Salt Marshes as Groundwater Buffers for Development: A Survey of South Carolina Salt Marsh Basins

Alicia M Wilson,Erik M Smith,Meghan Shanahan

doi:10.3389/frwa.2021.770819

Alicia M Wilson, Erik M Smith + Show 1 more

Open Access

https://doi.org/10.3389/frwa.2021.770819

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Abstract

Salt marshes serve as zones of intense groundwater mixing and reaction between freshwater uplands and estuaries. This raises the question of whether the impacts of upland development on nutrient and carbon species can be transmitted through salt marshes via groundwater, or whether salt marshes can buffer estuarine waters from coastal development. We sampled groundwater from fifteen tidal creek basins in South Carolina to test for compositional differences associated with development and marsh width. Groundwater samples from near creekbanks and below freshwater uplands were analyzed for salinity, total dissolved nitrogen and phosphorus, and dissolved organic carbon. Analyses revealed significantly higher TDN and TDP concentrations in creekbank samples from developed watersheds, independent of the season. Analyses of upland samples revealed significantly lower DOC concentrations in developed uplands, again independent of season. These results support the hypothesis that development can affect groundwater compositions in coastal groundwater and therefore may affect coastal nutrient and carbon fluxes. However, results also revealed significant linear correlations between marsh width, salinity, and nutrient concentrations in some marshes. These results suggest that salt marshes can act as buffers for development, and specifically suggests that the buffering capacity of salt marshes increases with width. Narrow or trenched salt marshes are far less likely to be effective buffers.

Highlights

Tidal salt marshes lie at the intersection of land and coastal surface waters
Samples from 2016 generated a large new database of creekbank groundwater nutrient compositions (Figure 3, Supplementary Table 1) but revealed few significant differences related to marsh width or the degree of upland development (Supplementary Table 2)
In some cases the salinity of creekbank groundwater samples increased from headwaters to the mouth of the creeks (Figure 4), which obscured possible relationships between marsh width and salinity

Summary

Introduction

Tidal salt marshes lie at the intersection of land and coastal surface waters. Tidal creeks in watersheds with higher fractions of impervious surfaces showed, among other indicators, higher nutrient concentrations and larger ranges in salinity (Holland et al, 2004; Sanger et al, 2015). These findings are consistent with increased nutrient inputs and increases in the ratio of runoff to infiltration in developed watersheds.

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