The three ponds are located in the high marsh (1.43-1.46 m above North American Vertical Datum of 1988) of the Plum Island Ecosystems – Long Term Ecological Research (PIE-LTER) site. Leaves from S. alterniflora, S. patens, R. maritima, and Ulva were collected in August 2014 from across the marsh platform and all three ponds; tissues were combined in combusted (450 ℃) glass jars to form species-specific composite samples. Benthic microalgae (BMA) were collected from ponds 1 and 2, as part of a separate study (July 2016), by placing combusted glass slides on surface sediments for one week and scraping attached material onto combusted glass fiber filters. All samples were frozen (-20 ℃) until analysis for TOC, TN, δ¹³C, and δ¹⁵N. Samples were prepared for elemental and isotopic analyses by drying to constant mass (60 °C) and fuming with hydrochloric (HCl) acid to remove carbonates (Hedges and Stern 1984). Analyses were performed by the Stable Isotope Laboratory at the Marine Biological Laboratory (Woods Hole, MA). Isotopic data are reported in the conventional δ-notation in units of per mil (‰).