organic carbon pool stored in permafrost becomes increasingly vulnerable to decomposition or transport. The transfer of newly mobilized carbon ...to the atmosphere and its potential influence upon climate change will largely depend on the degradability of carbon delivered to aquatic ecosystems. Dissolved organic carbon (DOC) is a key regulator of aquatic metabolism, yet knowledge of the mechanistic controls on DOC biodegradability is currently poor due to a scarcity of long-term data sets, limited spatial coverage of available data, and methodological diversity. Here, we performed parallel biodegradable DOC (BDOC) experiments at six Arctic sites (16 experiments) using a standardized incubation protocol to examine the effect of methodological differences commonly used in the literature. We also synthesized results from 14 aquatic and soil leachate BDOC studies from across the circum-arctic permafrost region to examine pan-arctic trends in BDOC. An increasing extent of permafrost across the landscape resulted in higher DOC losses in both soil and aquatic systems. We hypothesize that the unique composition of (yedoma) permafrost-derived DOC combined with limited prior microbial processing due to low soil temperature and relatively short flow path lengths and transport times, contributed to a higher overall terrestrial and freshwater DOC loss. Additionally, we found that the fraction of BDOC decreased moving down the fluvial network in continuous permafrost regions, i.e. from streams to large rivers, suggesting that highly biodegradable DOC is lost in headwater streams. We also observed a seasonal (January-December) decrease in BDOC in large streams and rivers, but saw no apparent change in smaller streams or soil leachates. We attribute this seasonal change to a combination of factors including shifts in carbon source, changing DOC residence time related to increasing thaw-depth, increasing water temperatures later in the summer, as well as decreasing hydrologic connectivity between soils and surface water as the thaw season progresses. Our results suggest that future climate warming-induced shifts of continuous permafrost into discontinuous permafrost regions could affect the degradation potential of thaw-released DOC, the amount of BDOC, as well as its variability throughout the Arctic summer. We lastly recommend a standardized BDOC protocol to facilitate the comparison of future work and improve our knowledge of processing and transport of DOC in a changing Arctic.
Northern high‐latitude rivers transport large amounts of terrestrially derived dissolved organic matter (DOM) from boreal and arctic ecosystems to coastal areas and oceans. Current knowledge of the ...biodegradability of DOM in these rivers is limited, particularly for large rivers discharging to the Arctic Ocean. We conducted a seasonally comprehensive study of biodegradable dissolved organic carbon (BDOC) dynamics in the Yukon River and two of its tributaries in Alaska, USA. Distinct seasonal patterns of BDOC, consistent across a wide range of watershed size, indicate BDOC is transported year‐round. Relative biodegradability (%BDOC) was greatest during winter, and decreased into spring and summer. Due to large seasonal differences in DOC concentration, the greatest concentrations of BDOC (mg C L−1) occurred during spring freshet, followed by winter and summer. While chemical composition of DOM was an important driver of BDOC, the overriding control of BDOC was mineral nutrient availability due to wide shifts in carbon (C) and nitrogen (N) stoichiometry across seasons. We calculated seasonal and annual loads of BDOC exported by the Yukon River by applying measured BDOC concentrations to daily water discharge values, and also by applying an empirical correlation between %BDOC and the ratio of DOC to dissolved inorganic N (DIN) to total DOC loads. The Yukon River exports ∼0.2 Tg C yr−1 as BDOC that is decomposable within 28 days. This corresponds to 12–18% of the total annual DOC export. Furthermore, we calculate that the six largest arctic rivers, including the Yukon River, collectively export ∼2.3 Tg C yr−1 as BDOC to the Arctic Ocean.
Key Points
Biodegradable DOC is transported by high‐latitude rivers year‐round
Biodegradable DOC is driven by inorganic nitrogen availability
Major arctic rivers export approximately 2.3 Tg C yr‐1 as Biodegradable DOC
Approximately 1700 Pg of soil carbon (C) are stored in the northern circumpolar permafrost zone, more than twice as much C than in the atmosphere. The overall amount, rate, and form of C released to ...the atmosphere in a warmer world will influence the strength of the permafrost C feedback to climate change. We used a survey to quantify variability in the perception of the vulnerability of permafrost C to climate change. Experts were asked to provide quantitative estimates of permafrost change in response to four scenarios of warming. For the highest warming scenario (RCP 8.5), experts hypothesized that C release from permafrost zone soils could be 19–45 Pg C by 2040, 162–288 Pg C by 2100, and 381–616 Pg C by 2300 in CO
2
equivalent using 100-year CH
4
global warming potential (GWP). These values become 50 % larger using 20-year CH
4
GWP, with a third to a half of expected climate forcing coming from CH
4
even though CH
4
was only 2.3 % of the expected C release. Experts projected that two-thirds of this release could be avoided under the lowest warming scenario (RCP 2.6). These results highlight the potential risk from permafrost thaw and serve to frame a hypothesis about the magnitude of this feedback to climate change. However, the level of emissions proposed here are unlikely to overshadow the impact of fossil fuel burning, which will continue to be the main source of C emissions and climate forcing.
The fate of carbon (C) contained within permafrost in boreal forest environments is an important consideration for the current and future carbon cycle as soils warm in northern latitudes. Currently, ...little is known about the microbiology or chemistry of permafrost soils that may affect its decomposition once soils thaw. We tested the hypothesis that low microbial abundances and activities in permafrost soils limit decomposition rates compared with active layer soils. We examined active layer and permafrost soils near Fairbanks, AK, the Yukon River, and the Arctic Circle. Soils were incubated in the lab under aerobic and anaerobic conditions. Gas fluxes at −5 and 5 °C were measured to calculate temperature response quotients (Q10). The Q10 was lower in permafrost soils (average 2.7) compared with active layer soils (average 7.5). Soil nutrients, leachable dissolved organic C (DOC) quality and quantity, and nuclear magnetic resonance spectroscopy of the soils revealed that the organic matter within permafrost soils is as labile, or even more so, than surface soils. Microbial abundances (fungi, bacteria, and subgroups: methanogens and Basidiomycetes) and exoenzyme activities involved in decomposition were lower in permafrost soils compared with active layer soils, which, together with the chemical data, supports the reduced Q10 values. CH4 fluxes were correlated with methanogen abundance and the highest CH4 production came from active layer soils. These results suggest that permafrost soils have high inherent decomposability, but low microbial abundances and activities reduce the temperature sensitivity of C fluxes. Despite these inherent limitations, however, respiration per unit soil C was higher in permafrost soils compared with active layer soils, suggesting that decomposition and heterotrophic respiration may contribute to a positive feedback to warming of this eco region.
Ice wedge degradation is a widespread occurrence across the circumpolar Arctic causing extreme spatial heterogeneity in water distribution, vegetation, and energy balance across landscapes. These ...heterogeneities influence carbon dioxide (CO2) and methane (CH4) fluxes, yet there is little understanding of how they effect change in landscape‐level carbon (C) gas flux over time. We measured CO2 and CH4 fluxes in an area undergoing ice wedge degradation near Prudhoe Bay, Alaska, and combined with repeat imagery analysis to estimate seasonal landscape‐level C flux response to geomorphic change. Net CO2 and CH4 emissions changed by −25% and +42%, respectively, resulting in a 14% increase in seasonal CO2‐C equivalent emissions over 69 years as ice wedge degradation formed water‐filled troughs. The dynamic ice wedge degradation/stabilization process can cause significant changes in CO2 and CH4 fluxes over time, and the integration of this process is important to forecasting landscape‐level C fluxes in permafrost regions abundant in ice wedges.
Plain Language Summary
Large wedge‐shaped masses of ice commonly occur in Arctic tundra just below the ground surface. These ice wedges form polygon patterns observable by aircraft and remote sensing. Warming temperatures are causing ice wedges to melt, forming troughs that widen, deepen, and flood over time to become small ponds. Some ice wedges reform over decades as troughs become drier and seasonal frost persists belowground. We conducted field measurements to quantify how melting and regrowing ice wedges affect the exchange of greenhouse gases carbon dioxide (CO2) and methane (CH4) between ground and pond surfaces and Earth's atmosphere in an area of Arctic Alaska where ice wedges are melting, and some are regrowing. Then we combined field measurement results with study area aerial photos from nine different years during 1949–2018 to assess how ice wedge distribution and greenhouse gas exchange has changed over the past seven decades. CH4 emissions to the atmosphere increased over time as ice wedges melted, but at the same time, more CO2 was taken out of the atmosphere by increased localized plant growth, counteracting some of the warming effect of CH4. The balance of these simultaneous changes indicates a net potential of melting ice wedges to add to a warming climate.
Key Points
Mean seasonal CO2 and CH4 fluxes were significantly different across ice wedge degradation/stabilization stages
Ice wedge degradation decreased seasonal net CO2 emissions and increased CH4 emissions at the landscape scale over 69 years
Seasonal CO2‐C equivalent flux was strongly correlated with abundance of water‐filled troughs formed from advanced ice wedge degradation
Rivers and streams export inorganic and organic carbon derived from contributing landscapes and so downstream carbon fluxes are important quantitative indicators of change in ecosystem function and ...for the full accounting of terrestrial carbon budgets. Carbon concentration‐discharge (C‐Q) relationships in rivers provide important information about carbon source and behavior in watersheds and are useful for estimating carbon export. However, C‐Q relationships are complex in large river systems because of spatial and temporal heterogeneity in carbon dynamics across the watershed and river networks. We quantified dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC) fluxes in the Upper Mississippi River basin and investigated their relationships with land cover and hydrology. The magnitude of dissolved carbon yields ranged widely among stations, 0.6–5.7 g DOC m−2 yr−1 and 2.9–11.8 g DIC m−2 yr−1. Spatial patterns in carbon fluxes were strongly related to land cover, with agricultural sites having high DIC/low DOC exports and forested and wetland areas having the opposite. DIC was always negatively related to discharge (Q), while the DOC‐Q relationship varied with land cover. Differential behavior of carbon across the basin resulted in Q having a weak relationship with DOC and DIC at the basin outlet. Hence, there is a need to improve understanding of headwater terrestrial‐to‐aquatic carbon connections in order to improve basin‐to‐continental‐scale carbon export estimates. Our results demonstrate that quantitative understanding of carbon export by large rivers can be improved by incorporating stream network information, such as the timing, location, and source of constituent flux, rather than relying solely upon relationships between constituent behavior and seasonality or discharge at the basin outlet.
Plain Language Summary
Riverine carbon export is important to water quality and to fully account for regional carbon budgets. Carbon export can be estimated by observing conditions at the basin outlet. However, large river basins often have heterogeneous land cover and different areas of the basin export carbon very differently. In this study, we examined carbon export in the Upper Mississippi River Basin and in contrasting subbasins dominated by either forested/wetland or agricultural/urban land cover. We found different behavior and intensity of carbon export from the subbasins, which affected our understanding of carbon flux and dynamics at the basin outlet. This work underscores the need to integrate knowledge of subbasin behavior into studies of basin‐scale solute transport dynamics.
Key Points
Dissolved organic and inorganic carbon fluxes were estimated for the Upper Mississippi River Basin in north‐central USA
Carbon fluxes varied strongly among land cover types with stark contrasts between agricultural areas and forested/wetland areas
Accounting for spatial drivers is important for understanding carbon flux dynamics in large, heterogeneous river basins
The fate of terrestrially-derived dissolved organic carbon (DOC) is important to carbon (C) cycling in both terrestrial and aquatic environments, and recent evidence suggests that climate warming is ...influencing DOC dynamics in northern ecosystems. To understand what determines the fate of terrestrial DOC, it is essential to quantify the chemical nature and potential biodegradability of this DOC. We examined DOC chemical characteristics and biodegradability collected from soil pore waters and dominant vegetation species in four boreal black spruce forest sites in Alaska spanning a range of hydrologic regimes and permafrost extents (Well Drained, Moderately Well Drained, Poorly Drained, and Thermokarst Wetlands). DOC chemistry was characterized using fractionation, UV-Vis absorbance, and fluorescence measurements. Potential biodegradability was assessed by incubating the samples and measuring CO₂ production over 1 month. Soil pore water DOC from all sites was dominated by hydrophobic acids and was highly aromatic, whereas the chemical composition of vegetation leachate DOC varied significantly with species. There was no seasonal variability in soil pore water DOC chemical characteristics or biodegradability; however, DOC collected from the Poorly Drained site was significantly less biodegradable than DOC from the other three sites (6% loss vs. 13-15% loss). The biodegradability of vegetation-derived DOC ranged from 10 to 90% loss, and was strongly correlated with hydrophilic DOC content. Vegetation such as Sphagnum moss and feathermosses yielded DOC that was quickly metabolized and respired. In contrast, the DOC leached from vegetation such as black spruce was moderately recalcitrant. Changes in DOC chemical characteristics that occurred during microbial metabolism of DOC were quantified using fractionation and fluorescence. The chemical characteristics and biodegradability of DOC in soil pore waters were most similar to the moderately recalcitrant vegetation leachates, and to the microbially altered DOC from all vegetation leachates.
The large amount of soil carbon in boreal forest ecosystems has the potential to influence the climate system if released in large quantities in response to warming. Thus, there is a need to better ...understand and represent the environmental sensitivity of soil carbon decomposition. Most soil carbon decomposition models rely on empirical relationships omitting key biogeochemical mechanisms and their response to climate change is highly uncertain. In this study, we developed a multi-layer microbial explicit soil decomposition model framework for boreal forest ecosystems. A thorough sensitivity analysis was conducted to identify dominating biogeochemical processes and to highlight structural limitations. Our results indicate that substrate availability (limited by soil water diffusion and substrate quality) is likely to be a major constraint on soil decomposition in the fibrous horizon (40-60% of soil organic carbon (SOC) pool size variation), while energy limited microbial activity in the amorphous horizon exerts a predominant control on soil decomposition (>70% of SOC pool size variation). Elevated temperature alleviated the energy constraint of microbial activity most notably in amorphous soils, whereas moisture only exhibited a marginal effect on dissolved substrate supply and microbial activity. Our study highlights the different decomposition properties and underlying mechanisms of soil dynamics between fibrous and amorphous soil horizons. Soil decomposition models should consider explicitly representing different boreal soil horizons and soil-microbial interactions to better characterize biogeochemical processes in boreal forest ecosystems. A more comprehensive representation of critical biogeochemical mechanisms of soil moisture effects may be required to improve the performance of the soil model we analyzed in this study.
Climate warming is having a dramatic effect on the vegetation distribution and carbon cycling of terrestrial subarctic and arctic ecosystems. Here, we present hydrologic evidence that warming is also ...affecting the export of dissolved organic carbon and bicarbonate (DOC and HCO3−) at the large basin scale. In the 831,400 km2 Yukon River basin, water discharge (Q) corrected DOC export significantly decreased during the growing season from 1978–80 to 2001–03, indicating a major shift in terrestrial to aquatic C transfer. We conclude that decreased DOC export, relative to total summer through autumn Q, results from increased flow path, residence time, and microbial mineralization of DOC in the soil active layer and groundwater. Counter to current predictions, we argue that continued warming could result in decreased DOC export to the Bering Sea and Arctic Ocean by major subarctic and arctic rivers, due to increased respiration of organic C on land.
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