Comparing Restored vs. Historic Salt Marshes

For centuries, diking and drainage of salt marshes was done to protect coastal lands against natural hazards and to convert such lands to agricultural use. This activity resulted in marshes being separated from the tides, causing their soils to decompose and to lose elevation. This practice also reduced the flow of carbon and nutrients both into and out of marshes. As a result, diked lands transitioned from saline to freshwater ecosystems, causing negative impacts on estuarine food webs.   

To address this situation, efforts are being made to restore drained and diked coastal areas back into fully functional salt marshes. Where successful, restoration increases carbon production in marsh plant communities, revitalizing food webs for wildlife and mitigating global warming through long-term carbon storage in marsh sediments.

To gauge the success of a salt marsh restoration, Dr. Judith Drexler, from the USGS California Water Science Center, took a team of scientists to the Nisqually River Delta near Olympia, Washington in April of 2015 to study a restored marsh and a neighboring historic (unaltered) salt marsh.  

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The heart of the Nisqually River Delta was drained and diked for agriculture in the 1880s. This change led to land‐surface subsidence (loss of elevation) due to both the microbial oxidation of soils and agricultural practices. To determine the resilience of the restored salt marsh called Six Gill Slough, marsh formation processes, including vertical accretion of sediment and carbon accumulation, needed to be measured.  

In their field research, Dr. Drexler and her team collected six sediment cores in both Six Gill Slough and an adjacent historic marsh, Animal Slough.  Cores from the two marshes were analyzed and compared for dry bulk density, percent organic carbon content, and other factors to determine how similarly the restored salt marsh was forming compared to the historic marsh.

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Sharp changes were noted in bulk density in surface layers of Six Gill Slough cores. These changes were reliable indicators for the onset of restoration.

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The results from the study show that carbon accumulation rates at Six Gill Slough are somewhat greater than at Animal Slough, but the difference is not significant. Nevertheless, the measurements demonstrate that a sparsely vegetated, restored salt marsh (Six Gill Slough) can quickly begin to accumulate carbon.   

The study also shows that restored and historic marshes can have similar carbon accumulation rates just six years after restoration occurred. 

For the Six Gill Slough marsh to ultimately develop all the characteristics and functions of a typical salt marsh, the elevation of the marsh needs to be raised through continued vertical accretion and/or sediment amendment.  Once the elevation is raised, salt marsh vegetation will be able to expand throughout the site, creating important habitat for local plants and animals.   

For further information, read Dr. Drexler’s article in the journal Restoration Ecology: Carbon accumulation and vertical accretion in a restored versus historic salt marsh in southern Puget Sound, Washington, United States.