Particles and Aeration at Mire‐Stream Interfaces Cause Selective Removal and Modification of Dissolved Organic Matter

Abstract

AbstractPeatlands are dominant sources of dissolved organic matter (DOM) to boreal inland waters and play important roles in the aquatic carbon cycle. Yet before peat‐derived DOM enters aquatic networks, it needs to pass through peat‐stream interfaces that are often characterized by transitions from anoxic or hypoxic to oxic conditions. Aeration at these interfaces may trigger processes that impact the DOM pool, and its fate downstream. Here we experimentally assessed how the aeration of iron‐ and organic‐rich mire‐waters influences biodegradation, particle‐formation, and modification of DOM. In addition, we investigated how suspended peat‐derived particles from mires may influence these processes. We found that within 5 days of aeration, 20% of the DOM transformed into particulate organic matter (POM). This removal was likely due to combination of mechanisms including coprecipitation with oxidized iron, aggregation, and DOM‐adsorption onto peat‐derived particles. Peat‐derived particles promoted microbial activity, but biodegradation was a minor loss mechanism of DOM removal. Interestingly, microbial respiration accounted for only half of the oxygen loss, suggesting substantial nonrespiratory oxygen consumption. The differences observed in DOM characteristics between anoxic and aerated treatments suggest that hydrophilic, aromatic DOM coprecipitated with iron oxides in aerated samples, and the corresponding C:N analysis of generated POM revealed that these organic species were nitrogen‐poor. Meanwhile, POM formed via adsorption onto peat‐derived particles generated from nonaromatic DOM and more nitrogen‐rich species. Hence, selective removal of DOM, dissolved iron, and thus oxygen may be important and overlooked processes in mire‐dominated headwater systems.