Whitmire, S. 2003. Anaerobic biogeochemical functions of Michigan wetlands and the influence of water source. Ph.D. Dissertation, Michigan State University, East Lansing, Michigan, USA.
Biogeochemical transformations in wetlands impact water quality and nutrient transport across landscapes as well as greenhouse gas fluxes. Anaerobic microbial processes, such as denitrification and methanogenesis, play particularly important roles in the biogeochemical functions of wetlands. I explored the influence of anaerobic microbial processes on wetland biogeochemistry in surficial sediments of southwest Michigan wetlands.
I examined porewater chemistry profiles in sediments of 12 wetlands to see if there was evidence for thermodynamic constraints on anaerobic microbial processes and to see if the wetland’s water source impacted the porewater chemistry. Although the large spatial and temporal variability in porewater chemistry obscured patterns among wetlands, wetland porewaters were generally depleted in nitrate and sulfate relative to source waters, and the presence of sulfide in porewaters confirmed that sulfate reduction occurred at most sites. Porewater chemistry showed little relationship to water source.
To further investigate anaerobic microbial decomposition in wetland sediments, I measured ambient rates of denitrification, sulfate reduction, iron reduction, methanogenesis and acetate turnover in three groundwater- and three precipitation-fed wetlands. Denitrification was not detected in any wetland. Iron reduction was measurable mainly in precipitation-fed wetlands, while sulfate reduction rates were greater in groundwater-fed wetlands than in precipitation-fed wetlands. Methanogenesis was measurable in all wetlands, with no differences between wetlands with contrasting water sources, indicating that methanogenesis was important irregardless of water source. Acetate turnover rates were comparable to those measured in other productive sediment environments.
Push-pull experiments provided estimates of potential in-situ rates of denitrification and sulfate reduction. All wetlands examined showed the same potential to rapidly remove nitrate and removal was dependent on concentration. Sulfate was also taken up from injected groundwater in all wetlands, but only after nitrate was depleted. The sulfate reduction rates in groundwater-fed wetlands were independent of sulfate concentrations, while the rates in precipitation-fed wetlands were dependent on sulfate concentrations.
Results from these studies indicate that water source can influence the biogeochemical functions of wetlands, but that other factors could influence anaerobic decomposition as well.
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