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Seasonal variation in apparent conductivity and soil salinity at two Narragansett Bay, RI salt marshes
Citation:
Mckinney, Rick, Alana Hanson, Roxannel Johnson, AND M. Charpentier. Seasonal variation in apparent conductivity and soil salinity at two Narragansett Bay, RI salt marshes. PeerJ. PeerJ Inc., Corte Madera, CA, 7:e8074, (2019). https://doi.org/10.7717/peerj.8074
Impact/Purpose:
Coastal wetlands, or vegetated areas along a bay or cove, provide many benefits such as helping to protect nearby communities from storm damage. These areas are currently being negatively impacted by many factors, including too many nutrients. One of the symptoms of the damage to coastal wetlands is a change in patterns of salinity, or degree of saltiness, in wetland soils, and environmental managers can use soil salinity information to help inform their efforts to protect and restore coastal wetlands. However, soil salinity measurements by traditional methods are difficult and time consuming, and as a result soil salinity data is lacking for many areas. We are working to adapt a simplified and less time-consuming method, currently used in commercial agriculture, to measure soil salinity in coastal wetlands. As part of this effort we used the simplified method to measure soil salinity at two coastal wetlands in Narragansett Bay, RI, over the course of several years, to identify factors that may interfere with the measurements. We identified several factors that, when accounted for, improved the accuracy of salinity measurements at our wetland sites. Our results suggest the simplified method may be able to provide environmental managers with needed information about patterns of soil salinity in coastal wetlands. Managers can use this information to identify specific parts of a wetland that may be more susceptible to degradation, or to help track the progress of restoration efforts such as those which involve adding additional soil to wetlands to protect against rising sea levels.
Description:
Measurement of the apparent conductivity of salt marsh sediments using electromagnetic induction (EMI) is a rapid alternative to traditional methods of salinity determination that can be used to map soil salinity across a marsh surface. Soil salinity measures can provide information about marsh processes, since salinity is important in determining the structure and function of tidally influenced marsh communities. While EMI has been shown to accurately reflect salinity to a specified depth, more information is needed on the potential for spatial and temporal variability in apparent conductivity measures that may impact the interpretation of salinity data. In this study we mapped soil salinity at two salt marshes in the Narragansett Bay, RI estuary monthly over the course of several years to examine spatial and temporal trends in marsh salinity. Mean monthly calculated salinity was 25.8 ± 5.5 ppt at Narrow River marsh (NAR), located near the mouth of the Bay, and 17.7 ± 5.3 ppt at Passeonkquis marsh (PAS) located in the upper Bay. Salinity varied seasonally with both marshes, showing the lowest values (16.3 and 8.3, respectively) in April and highest values (35.4 and 26.2, respectively) in August. Contour plots of calculated salinities showed that while the mean whole-marsh calculated salinity at both sites changed over time, within-marsh patterns of higher versus lower salinity were maintained at NAR but changed over time at PAS. Calculated salinity was significantly negatively correlated with elevation at NAR during a sub-set of 12 sample events, but not at PAS. Best-supported linear regression models for both sites included one-month and 6-month cumulative rainfall, and tide state as potential factors driving observed changes in calculated salinity. Mapping apparent conductivity of salt marsh sediments may be useful both identifying within-marsh micro-habitats, and documenting marsh-wide changes in salinity over time.
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DOI: Seasonal variation in apparent conductivity and soil salinity at two Narragansett Bay, RI salt marshes