Semester of Graduation

Spring 2021

Degree

Master of Science (MS)

Department

Oceanography and Coastal Science

Document Type

Thesis

Abstract

Coastal wetlands are experiencing threats to their long-term sustainability brought about by the combined effects of relative sea-level rise and human modifications to hydrology, sediment delivery and nutrient loading. Restoration and management strategies can include adding sediment to the surface of deteriorating marshes to facilitate positive feedbacks among elevation, plant productivity, sediment trapping, and accretion; however, if delivered using nitrate-enriched river waters, belowground biomass and soil organic matter pools may be negatively affected, resulting in the acceleration of wetland loss. Overall, there is limited information on the combined effects of nutrient loading, sedimentation, and flooding dynamics in regulating feedbacks among marsh plant community composition, productivity, and soil organic matter accumulation. For this study, we tested the hypothesis that elevation, sedimentation, and nutrient-enrichment interact to affect wetland community composition and plant productivity. Plant productivity was generally predicted to be greater with a combination of less flooding, greater sedimentation and nutrient-enrichment. To test these hypotheses, 96 marsh soil plugs were collected from a Sagittaria lancifolia-dominated oligohaline marsh and placed them into a greenhouse mesocosm. Plugs experienced a simulated diurnal tide of 15 cm and were subjected to a combination of three treatments for two growing seasons (n = 8): 1) an elevation treatment of low (20 cm below MHW) or high (5 cm below MHW) 2) sediment deposition treatment of 0 (control), 5, or 10 cm and 3) no nutrient-enrichment (control) or a nutrient-enriched treatment of 2.0 mg/L N as (CaNO3)2 and 0.3 mg/L P as PO4. Measurements included water column and porewater nutrient concentrations, species richness, species-specific stem densities and heights, aboveground biomass, root ingrowth, soil organic matter content, and bulk density. We found that wetland plant productivity and communities are limited by flooding dynamics, but sediment and nutrient enrichments can improve soil environmental conditions that stimulate plant growth. Interaction between reduced flooding and nutrient enrichment generally increase aboveground productivity but did the opposite when flooding durations increase. Interestingly, reduced flooding regimes interacted with nutrient enriched conditions to created positive effects on ingrowth fine and large roots at specific soil depths. To our knowledge this is the first study to describe this interaction with soil deposition. While no three-way interaction was observed, this study provides valuable information on how flooding, sediment deposition, and nutrient loading affects wetland plant community dynamics.

Committee Chair

Tracy Quirk

DOI

10.31390/gradschool_theses.5272

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