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ABSTRACT To better understand linkages between hydrology and ecosystem carbon flux in northern aquatic ecosystems, we evaluated the relationship between plant communities, biofilm development, and carbon dioxide (CO₂) exchange following long‐term changes in hydrology in an Alaskan fen. We quantified seasonal variation in biofilm composition and CO₂exchange in response to lowered and raised water table position (relative to a control) during years with varying levels of background dissolved organic carbon (DOC). We then used nutrient‐diffusing substrates (NDS) to evaluate cause–effect relationships between changes in plant subsidies (i.e., leachates) and biofilm composition among water table treatments. We found that background DOC concentration determined whether plant subsidies promoted net autotrophy or heterotrophy on NDS. In conditions where background DOC was ≤ 40 mg L⁻¹, plant subsidies promoted an autotrophic biofilm. Conversely, when background DOC concentration was ≥ 50 mg L⁻¹, plant subsidies promoted heterotrophy. Greater light attenuation associated with elevated levels of DOC may have overwhelmed the stimulatory effect of nutrients on autotrophic microbes by constraining photosynthesis while simultaneously allowing heterotrophs to outcompete autotrophs for available nutrients. At the ecosystem level, conditions that favored an autotrophic biofilm resulted in net CO₂uptake among all water table treatments, whereas the site was a net source of CO₂to the atmosphere in conditions that supported greater heterotrophy. Taken together, these findings show that hydrologic history interacts with changes in dominant plant functional groups to alter biofilm composition, which has consequences for ecosystem CO₂exchange. 

ABSTRACT Aquatic biofilms are an understudied component of northern peatlands and are expected to play a more prominent role in ecosystem processes in areas where aquatic habitat is expanding. The goal of this study was to investigate how hydrologic history influences biofilm diversity and functional genes. This study was conducted in a long‐term water table manipulation that simulates drought (lowered water table treatment) and flooding (raised water table treatment) conditions relative to a control treatment (no manipulation). We used a combination of metabarcoding and metagenomic approaches to (1) examine the diversity of eukaryotic algae, cyanobacteria, bacteria and fungi within the biofilm and (2) identify functional genes associated with alternating wet‐dry transitional states. Historical flooding, but not drought, led to broad changes in composition and functional genes, especially those associated with carbon metabolism and nitrogen cycling. Differences were related to changes in relative abundance rather than the presence/absence of individual taxa or genes. Hydrologic history influenced community diversity by reducing interspecific competition or by alleviating resource limitation. These findings show that hydrologic history regulates species membership of the community (and thereby associated genes) but differences in water chemistry and interspecific interactions alter the relative abundance of species and their functional potential. 

Great Lakes coastlines are mosaics of wetland, stream, and lake habitats, characterized by a high degree of spatial heterogeneity that may facilitate the co-occurrence of seemingly incompatible biogeochemical processes due to variation in environmental factors that favor each process. We measured nutrient limitation and rates of N2 fixation and denitrification along transects in 5 wetland - stream - lake ecotones with different nutrient loading in Lakes Superior and Huron and hypothesized that rates of both processes would be related to nutrient limitation status, habitat type, and environmental characteristics including temperature, nutrient concentrations, and organic matter quality. This data package includes information on sampling sites, dates and locations; rates of N fixation and denitrification measured at each site, date and transect location; and biomass information from nutrient diffusing substrates deployed on the study transects. 

Sphagnum-dominated peatlands store more carbon than all of Earth’s forests, playing a large role in the balance of carbon dioxide. However, these carbon sinks face an uncertain future as the changing climate is likely to cause water stress, potentially reducing Sphagnum productivity and transitioning peatlands to carbon sources. A mesocosm experiment was performed on thirty-two peat cores collected from two peatland landforms: elevated mounds (hummocks) and lower, flat areas of the peatland (hollows). Both rainfall treatments and water tables were manipulated, and CO2 fluxes were measured. Other studies have observed peat subsiding and tracking the water table downward when experiencing water stress, thought to be a self-preservation technique termed ‘Mire-breathing’. However, we found that hummocks tended to compress inwards, rather than subsiding towards the lowered water table as significantly as hollows. Lower peat height was linearly associated with reduced gross primary production (GPP) in response to lowered water tables, indicating that peat subsidence did not significantly enhance the resistance of GPP to drought. Conversely, Sphagnum peat compression was found to stabilize GPP, indicating that this mechanism of resilience to drought may transmit across the landscape depending on which Sphagnum landform types are dominant. This study draws direct connections between Sphagnum traits and peatland hydrology and carbon cycling. 

Abstract Permafrost degradation in peatlands is altering vegetation and soil properties and impacting net carbon storage. We studied four adjacent sites in Alaska with varied permafrost regimes, including a black spruce forest on a peat plateau with permafrost, two collapse scar bogs of different ages formed following thermokarst, and a rich fen without permafrost. Measurements included year‐round eddy covariance estimates of net carbon dioxide (CO₂), mid‐April to October methane (CH₄) emissions, and environmental variables. From 2011 to 2022, annual rainfall was above the historical average, snow water equivalent increased, and snow‐season duration shortened due to later snow return. Seasonally thawed active layer depths also increased. During this period, all ecosystems acted as slight annual sources of CO₂(13–59 g C m⁻² year⁻¹) and stronger sources of CH₄(11–14 g CH₄ m⁻²from ~April to October). The interannual variability of net ecosystem exchange was high, approximately ±100 g C m⁻² year⁻¹, or twice what has been previously reported across other boreal sites. Net CO₂release was positively related to increased summer rainfall and winter snow water equivalent and later snow return. Controls over CH₄emissions were related to increased soil moisture and inundation status. The dominant emitter of carbon was the rich fen, which, in addition to being a source of CO₂, was also the largest CH₄emitter. These results suggest that the future carbon‐source strength of boreal lowlands in Interior Alaska may be determined by the area occupied by minerotrophic fens, which are expected to become more abundant as permafrost thaw increases hydrologic connectivity. Since our measurements occur within close proximity of each other (≤1 km²), this study also has implications for the spatial scale and data used in benchmarking carbon cycle models and emphasizes the necessity of long‐term measurements to identify carbon cycle process changes in a warming climate. 

Abstract Shifts in plant functional groups associated with climate change have the potential to influence peatland carbon storage by altering the amount and composition of organic matter available to aquatic microbial biofilms. The goal of this study was to evaluate the potential for plant subsidies to regulate ecosystem carbon flux (CO 2 ) by governing the relative proportion of primary producers (microalgae) and heterotrophic decomposers (heterotrophic bacteria) during aquatic biofilm development in an Alaskan fen. We evaluated biofilm composition and CO 2 flux inside mesocosms with and without nutrients (both nitrogen and phosphorus), organic carbon (glucose), and leachates from common peatland plants (moss, sedge, shrub, horsetail). Experimental mesocosms were exposed to either natural sunlight or placed under a dark canopy to evaluate the response of decomposers to nutrients and carbon subsidies with and without algae, respectively. Algae were limited by inorganic nutrients and heterotrophic bacteria were limited by organic carbon. The quality of organic matter varied widely among plants and leachate nutrient content, more so than carbon quality, influenced biofilm composition. By alleviating nutrient limitation of algae, plant leachates shifted the biofilm community toward autotrophy in the light-transparent treatments, resulting in a significant reduction in CO 2 emissions compared to the control. Without the counterbalance from algal photosynthesis, a heterotrophic biofilm significantly enhanced CO 2 emissions in the presence of plant leachates in the dark. These results show that plants not only promote carbon uptake directly through photosynthesis, but also indirectly through a surrogate, the phototrophic microbes.