Stream sediment decomposition in contrasting glacial sand, peat and muck reaches of a wetland stream

Low-gradient peatland streams often have unconventional channel forms which are hypothesized to be driven by variability in groundwater discharge and organic matter production and decomposition. These aquatic systems are also regionally important sources of CO₂ and CH₄, and knowledge of spatial controls of decomposition could help constrain the regional carbon budget. We investigated how sediment texture, organic matter content, and groundwater exchange affect the metabolism of stream sediments in a wetland stream in northern Wisconsin, U.S. We compared an organic-rich wetland reach with adjacent glacial sand reaches and also identified six cross-sections in the wetland where fine, unconsolidated organic sediments (muck) were positioned adjacent to a solid peat bed. Based on sediment temperature anomalies and substantial previous work on groundwater hydrology, we found that loose organic sediments were always associated with groundwater upwelling, which was not seen in the adjacent peat. Sediments from sand and muck cores were incubated aerobically, and at each wetland cross-section we incubated cores under both aerobic and anaerobic conditions. Aerobic respiration rates in muck sediments were higher than for glacial sands, and there were substantial localized differences in the wetland sediments associated with groundwater upwelling. We found that aerobic and anaerobic respiration were higher in muck sediments than in peat (nearly threefold, p<0.05) which confirms that decomposition is more rapid in sediments receiving constant groundwater discharge. Sediment organic matter was lower in muck compared to solid peat sediments, but much higher than in sand. The resulting variation in sediment bioavailability and quantity support the hypothesis that biological processes, rather than purely physical processes, dictate the unique cross-sectional and planform geometry of peatland stream channels. Further, these patterns can be used to explain previously documented reach and regional scale variability in stream greenhouse gas emissions.