Unveiling the molecular composition of dissolved organic phosphorus in the freshwater-estuarine continuum using ultrahigh-resolution FT-ICR-MS.

Dissolved organic phosphorus (DOP) plays a critical role in the aquatic phosphorus cycle. However, its molecular composition and controlling factors along the freshwater-estuarine continuum remain largely unknown. To address this issue, we employed Fourier transform-ion cyclotron resonance mass spectrometry (FT-ICR-MS) to characterize the seasonal and spatial dynamics of DOP molecular composition in the subtropical Minjiang River. Our analysis detected a total of 100-271 DOP formulas for each station, with higher %CHOP and %CHONP in summer and higher %CHOSP in winter. The high %CHOSP occurred in the freshwater and decreased rapidly in the estuary, suggesting that the CHOSP molecules were likely derived from anthropogenic inputs, algal production, and sulfurization with weakened dilution at a low runoff in winter. We further compared the molecular composition of DOP with that of the bulk dissolved organic matter (DOM), and found lower percentages of lignin-like compounds, carboxyl-rich alicyclic molecules, and highly unsaturated phenolics but higher percentages of protein-like compounds and labile formulas for DOP, suggesting a higher bioavailability of DOP. Moreover, to unveil the controlling factors of DOP molecular indices, their correlations with key environmental factors (salinity, total suspended matter, and Chlorophyll a) were analyzed, which suggested the mixing of terrestrial aromatic and marine nitrogen/protein-rich compounds, along with the algal production of labile compounds that were partly dependent on light availability. Finally, principal component analysis of DOP molecular indices revealed higher lability in summer than in winter and more oxygen-containing compounds in freshwater than in the estuary in winter, which were probably related to the changes of DOP sources and transformations. In conclusion, these results provided insights into the molecular composition and controlling mechanisms of DOP in the river-coastal continuum, which are valuable for understanding the environmental behaviors of DOP and improving water quality management.