Date of Award

Winter 2018

Project Type

Thesis

Program or Major

Natural Resources

Degree Name

Master of Science

First Advisor

David Burdick

Second Advisor

Cathleen Wigand

Third Advisor

Gregg Moore

Abstract

The survival of salt marshes depends largely on their ability to build in elevation, thereby preventing increases in flooding due to sea-level rise, but the rate of marsh elevation gain depends on processes that are not well-understood, i.e. belowground productivity, sedimentation, and subsidence. The application of sediment to the marsh surface (thin-layer deposition) is a potential mitigation tool for increased flooding, but its effects on plant growth and elevation gain are understudied, especially in New England marshes. A marsh organ experiment was constructed and installed in the field to examine the effects of tidal flooding and thin-layer deposition on productivity. Feldspar Marker Horizons (MHs) were placed in the marsh to determine the effect of flooding on sedimentation rates and Surface Elevation Tables (SETs) were measured to determine elevation change. Without sediment addition, we found that belowground productivity decreased linearly as flooding increased for both Spartina patens and Spartina alterniflora. Belowground volume of S. alterniflora at the top elevation was around 4 times the amount of the lowest, most-flooded elevation. Planted treatments subsided significantly less than unplanted controls, indicating the importance of plants in reducing marsh subsidence. MHs showed that sedimentation decreased as elevation and distance from the creek increased, and SET measurements indicated high marshes in the Great Bay of New Hampshire are losing elevation relative to sea level at an average rate of 2.1 mm/year. This high marsh accretion deficit, combined with low sedimentation rates and a predicted decrease in productivity due to sea-level rise suggest that S. patens and other high marsh species will be replaced by S. alterniflora as flooding increases. Thin-layer deposition may help to slow this conversion, but our results show no significant effect of sediment addition on NPP in either S. patens or S. alterniflora over a 2 month period. More studies are needed to determine the long-term effects of thin-layer deposition in New England.

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