Defense Date


Document Type


Degree Name

Master of Science



First Advisor

Rima Franklin, Ph.D


Saltwater intrusion due to global change is expected to have a detrimental effect on the biogeochemistry of tidal freshwater wetlands. Of particular concern is that fact that salinization can alter the role of these ecosystems in the global carbon cycling by causing shifts in microbial metabolism that alter greenhouse gas emissions and increase carbon mineralization rates. However, our understanding of how wetland microbial community dynamics will respond to saltwater intrusion is limited. To address this knowledge gap and increase our understanding of how microbial communities in tidal freshwater wetlands change over time (1, 3, 12, and 49 weeks) under elevated salinity conditions, an in situ soil transplant was conducted. Throughout the 49 weeks of saltwater exposure, salinity had no effect on soil quality (organic matter content and C:N ratio). In contrast, the concentration of porewater ion species (SO4-2, NO3-, and NH4+) considerably increased. The activity of hydrolytic enzymes, (ß-1,4-glucosidase and 1,4-ß-cellobiohydrolase) gradually decreased with prolonged exposure to saline conditions; by the final sampling event (49 weeks), activity was reduced by ~70% in comparison to the freshwater controls. Short term exposure to salinity (3 and 12 weeks) had a greater effect on phenol oxidase, decreasing activity by 10-20%. Saltwater exposure had an immediate (1 week) effect on potential rates of carbon mineralization; overall, carbon dioxide production doubled and methane production decreased by ~20-fold. These changes in gas production were correlated to increased salinity and to changes in the abundance of methanogens and sulfate reducing bacteria, suggesting a shift in the terminal step in organic matter degradation from methanogenesis to sulfate reduction. Principal component analysis revealed distinct changes in soil environmental conditions and carbon metabolism within weeks, but the response of the microbial community was slower (months to a year). Taken together, results from this study indicate that the response of tidal freshwater wetlands to salinization is driven by complex interactions of microbial related processes and environmental changes that are dependent on the duration of exposure. Assessing the impact of environmental perturbation on ecosystem function may be better achieved by complementary analysis of both microbial community structure and function.


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