Seasonal changes of dissolved organic matter chemistry and its linkage with greenhouse gas emissions in saltmarsh surface water and porewater interactions.

ABSTRACT

Dissolved organic matter (DOM) is one of the largest reactive reservoirs of carbon on earth. Saltmarshes play an essential role in shaping the fate of DOM and greenhouse gas (GHG) production in surface water and groundwater interactions in coastal areas. However, the coupling mechanism between DOM and GHG production is poorly understood. In this study, DOM in both surface water and porewater were analyzed by 3D excitation-emission-matrix spectroscopy under different seasonal and tidal conditions in a saltmarsh. Protein-like DOM was likely to produce CH4, while humic-like DOM tended to produce CO2. CH4 concentration was highly enriched in porewater because increasing fresh groundwater flow introduced small-sized protein-like DOM. Based on the mass balance model, >98.5% of CH4 was oxidated to CO2 in sediment-water interface. The degradation of sediment-derived DOC (especially humic-like DOM) contributes ∼80% of the total amount of CO2 in surface water. Both hydrodynamics and chemical reactions are suggested to influence greenhouse gas (GHG) emissions. Hydrodynamics (e.g., tidal pumping) are controlling factors in short timescales (hourly/weekly) while chemical reactions become crucial in influencing DOM chemistry and related degradation rate on seasonal scales. These findings emphasize the importance of the coupling mechanism at different time scales between DOM characteristics and GHG emissions in saltmarshes.