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Globally 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. Plain Language Summary Peatlands are important ecosystems in both the global carbon (C)cycle and the Earth's climate system owing to their ability to store vast quantities of C taken from the atmosphere. Peat C stays locked up in these ecosystems largely owing to cool and wet conditions, and as such, these C stores are vulnerable to release back to the atmosphere if the climate or water levels change. Water table level and plant species composition have a combined effect on C and nitrogen (N) cycling in peatlands. We manipulated both factors in an experimental setting composed of 24 large bins into which we put intact peatland miniecosystems. Sedges play a big role in producing N compounds below the peat surface, and this work suggests these compounds can actually be synthesized into larger, less accessible compounds. In addition, activities of sedge roots and other dominant plant communities (such as shrubs in the heath family of plants) may interact in the synthesis of oxidized, larger molecules. These molecules can allow microbes to continue to decompose peat even in the absence of oxygen. This could increase the release of greenhouse gas carbon dioxide to the atmosphere, while reducing inputs of the stronger greenhouse gas methane.

Northern peatlands act as globally important carbon (C) sinks (Nichols & Peteet, 2019). Changes in climate and large-scale drainage for agriculture and forestry increase the susceptibility of these C sinks to oxidation, which has the potential to shift some peatlands from being net C sinks to sources (Dinsmore et al., 2010;Trettin et al., 2006), although the warming and drying-to-C-loss scenario is far from certain across all boreal peatlands (Charman et al., 2013;Gallego-Sala et al., 2018). Beyond simply acting as net C sinks, peatlands play

Research Impact Statement: Interactions between Lake Superior and its tributaries are frequently ignored, and understanding the Lake Superior ecosystem requires a nuanced consideration of these interactions in time and space.ABSTRACT: Lake Superior receives inputs from approximately 2,800 tributaries that provide nutrients and dissolved organic matter (DOM) to the nearshore zone of this oligotrophic lake. Here, we review the magnitude and timing of tributary export and plume formation in Lake Superior, how these patterns and interactions may shift with global change, and how emerging technologies can be used to better characterize tributary-lake linkages. Peak tributary export occurs during snowmelt-driven spring freshets, with additional pulses during raindriven storms. Instream processing and transformation of nitrogen, phosphorus, and dissolved organic carbon (DOC) can be rapid but varies seasonally in magnitude. Tributary plumes with elevated DOC concentration, higher turbidity, and distinct DOM character can be detected in the nearshore during times of high runoff, but plumes can be quickly transported and diluted by in-lake currents and mixing. Understanding the variability in size and load of these tributary plumes, how they are transported within the lake, and how long they persist may be best addressed with environmental sensors and remote sensing using autonomous and unmanned vehicles. The connections between Lake Superior and its tributaries are vulnerable to climate change, and understanding and predicting future changes to these valuable freshwater resources will require a nuanced and detailed consideration of tributary inputs and interactions in time and space.

Increasing concentrations of dissolved organic carbon (DOC) have been identified in many freshwater systems over the last three decades. Studies have generally nominated atmospheric deposition as the key driver of this trend, with changes in climatic factors also contributing. However, there is still much uncertainty concerning net effects of these drivers on DOC concentrations and export dynamics. Changes in climate and climate mediated snowfall dynamics in northern latitudes have not been widely considered as causal factors of changes in long-term DOC trends, despite their disproportionate role in annual DOC export. We leveraged long-term datasets (1988–2013) from a first-order forested tributary of Lake Superior to understand causal factors of changes in DOC concentrations and exports from the watershed, by simultaneously evaluating atmospheric deposition, temperature, snowmelt timing, and runoff. We observed increases in DOC concentrations of approximately 0.14 mg C l−1 yr−1 (mean = 8.12 mg C l−1) that were related with declines in sulfate deposition (0.03 mg SO 4 2 − l−1 yr−1). Path analysis revealed that DOC exports were driven by runoff related to snowmelt, with peak snow water equivalences generally being lower and less variable in the 21st century, compared with the 1980s and 1990s. Mean temperatures were negatively related (direct effects) to maximum snow water equivalences (−0.71), and in turn had negative effects on DOC concentrations (−0.58), the timing of maximum discharge (−0.89) and DOC exports (indirect effect, −0.41). Based on these trends, any future changes in climate that lessen the dominance of snowmelt on annual runoff dynamics—including an earlier peak discharge—would decrease annual DOC export in snowmelt dominated systems. Together, these findings further illustrate complex interactions between climate and atmospheric deposition in carbon cycle processes, and highlight the importance of long-term monitoring efforts for understanding the consequences of a changing climate.