Triplicate core samples were collected at three different distances from the island’s edge at sites on the North, East, South, and West sides of the island. These core samples were then broken down into 10 centimeter (cm) sections and these sections were then further divided into two subsets. The samples in the first subset were used for the soil physiochemical analysis. The samples in the second set were centrifuged to yield pore water samples that were spectroscopically analyzed.

Sample Collection: Thirty-six cores, which were 0.5 – 1 meters long, were collected on four sides (cardinal directions) of the island with a manual push-coring method using an acrylic tube (1.7 m long x 7.6 cm diameter). On each side, cores were extracted in triplicates at 16 m outland, the edge of the marsh (0 m), and 2 m inland. Upon extraction in the field, each core was sectioned in 10 cm sections, sealed in polyethylene bags, and stored on ice during transportation back to the lab, where they were stored at 4 degrees C in the dark until analysis.

Surface water samples were collected 10 cm below the surface at the edge of the marsh and 16 m outland following the procedures in Haywood et al. (2018).

Soil Physiochemical Analysis: The 10 cm sections were homogenized and weighed prior to analysis to determine gravimetric moisture content, bulk density, percent organic matter, and total carbon.

Gravimetric Moisture Content: The entire 10 cm section (~30 grams) was oven-dried at 60 degrees C until constant weight was achieved.

Bulk Density: The total weight of the dried 10 cm section of core was divided by the volume of the 10 cm section (385 cm3)

Percent Organic Matter: 1 gram of the dried 10 cm section of core was ground using a mortar and pestle. It was combusted in a Thermolyne furnace (Thermofischer Scientific, Waltham, MA) at 550 degrees C for 4 hours. After cooling, the sample was weighed. The difference in weight following combustion determined percent organic matter.

Total Carbon: 8 milligrams (mg) of the dried 10 cm section of core was used to determine total carbon. These samples were placed into a Shimadzu Total Organic Carbon Analyzer with SSM-5000 A (Shimadzu, Columbia, MD) and combusted at 900 degrees C for 13 minutes.

Porewater Analysis: To remove pore water from the wet soil samples, samples were centrifuged at 2578 g for 10 minutes. The pore water was decanted and filtered through a 0.45-micrometer x 22-millimeter Nylon syringe filter. After filtration, the pore water was stored at 4 degrees C in the dark until analysis. To determine the dissolved organic carbon, a Total Organic Carbon Analyzer (TOC-L) (Shimazdu, Columbia, MD) was used. A Cary 100 Spectrophotometer (Varian Inc., Palo Alto, CA) was used to collect UV–Vis absorbance spectra, and a Spex Fluorolog-3 spectrofluorometer (HORIBA Scientific, Edison, NJ) was used to collect fluorescence, methods based on Haywood et al. (2018).

The UV–Vis absorbance spectra were collected from 200 to 600 nm using a 0.5 nm bandpass and a 1 cm quartz cell, on a Cary 100 spectrophotometer.

Fluorescence EEMs were collected using a 1 cm quartz cell with excitation wavelengths of 250 to 550 nm and emission wavelengths of 250 to 600 nm with 5 nm increments for both on a Spex Fluorolog-3 spectrofluorometer. Along with sample EEMs, blank EEMs of Milli-Qnwater were collected daily.

To minimize temperature effects, samples were allowed to reach room temperature and shielded from light prior to analysis.