Tidal Effects on Dissolved Organic Matter Dynamics in a Brackish Water Front Adjacent to Yangtze River Estuary

A brackish water front, where river water meets seawater, is a hotspot for biogeochemical processes. In this study, we examined the quantity and composition of dissolved organic matter (DOM) over a 24 h tidal cycle at a brackish water front near the Yangtze River estuary. Utilizing elemental analysis, fluorescence and ultraviolet spectroscopy, and ultra-high-resolution mass spectrometry, we observed rapid fluctuations in DOM throughout the tidal cycle. The dissolved organic carbon (DOC) and total nitrogen (TN) concentrations ranged from 0.70 to 1.5 mg/L and 0.43 to 0.94 mg/L, respectively. Water samples during low tide exhibited a higher fractional abundance of CHON (17.2 ± 0.1% vs. 14.6 ± 0.4%), CHOS (14.6 ± 4.5% vs. 9.1 ± 3.1%), and CHONS (1.6 ± 0.5% vs. 0.5 ± 0.3%) formulas, and a higher aromatization and average molecular weight, which is consistent with a stronger terrestrial influence. In contrast, at high tide, the water samples contained a greater abundance of CHO compounds (75.7 ± 3.8% vs. 66.5 ± 4.1%), a humic-like fluorescent C1 component, and carboxyl-rich alicyclic molecules (CRAMs), indicating a greater release of refractory DOM from resuspended sediments. However, variations in the DOC concentrations and several optical spectral parameters did not correlate with the changes in the salinity and tidal height. The results of the principal component analysis revealed different controls on specific fractions of DOM that are related to variable DOM sources or biogeochemical processes. The complexity of DOM dynamics underscores the necessity of elucidating DOM compositions at varying levels to enhance our understanding of carbon cycling in estuarine and coastal ecosystems.