Northern Peatlands Research Articles

Mercury dynamics in the pore water of peat columns during experimental freezing and thawing

Biogeochemical processes in northern peatland ecosystems are influenced by seasonal temperature fluctuations that are changing with the climate. Methylmercury (MeHg), commonly produced in peatlands, affects downstream waters; therefore, it is important to understand how temperature transitions affect mercury (Hg) dynamics. We investigated how the freeze-thaw cycle influences belowground peat pore water total Hg (THg), MeHg, and dissolved organic carbon (DOC). Four large, intact peat columns were removed from an ombrotrophic peat bog and experimentally frozen and thawed. Pore water was sampled across seven depths in the peat columns during the freeze-thaw cycle and analyzed for THg, MeHg, and DOC concentrations. Freezing results showed increased concentrations of THg below the ice layers and limited change in MeHg concentrations. During thawing, THg concentrations significantly increased, whereas MeHg concentrations decreased. Limited bromide movement and depth decreases in THg and DOC concentrations were associated with increased bulk density and degree of humification in the peat. The experiment demonstrates the effects of the freeze-thaw cycle on Hg concentrations in northern peatlands. Changes to freeze-thaw cycles with climate change may exacerbate Hg cycling and transport processes in peatland environments.

Decreased carbon accumulation feedback driven by climate-induced drying of two southern boreal bogs over recent centuries.

Northern boreal peatlands are important ecosystems in modulating global biogeochemical cycles, yet their biological communities and related carbon dynamics are highly sensitive to changes in climate. Despite this, the strength and recent direction of these feedbacks are still unclear. The response of boreal peatlands to climate warming has received relatively little attention compared with other northern peatland types, despite forming a large northern hemisphere-wide ecosystem. Here, we studied the response of two ombrotrophic boreal peatlands to climate variability over the last c. 200years for which local meteorological data are available. We used remains from plants and testate amoebae to study historical changes in peatland biological communities. These data were supplemented by peat property (bulk density, carbon and nitrogen content), 14 C, 210 Pb and 137 Cs analyses and were used to infer changes in peatland hydrology and carbon dynamics. In total, six peat cores, three per study site, were studied that represent different microhabitats: low hummock (LH), high lawn and low lawn. The data show a consistent drying trend over recent centuries, represented mainly as a change from wet habitat Sphagnum spp. to dry habitat S. fuscum. Summer temperature and precipitation appeared to be important drivers shaping peatland community and surface moisture conditions. Data from the driest microhabitat studied, LH, revealed a clear and strong negative linear correlation (R2 =.5031; p<.001) between carbon accumulation rate and peat surface moisture conditions: under dry conditions, less carbon was accumulated. This suggests that at the dry end of the moisture gradient, availability of water regulates carbon accumulation. It can be further linked to the decreased abundance of mixotrophic testate amoebae under drier conditions (R2 =.4207; p<.001). Our study implies that if effective precipitation decreases in the future, the carbon uptake capacity of boreal bogs may be threatened.

Interacting effects of vegetation components and water level on methane dynamics in a boreal fen

Abstract. Vegetation and hydrology are important controlling factors in peatland methane dynamics. This study aimed at investigating the role of vegetation components, sedges, dwarf shrubs, and Sphagnum mosses, in methane fluxes of a boreal fen under natural and experimental water level drawdown conditions. We measured the fluxes during growing seasons 2001–2004 using the static chamber technique in a field experiment where the role of the ecosystem components was assessed via plant removal treatments. The first year was a calibration year after which the water level drawdown and vegetation removal treatments were applied. Under natural water level conditions, plant-mediated fluxes comprised 68 %–78 % of the mean growing season flux (1.73±0.17 g CH4 m−2 month−1 from June to September), of which Sphagnum mosses and sedges accounted for one-fourth and three-fourths, respectively. The presence of dwarf shrubs, on the other hand, had a slightly attenuating effect on the fluxes. In water level drawdown conditions, the mean flux was close to zero (0.03±0.03 g CH4 m−2 month−1) and the presence and absence of the plant groups had a negligible effect. In conclusion, water level acted as a switch; only in natural water level conditions did vegetation regulate the net fluxes. The results are relevant for assessing the response of fen peatland fluxes to changing climatic conditions, as water level drawdown and the consequent vegetation succession are the major projected impacts of climate change on northern peatlands.

The capacity of northern peatlands for long-term carbon sequestration

Abstract. Northern peatlands have been a persistent natural carbon sink since the Last Glacial Maximum. The continued growth and expansion of these carbon-rich ecosystems could offset a large portion of anthropogenic carbon emissions before the end of the present interglacial period. Here we used an impeded drainage model and gridded data on the depth to bedrock and the fraction of histosol-type soils to evaluate the limits to the growth of northern peatland carbon stocks. Our results show that the potential carbon stock in northern peatlands could reach a total of 875±125 Pg C before the end of the present interglacial, which could, as a result, remove 330±200 Pg C of carbon from the atmosphere. We argue that northern peatlands, together with the oceans, will potentially play an important role in reducing the atmospheric carbon dioxide concentration over the next 5000 years.

Detecting northern peatland vegetation patterns at ultra‐high spatial resolution

AbstractWithin northern peatlands, landscape elements such as vegetation and topography are spatially heterogenic from ultra‐high (centimeter level) to coarse scale. In addition to within‐site spatial heterogeneity, there is evident between‐site heterogeneity, but there is a lack of studies assessing whether different combinations of remotely sensed features and mapping approaches are needed in different types of landscapes. We evaluated the value of different mapping methods and remote sensing datasets and analyzed the kinds of differences present in vegetation patterns and their mappability between three northern boreal peatland landscapes in northern Finland. We utilized field‐inventoried vegetation plots together with spectral, textural, topography and vegetation height remote sensing data from 0.02‐ to 3‐m pixel size. Remote sensing data included true‐color unmanned aerial vehicle images, aerial images with four spectral bands, aerial lidar data and multiple PlanetScope satellite images. We used random forest regressions for tracking plant functional type (PFT) coverage, non‐metric multidimensional scaling ordination axes and fuzzy k‐medoid plant community clusters. PFT regressions had variable performance for different study sites (R2 −0.03 to 0.69). Spatial patterns of some spectrally or structurally distinctive PFTs could be predicted relatively well. The first ordination axis represented wetness gradient and was well predicted using remotely sensed data (R2 0.64 to 0.82), but the other three axes had a less straightforward explanation and lower mapping performance (R2 −0.09 to 0.53). Plant community clusters were predicted most accurately in the sites with clear string‐flark topography but less accurately in the flatter site (R2 0.16–0.82). The most important remote sensing features differed between dependent variables and study sites: different topographic, spectral and textural features; and coarse‐scale and fine‐scale datasets were the most important in different tasks. We suggest that multiple different mapping approaches should be tested and several remote sensing datasets used when maps of vegetation are produced.

Controls of Sphagnum growth and the role of winter

AbstractSphagnum is the major genus in northern peatlands that contributes to peat formation and carbon sequestration. Sphagnum growth in summer has been fairly well studied but the information about growth in autumn and winter is limited. Therefore, we studied how the growth of Sphagnum is seasonally distributed with a particular interest on possible winter growth. The linear increment and biomass production of three Sphagum species was measured in three Northern European bogs over a year. In all sites, our results indicate the highest annual linear increment in S. angustifolium (28 mm), followed by S. magellanicum (20 mm) and S. fuscum (13 mm), but the biomass production was fairly even among the species (189, 192 and 215 g m−2, respectively). Both linear increment and biomass production depended mostly on meteorological parameters rather than ecophysiological or microsite properties. The seasonal measurements revealed a significant linear increment and biomass production during the winter that accounted for ca. 10% and ca. 5% from the annual values, respectively. Moreover, the mean daily rates of linear increment in autumn often exceeded the increment in summer. Our results thus indicate the ability for year‐around growth of Sphagna if the conditions are favorable, including during boreal winter.

Investigating drivers of microbial activity and respiration in a forested bog

Northern peatlands store nearly one-third of terrestrial carbon (C) stocks while covering only 3% of the global landmass; nevertheless, the drivers of C cycling in these often-waterlogged ecosystems are different from those that control C dynamics in upland forested soils. To explore how multiple abiotic and biotic characteristics of bogs interact to shape microbial activity in a northern, forested bog, we added a labile C tracer (13C-labeled starch) to in situ peat mesocosms and correlated heterotrophic respiration with natural variation in several microbial predictor variables, such as enzyme activity and microbial biomass, as well as with a suite of abiotic variables and proximity to vascular plants aboveground. We found that peat moisture content was positively correlated with respiration and microbial activity, even when moisture levels exceeded total saturation, suggesting that access to organic matter substrates in drier environments may be limiting for microbial activity. Proximity to black spruce trees decreased total and labile heterotrophic respiration. This negative relationship may reflect the influence of tree evapotranspiration and peat shading effects; i.e., microbial activity may decline as peat dries and cools near trees. Here, we isolated the response of heterotrophic respiration to explore the variation in, and interactions among, multiple abiotic and biotic drivers that influence microbial activity. This approach allowed us to reveal the relative influence of individual drivers on C respiration in these globally important C sinks.

Investigation of the climatological impacts of agricultural management and abandonment on a boreal bog in western Newfoundland, Canada

We compared greenhouse gas (GHG) fluxes and albedo of a pristine boreal bog and an adjacent abandoned peatland pasture in western Newfoundland, Canada to estimate the magnitude of radiative forcing (RF) created by agricultural drainage and abandonment. Our results indicated that these anthropogenic activities induced a climate cooling effect (negative RF), with the magnitude of the RF caused by the albedo change comparable to that induced by altered GHGs. Although the albedo-induced RF was positive in winter and negative in summer, the summer effect dominated because of greater solar radiation received. The climate cooling effect of GHGs change was due to an increase in the carbon dioxide sink capacity and a reduction in methane emissions under lower water table levels following agricultural drainage and abandonment. Calculation of sustained-flux global warming/cooling potentials also supported this finding. Our results show that the overall increase in albedo resulting from agricultural drainage and abandonment contributes significantly to the negative RF, strengthening the cooling effect due to the changing GHG fluxes. Therefore, changes in albedo due to altered vegetation coverage and hydrology and GHG fluxes should be considered when assessing the climatic impacts from land-use change in northern peatland.

Inferring Methane Production by Decomposing Tree, Shrub, and Grass Leaf Litter in Bog and Rich Fen Peatlands

Plant litter provides a fresh source of energy and nutrients to fuel microbial activity in soil, and in northern peatlands this can be leaf litter from mosses, graminoids, shrubs, and / or trees. Because Sphagnum and other mosses decompose slowly, vascular plant litter assumes a principal role, but its role in microbial methane production is unclear. Therefore, we examined decomposition of leaf litter from nine species, including trees, shrubs, and graminoids, using litterbags positioned for up to 2.5 years in two raised bogs and in two rich fens. Across species leaf litter quality varied for concentrations of nitrogen, soluble organic matter, and cell wall composition. After 2.5 years of decay the amount of leaf litter mass remaining ranged from 43 to 63% in the bogs versus 17 to 71% in the rich fens. Thus, site conditions interacted with litter quality to determine decay rates but with idiosyncratic patterns. Leaf mass remaining after 0.5, 1.5 and 2.5 years of decay was incubated in vitro, without soil, to assess its ability to support methane production and concomitant anaerobic carbon dioxide respiration. Residue from all nine species supported methane production, with the greatest rates (up to 5000 nmol g-1 day-1) in tissue with high concentrations of pectin. Rates were 2- to 700-times greater for the leaf material that decomposed in the rich fens than in the bogs. Production rates were more variable for methane than for anaerobic respiration. As seen for mass loss, differences in litter quality predicted variation in gas production rates but differently in the bogs than in the rich fens. The results underscore the importance of vascular plant litter in the biogeochemistry of carbon and methane in peatlands and why vegetation, plant species composition, and peatland type must be described to put peatland ecosystems into global budgets of carbon and methane.

Reduction‐Oxidation Potential and Dissolved Organic Matter Composition in Northern Peat Soil: Interactive Controls of Water Table Position and Plant Functional Groups

AbstractGlobally important carbon (C) stores in northern peatlands are vulnerable to oxidation in a changing climate. A growing body of literature draws attention to the importance of dissolved organic matter (DOM) in governing anaerobic metabolism in organic soil, but exactly how the reduction‐oxidation (redox) activities of DOM, and particularly the phenolic fraction, are likely to change in an altered climate remain unclear. We used large mesocosms in the PEATcosm experiment to assess changes in peatland DOM and redox potential in response to experimental manipulations of water table (WT) position and plant functional groups (PFGs). WT position and PFGs interacted in their effects on redox potential and quantity and quality of DOM. Phenolics were generally of higher molecular weight and more oxidized with sedges in lowered WTs. Altered DOM character included changes in dissolved nitrogen (N), with higher N:[phenolics] with higher E4:E6 (absorbance ratio λ = 465:665) DOM in the lowered WT and sedge PFG treatments. Conversely, biomolecular assignments to amino‐sugars were largely absent from low‐WT treatments. Low WT resulted in the creation of unique N compounds, which were more condensed (lower H:C), that changed with depth and PFG. The accumulation of oxidized compounds with low WT and in sedge rhizospheres could be very important pools of electron acceptors beneath the WT, and their mechanisms of formation are discussed. This work suggests the effects of changes in vegetation communities can be as great as WT position in directly and interactively mediating peat redox environment and the redox‐activity of DOM.

Organic Carbon Flux in Ditches during the Growing Season in a Drained Alpine Peatland

AbstractThe carbon budget dynamics of peatlands plays a crucial role in both the response of peatland ecosystems to environmental change and the effect of environmental change on peatlands. Studies on northern peatlands have suggested that the reliable assessment of the carbon budget must include the carbon flux at the land‐stream interface. But this argument has not been proved in the Qinghai‐Tibet Plateau, the world's largest alpine system. To address the knowledge gap, in the growing season (May to October) of 2016, we chose Riganqiao peatland, a typical peatland in the Zoige Basin of the Qinghai‐Tibet Plateau which has been intensely drained via ditches since the 1960s. The objective of this study was to measure the organic carbon export from ditches, and to estimate what proportion of the carbon budget can be attributed to this export. Results showed that, during the growing season in 2016, total organic carbon flux in ditches was 2,345 t (8.6 g m−2), of which particulate organic carbon accounted for 12% and dissolved organic carbon for 88%. This flux decreased the carbon budget of Riganqiao peatland by 11.2% (from +76.8 g m−2 to +68.2 g m−2), making it a weakened carbon sink. This result revealed that organic carbon export from ditches plays an important role in the carbon balance in peatland of the Qinghai‐Tibet Plateau.

Peatlands may hold double the carbon

Peatlands are the planet’s biggest terrestrial carbon sink, and understanding how they store and cycle carbon is critical for understanding climate change. But Jonathan Nichols, a biogeochemist at Columbia University’s Lamont-Doherty Earth Observatory, noticed a pattern when reading about peatlands: everyone cites the same statistics over and over again. He began to wonder whether those numbers were “the best we can do,” he says. Nichols and coauthor Dorothy Peteet of NASA Goddard Institute for Space Studies created a probabilistic distribution that included data from peatlands that had been left out of previous studies because their data were too sparse. The new approach nearly doubled the estimated amount of carbon held in northern peatlands (Nat. Geosci. 2019, DOI: 10.1038/s41561-019-0454-z). The study also has implications for paleoclimatology; it suggests that the expansion of peatlands and their carbon accumulation rate increased much more quickly after the end of the last ice age than

Microbial Community Structure and Methane Cycling Potential along a Thermokarst Pond-Peatland Continuum.

The thawing of ice-rich permafrost soils in northern peatlands leads to the formation of thermokarst ponds, surrounded by organic-rich soils. These aquatic ecosystems are sites of intense microbial activity, and CO2 and CH4 emissions. Many of the pond systems in northern landscapes and their surrounding peatlands are hydrologically contiguous, but little is known about the microbial connectivity of concentric habitats around the thermokarst ponds, or the effects of peat accumulation and infilling on the microbial communities. Here we investigated microbial community structure and abundance in a thermokarst pond-peatland system in subarctic Canada. Several lineages were ubiquitous, supporting a prokaryotic continuum from the thermokarst pond to surrounding peatlands. However, the microbial community structure shifted from typical aerobic freshwater microorganisms (Betaproteobacteria and Alphaproteobacteria) in the pond towards acidophilic and anaerobic lineages (Acidobacteria and Choroflexi) in the connected peatland waters, likely selected by the acidification of the water by Sphagnum mosses. Marked changes in abundance and community composition of methane cycling microorganisms were detected along the thermokarst pond-peatland transects, suggesting fine tuning of C-1 carbon cycling within a highly connected system, and warranting the need for higher spatial resolution across the thermokarst landscape to accurately predict net greenhouse gas emissions from northern peatlands.

The Difference in Light use Efficiency between an Abandoned Peatland Pasture and an Adjacent Boreal Bog in Western Newfoundland, Canada

Light use efficiency (LUE) is a critical parameter to estimate the gross primary production (GPP) for terrestrial ecosystems from remotely sensed data. Only a few studies have estimated the LUE for boreal peatlands and none has calculated the LUE for a drained boreal peatland due to the limited availability of direct measurement of GPP and absorbed photosynthetically active radiation (APAR), which are required to calculate the LUE. In this study, we used eddy covariance (EC) measurement of GPP and APAR at a drained peatland pasture and an adjacent boreal bog to calculate the LUE. The pasture had a significantly higher LUE (0.034 ± 0.0109 mol mol-1) than the bog (0.028 ± 0.098 mol mol-1). The LUE showed significant year-to-year variation at the pasture, but not at the bog. The LUE variation strongly correlated with air temperature and water table depth at both sites, but did not correlate well with vapor pressure deficit. This study indicates that agricultural drainage significantly alters the LUE of boreal peatlands and that the drained peatlands need to be considered separately in LUE-based models for northern peatlands. Moreover, a longer period of study is needed to fully understand the temporal dynamics of LUE in natural peatlands.

Rapid expansion of northern peatlands and doubled estimate of carbon storage

Northern peatlands are an integral part of the global carbon cycle—a strong sink of atmospheric carbon dioxide and source of methane. Increasing anthropogenic carbon dioxide and methane in the atmosphere are thought to strongly impact these environments, and yet, peatlands are not routinely included in Earth system models. Here we present a quantification of the sink and stock of northern peat carbon from the last glacial period through the pre-industrial period. Additional data and new algorithms for reconstructing the history of peat carbon accumulation and the timing of peatland initiation increased the estimate of total northern peat carbon stocks from 545 Gt to 1,055 Gt of carbon. Further, the post-glacial increases in peatland initiation rate and carbon accumulation rate are more abrupt than previously reported. Peatlands have been a strong carbon sink throughout the Holocene, but the atmospheric partial pressure of carbon dioxide has been relatively stable over this period. While processes such as permafrost thaw and coral reef development probably contributed some additional carbon to the atmosphere, we suggest that deep ocean upwelling was the most important mechanism for balancing the peatland sink and maintaining the observed stability. Northern peatlands are estimated to store more than 1,000 Gt of carbon, almost doubling previous estimates, according to a reconstruction of historical peat carbon accumulation.

Peatlands as prolific carbon sinks

Northern peatlands store over 1,000 Gt of carbon, almost double previous estimates, according to a new analysis of peat core data. The fate of this peat carbon, however, is uncertain in a rapidly changing world.

Methane Production Pathway Regulated Proximally by Substrate Availability and Distally by Temperature in a High‐Latitude Mire Complex

AbstractProjected 21st century changes in high‐latitude climate are expected to have significant impacts on permafrost thaw, which could cause substantial increases in emissions to the atmosphere of carbon dioxide (CO2) and methane (CH4, which has a global warming potential 28 times larger than CO2 over a 100‐year horizon). However, predicted CH4 emission rates are very uncertain due to difficulties in modeling complex interactions among hydrological, thermal, biogeochemical, and plant processes. Methanogenic production pathways (i.e., acetoclastic [AM] and hydrogenotrophic [HM]) and the magnitude of CH4 emissions may both change as permafrost thaws, but a mechanistic analysis of controls on such shifts in CH4 dynamics is lacking. In this study, we reproduced observed shifts in CH4 emissions and production pathways with a comprehensive biogeochemical model (ecosys) at the Stordalen Mire in subarctic Sweden. Our results demonstrate that soil temperature changes differently affect AM and HM substrate availability, which regulates magnitudes of AM, HM, and thereby net CH4 emissions. We predict very large landscape‐scale, vertical, and temporal variations in the modeled HM fraction, highlighting that measurement strategies for metrics that compare CH4 production pathways could benefit from model informed scale of temporal and spatial variance. Finally, our findings suggest that the warming and wetting trends projected in northern peatlands could enhance peatland AM fraction and CH4 emissions even without further permafrost degradation.

Mechanisms for the Development of Microform Patterns in Peatlands of the Hudson Bay Lowland

Spatial surface patterns of hummocks, hollows, ridges, and pools (microtopography) are common features of many northern peatlands and are particularly distinct within the vast peatlands of the Hudson Bay Lowland (HBL), Canada. Hypotheses and models describe how small-scale feedbacks among vegetation, hydrology, and nutrients cause spatial differences in peat accumulation that enable microforms and surface patterns to develop over time. Empirical tests of the predictions from theoretical models of these proposed feedback mechanisms are limited, particularly in large peatland complexes such as the HBL. We investigate feedbacks controlling peatland structure and function in an ombrogenous bog and a minerogenous fen in the HBL. Our sites represent surface patterns found in many northern peatlands, specifically spatially irregular hummocks and hollows, and parallel ridges and pools that are perpendicular to slope. We found the occurrence of different spatial patterns depends on position within a peat landform, with these differences attributed to the ecohydrological setting. In turn, the ecohydrological setting, with different water table depths, nutrient availability, and species composition, influences the strength and direction of feedback mechanisms at the microform scale. Our data support the prediction of a positive feedback between plant productivity and acrotelm thickness for peat accumulation and hummock growth and that this may be enhanced by water ponding on slopes to form ridge–pool tracks. We did not find evidence to support the proposed feedback among evapotranspiration-driven transport of water and nutrients for the development of hummocks. Our results suggest a combination of mechanisms operating at various temporal and spatial scales is associated with the development of surface patterns in northern peatlands.

Assessing the peatland hummock–hollow classification framework using high-resolution elevation models: implications for appropriate complexity ecosystem modeling

Abstract. The hummock–hollow classification framework used to categorize peatland ecosystem microtopography is pervasive throughout peatland experimental designs and current peatland ecosystem modeling approaches. However, identifying what constitutes a representative hummock–hollow pair within a site and characterizing hummock–hollow variability within or between peatlands remains largely unassessed. Using structure from motion (SfM), high-resolution digital elevation models (DEMs) of hummock–hollow microtopography were used to (1) examine how much area needs to be sampled to characterize site-level microtopographic variation; and (2) examine the potential role of microtopographic shape/structure on biogeochemical fluxes using plot-level data from nine northern peatlands. To capture 95 % of site-level microtopographic variability, on average, an aggregate sampling area of 32 m2 composed of 10 randomly located plots was required. Both site- (i.e. transect data) and plot-level (i.e. SfM-derived DEM) results show that microtopographic variability can be described as a fractal at the submeter scale, where contributions to total variance are very small below a 0.5 m length scale. Microtopography at the plot level was often found to be non-bimodal, as assessed using a Gaussian mixture model (GMM). Our findings suggest that the non-bimodal distribution of microtopography at the plot level may result in an undersampling of intermediate topographic positions. Extended to the modeling domain, an underrepresentation of intermediate microtopographic positions is shown to lead to potentially large flux biases over a wide range of water table positions for ecosystem processes which are non-linearly related to water and energy availability at the moss surface. Moreover, our simple modeling results suggest that much of the bias can be eliminated by representing microtopography with several classes rather than the traditional two (i.e. hummock/hollow). A range of tools examined herein can be used to easily parameterize peatland models, from GMMs used as simple transfer functions to spatially explicit fractal landscapes based on simple power-law relations between microtopographic variability and scale.

Warming enhances the stimulatory effect of algal exudates on dissolved organic carbon decomposition

Abstract The current paradigm in peatland ecology is that the organic matter inputs from plant photosynthesis (e.g. moss litter) exceed that of decomposition, tipping the metabolic balance in favour of carbon (C) storage. Here, we investigated an alternative hypothesis, whereby exudates released by microalgae can actually accelerate C losses from the surface waters of northern peatlands by stimulating dissolved organic C (DOC) decomposition in a warmer environment expected with climate change. To test this hypothesis, we evaluated the biodegradability of fen DOC in a factorial design with and without algal DOC in both ambient (15°C) and elevated (20°C) water temperatures during a laboratory bioassay. When DOC sources were evaluated separately, decomposition rates were higher in treatments with algal DOC only than with fen DOC only, indicating that the quality of the organic matter influenced degradability. A mixture of substrates (½ algal DOC + ½ fen DOC) exceeded the expected level of biodegradation (i.e. the average of the individual substrate responses) by as much as 10%, and the magnitude of this effect increased to more than 15% with warming. Specific ultraviolet absorbance at 254 nm (SUVA254), a proxy for aromatic content, was also significantly higher (i.e. more humic) in the mixture treatment than expected from SUVA254 values in single substrate treatments. Accelerated decomposition in the presence of algal DOC was coupled with an increase in bacterial biomass, demonstrating that enhanced metabolism was associated with a more abundant microbial community. These results present an alternative energy pathway for heterotrophic consumers to breakdown organic matter in northern peatlands. Since decomposition in northern peatlands is often limited by the availability of labile organic matter, this mechanism could become increasingly important as a pathway for decomposition in the surface waters of northern peatlands where algae are expected to be more abundant in conditions associated with ongoing climate change.