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Are Tidal Salt Marshes Exposed to Nutrient Pollution more Vulnerable to Sea Level Rise?

Citation

Krause, Johannes; Watson, Elizabeth; Wigand, Cathleen; Maher, Nicole (2019), Are Tidal Salt Marshes Exposed to Nutrient Pollution more Vulnerable to Sea Level Rise?, Dryad, Dataset, https://doi.org/10.5061/dryad.crjdfn310

Abstract

Over the past four decades, Long Island, NY, USA, has lost coastal wetlands at a rate of 4% per decade due to submergence. In this study, we examined relationships between the rate of tidal salt marsh loss and environmental factors, including marsh elevation, tidal range, and wastewater exposure through analysis of stable isotope ratios of marsh soils and biota. Our goal was to identify factors that increase vulnerability of marshes to sea level rise, with a specific emphasis on the potential role of poor water quality in hastening marsh loss. Our results suggest that wastewater exposure may accelerate loss of intertidal marsh, but does not negatively impact high tidal marsh resilience to sea level rise. And while marsh elevation and tidal range were statistically significant predictors of marsh loss, they similarly displayed opposite relationships among marsh zones. This study suggests that different functional zones of coastal salt marshes may not respond similarly to global change factors, and that elevation may be an important factor mediating eutrophication effects to coastal salt marshes.

Methods

Collections of the Eastern mud snail (Tritia obsoletus) (50 site-1), Atlantic cordgrass (Spartina alterniflora) (5 green shoots site-1 >10m distant) and soil cores (0-5 cm, 5 collections site-1 >10m distant) were made at each study location and processed for stable carbon and nitrogen isotopes, which have previously been successfully employed as eutrophication indicators. Soil and macrophyte material was also analyzed for nutrient stoichiometric ratios (molar C/N). Snails were analyzed as whole tissue samples. All samples were dried, milled and introduced into a Vario Cube elemental analyzer interfaced to an Isoprime 100 isotope ratio mass spectrometer (IRMS).  Isotope ratios for carbon and nitrogen are reported in per mille notation as:

δaX (‰)= RsampleRstandard-1 × 1000 ‰

where R is the abundance ratio of the less common (a) to the more common isotope. All samples were analyzed in duplicate for stable isotopic and elemental composition with a mean difference between duplicates of 0.07 ‰ for δ15N and 0.1 ‰  δ13C, and 0.7 for the C/N ratio.

We generated an estimate of mean marsh elevation relative to mean high water at each study site. Digital elevation models (DEMs) with a 10-m resolution were obtained from the USGS 3D Elevation Program. These elevation data were derived by light detection and ranging (LIDAR) surveys, conducted in April 2013, before the growing season for marsh plants on Long Island. The elevation bias introduced by overwintering salt marsh vegetation was evaluated using ground-based real time kinematic GNSS measurements and found to be within the error reported for the DEM, therefore we did not apply a vegetation-specific elevation offset. To obtain marsh elevation statistics, we generated 1000 random points on all wetland areas that fell within a 2 km buffer around each of the sampling locations for eutrophication indicators. This sampling procedure allows for the exclusion of points that fall on features other than the marsh platform, such as tidal channels, upland, or roads. Mean elevations of each marsh are expressed relative to mean high water (MHW), utilizing the online vertical datum transformation of the National Oceanic and Atmospheric Administration.

Marsh loss at the 39 Long Island wetland sites was quantified by comparing past and recent aerial imagery. Changes in the areal extent of Long Island tidal wetlands, classified by vegetation type, were calculated based on analysis of high resolution aerial imagery (color near-infrared imagery) collected in 1974 and 2005 or 2008 by Cameron Engineering and Associates (2015). Vegetation classes included intertidal marsh, tidal fresh marsh, high marsh, and Phragmites marsh. The boundary between inter-tidal marsh and high marsh roughly corresponds with MHW, such that inter-tidal marsh is found below MHW and high marsh above MHW. To characterize change rates that were calculated for different time periods, loss rates were annualized. For each sampling station of eutrophication indicators, we generated a 2-km buffer, and intersected wetland extent polygons within this range to exclude wetlands far from indicator locations. Changes in the extent of intertidal marsh, high marsh, fresh marsh, and Phragmites were expressed as percent change relative to 1974. To avoid extremely small marsh areas skewing the results, the area of each class was only considered if it was greater than 1 m2 at a given site.