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Runnels mitigate marsh drowning in microtidal salt marshes
Elizabeth B Watson1, Wenley Ferguson2, Lena K Champlin1
1Department of Biodiversity, Earth and Environmental Science, Academy of Natural Sciences of Drexel University, Philadelphia, PA, United States.
Coastal marshes are drowning due to sea level rise. Installing runnels, or drainage channels, helped vegetation return but did not consistently increase species diversity in Rhode Island marshes.
Area of Science:
- Coastal Ecology
- Wetland Restoration
- Hydrology
Background:
- Northeastern U.S. coastal marshes are converting to open water due to sea level rise and altered tidal hydrology.
- Interior pond expansion in high-elevation marshes challenges existing predictive models for marsh loss.
- Runnels (tidal channel extensions) are a potential technique to restore drowning marshes, but data are limited.
Purpose of the Study:
- To evaluate the effectiveness of runnel installation for restoring vegetation in drowning coastal marshes.
- To monitor the impact of runnels on marsh hydrology, including groundwater tables and tidal range.
- To assess changes in plant communities, specifically high marsh species, following runnel implementation.
Main Methods:
- A Before-After Control-Impact (BACI) study was conducted over 5 years.
- Shallow runnels (0.15-0.30 m diameter/depth) were installed in two locations in the Pettaquamscutt River Estuary, Rhode Island.
- Vegetation and hydrologic parameters, including groundwater levels and tidal influence, were monitored.
Main Results:
- Runnel installation successfully promoted plant recolonization in the monitored marsh areas.
- Runnels reduced groundwater tables by 0.07-0.12 m and increased tidal range influence on marsh water tables at one site.
- While vegetation returned, runnels did not consistently increase the presence or diversity of high marsh species.
Conclusions:
- Restoring tidal hydrology via runnel installation shows promise for revegetation and extending the life of drowning coastal marshes.
- The study underscores the significance of rising groundwater tables in marsh drowning, particularly with expanding interior ponds.
- Further research is needed to optimize runnel design and placement for enhancing high marsh species recovery.
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