PNNL

Abstract

Coastal wetlands regulate the transport and transformation of dissolved organic matter (DOM) between land and ocean. It is known that DOM exported from tidal wetlands is chemically distinct from DOM produced in adjacent estuaries, yet its sources and compositional dynamics remain poorly resolved. Here, we investigated the optical and molecular composition of surface water DOM collected over multiple tidal cycles at three tidal marshes along a surface water salinity gradient in the Chesapeake Bay, using absorbance and fluorescence spectroscopy and high-resolution mass spectrometry. At all sites, ebb tides consistently exported marsh-derived DOM enriched in terrestrial humic-like components with larger, more degraded, aromatic (e.g., tannin- and condensed hydrocarbon-like) molecules. As water levels decreased during ebb tides, DOM composition gradually shifted from more protein- and lipid-like to more tannin-, and condensed hydrocarbon-like substances, suggesting a sequential release of compositionally distinct DOM pools that may be distributed along the lateral gradient in the landscape. Tidal variability was relatively minor at the low-salinity marsh due to limited hydrologic connectivity and greater riverine inputs. Our findings highlight the finer-scale tidal dynamics of surface water DOM, providing insights into how increasing hydrological alterations may affect coastal carbon cycling and its downstream impacts on ecosystem functioning.