The response of dissolved organic matter dynamics to flood events in tidal estuaries

Tidal estuaries serve as critical zones for biogeochemical connectivity between terrestrial and oceanic ecosystems. With climate change magnifying the impact of flood events on riverine system, dissolved organic matter (DOM) cycling, the largest reactive elemental pool in ecosystems, in tidal estuaries tend to be more complex and remain poorly understood. To address this gap, the response of DOM dynamics to flood events in a typical tidal estuary was explored. Results showed that although tidal waves constrained DOM chemistry under normal hydrological conditions, significant chemical changes of DOM were triggered by floods under flood-influenced hydrological conditions. Flood events resulted in lower salinity and pH but higher concentrations of nutrients and chlorophyll-a compared to normal period. Significantly higher concentrations of dissolved organic carbon (DOC) and total dissolved nitrogen (TDN) in flood period than normal period were also observed. Molecular analysis revealed increased DOM diversity, higher aromaticity, unsaturation, and molecular weight, indicating enhanced terrestrial inputs and reduced biodegradability in the flood period than the normal period. These variations are likely attributable to flood-induced hydrological impacts on river ecosystems (e.g., erosion of riparian soil and sediments addition). Furthermore, with the integration of our results and global flood information, significant input of the recalcitrant DOC induced by floods was uncovered in global rivers and estuaries, hinting the enhancement of carbon transportation from terrestrial to riverine ecosystems driven by floods. This work highlights the substantial impact of floods on DOM dynamics and carbon cycling in estuaries, with broader implications for coastal ecosystem resilience under changing climate regimes.