Variations in the Optical and Molecular Composition of Dissolved Organic Matter Exported from Coastal Wetlands.

Coastal wetlands regulate the transport and transformation of dissolved organic matter (DOM) between land and the ocean. It is known that DOM exported from tidal wetlands is chemically distinct from DOM in adjacent estuaries, yet its sources and compositional dynamics remain poorly resolved. Here, we investigated tidal variability in the optical and molecular composition of surface water DOM through absorbance and fluorescence spectroscopy and high-resolution mass spectrometry. Surface water samples were collected over multiple tidal cycles at three tidal marshes along a surface water salinity gradient in the Chesapeake Bay. At all sites, ebb tides consistently exported marsh-derived DOM enriched in terrestrial signatures with larger, more degraded, aromatic (e.g., tannin-like and condensed hydrocarbon-like) molecules. As water levels decreased during ebb tides, DOM composition gradually shifted from more protein-like and lipid-like to more 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 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 downstream ecosystem functioning.