Vegetation dependence on depth in a salt marsh, and implications for marsh drowning

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2024-06-15

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2023-12-15

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Abstract

Coastal salt marshes are among the world’s most important ecosystems with ecosystem services valued at over $193,000 per hectare (Costanza et al. 2014). Despite this, over 150,000 hectares of salt marshes have been lost globally in the last 20 years (Campbell et al. 2022). They face numerous threats, including drowning due to increasing rates of sea level rise (SLRR). However, marshes are able to grow vertically by enhancing inorganic sedimentation and creating organic sediments. Whether or not marshes can gain elevation at a rate that keeps up with increases in sea level rise depends, in part, on how marsh vegetation responds to changing water depths.

Here, we use field observations from two sites within an interconnected marsh system to evaluate two distinct models of marsh vegetation dynamics: a parabolic model, following Morris et al. (2002), where biomass increases and subsequently decreases with depth, and a logistic model, following Finotello et al. (2022), where biomass decreases with depth. We find that at one of our sites (Winyah Bay), Spartina alterniflora exhibits an increase in biomass with depth, while at the other (North Inlet), there is an initial increase in biomass with depth followed by a decrease beyond a biomass-optimizing depth. Both sites are consistent with a parabolic depth-biomass relationship, with the difference between them suggesting that Winyah Bay occupies a “stable” position on the parabola, where increases in SLRR will increase biomass and enhance the ability of the marsh to keep up with increases in SLRR. In contrast, vegetation at North Inlet occupies an “unstable” position where increases in SLRR would be followed by decreases in biomass. This decrease in production would reduce the ability of the marsh to gain elevation and could lead to marsh drowning.

We attribute these divergent responses to differences in characteristics of the inundating waters, with lower salinity and higher nutrient and sediment concentrations at Winyah Bay leading to increased plant growth and a more stable marsh platform. Our results broadly support a parabolic biomass-depth relationship and identify salinity and nutrient concentrations as additional variables that can affect marsh responses to increases in the rate of sea level rise.

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Blackford, Nathaniel (2023). Vegetation dependence on depth in a salt marsh, and implications for marsh drowning. Master's project, Duke University. Retrieved from https://hdl.handle.net/10161/29535.


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