Search

In subterranean estuaries (STE), fresh and saline groundwater introduce dissolved organic matter (DOM) of different origin. This DOM serves as substrate for microorganisms that thrive in the STE. In high-energy beaches with dynamic porewater advection, microbial communities face frequent changes in groundwater composition, even at several meters depth. It is unknown how DOM from deep STE groundwater (> 5 m depth) is transformed by prevailing microbial communities. To address this question, we performed sediment incubations in flow-through reactors (FTRs) with deep (6 m depth) STE groundwater of low (1.6) and high salinity (29.1). FTR setups were sampled daily for quantification of dissolved organic carbon (DOC), and at start and end (day 13) of the incubation for analysis of DOM composition, microbial cell numbers and community composition. Solid-phase extracted DOM was molecularly characterized via ultrahigh-resolution Fourier-transform ion cyclotron resonance mass spectrometry. Both groundwater types contained mainly reworked DOM. Corroborating its presumed origin, the fresh groundwater had a more terrestrial DOM signature with a higher proportion of aromatic compounds compared to the saline groundwater. Over the course of the incubation, DOC concentrations increased primarily due to leaching of sedimentary organic matter, providing an additional source of DOM. In all setups, the DOM composition changed significantly from start to end, and similarly for fresh and saline groundwater. From the ~2700 molecular formulae (MF) detected on day 0, 34-35% were removed during the incubations, demonstrating the potential of deep STE microbial communities to degrade recalcitrant DOM that is supplied with the advective porewater flow. However, a substantial portion of MF (63-64%) was retained in both groundwater types, indicating that a fraction of deep STE-DOM is resistant to removal. Properties of MF that were newly detected on day 13 (26-28%) were indicative of labile DOM. Some of these newly detected MF were also identified in sediment-leachates, suggesting that beach sediments are a source of fresh DOM for the STE microbial communities. It is likely that due to longer groundwater residence time in the STE, continuous leaching and microbial processing shift the molecular composition of released DOM from more labile to more recalcitrant DOM. [ABSTRACT FROM AUTHOR]

The export of dissolved organic matter (DOM) from rivers is essential for linking terrestrial and marine carbon reservoirs in the global carbon cycle. However, there is limited knowledge regarding how the molecular composition of riverine DOM changes under different hydrological conditions, especially during extreme rainfall events. Moreover, the factors beyond hydrology that impact DOM composition have not been well defined. To address these gaps, samples were collected from a human‐impacted medium‐sized subtropical monsoonal river across various hydrological conditions throughout a complete hydrological cycle. Utilizing high‐resolution mass spectrometry, it was discovered that the solid‐phase extractable DOM (SPE‐DOM) during the high‐flow (1 < runoff (Q): annual mean runoff (Qm) < 3) and extreme‐rain (Q:Qm > 3) periods exhibited a higher number of molecular formulae, lower H/C, higher O/C, and a higher proportion of carboxylic‐rich alicyclic molecules compared to the low‐flow period (LFP) (Q:Qm < 1). These alterations were attributed to input from more diverse sources, particularly a greater input from soil organic matter with higher oxidation degrees. Additionally, the P‐containing formulae were more enriched during the extreme‐rain period, likely from agricultural lands and sediment release. Conversely, the fraction of S‐containing formulae was significantly higher during the LFP, possibly due to the amplified influence of anthropogenic input. Furthermore, the DOM aromaticity did not fluctuate with runoff but was significantly associated with temperature. In summary, the study indicated that the composition of DOM varied significantly under different hydrological conditions, with temperature and anthropogenic activities identified as crucial factors influencing riverine DOM export. Plain Language Summary: Rivers play a crucial role in the global carbon cycle by transporting dissolved organic matter (DOM) from land to the ocean. With the increasing frequency of extreme rainfall events attributed to climate change, there is a growing interest among researchers in understanding how hydrological conditions influence the seasonal variation of DOM. In this study, we investigated a subtropical river in China to explore how the molecular composition of DOM changes under different hydrological conditions and the various factors that contribute to shaping DOM composition. Our findings revealed that the composition of DOM during periods of high discharge and extreme discharge were similar. Still, extreme water discharge, including rainfall events, led to an increased export of heteroatom‐containing DOM. In addition to riverine discharge, factors such as temperature and human activities, including agriculture and sewage discharge, influenced DOM composition from different perspectives. For instance, sewage discharge predominantly introduced S‐containing compounds, while agricultural activities were associated with abundant P‐containing compounds. This study highlights the importance of considering not only changes in sources related to hydrology but also the influences of temperature and human activities when examining how riverine DOM responds to climate pressures. Key Points: The S‐containing formulae were more enriched during the low‐flow periodDissolved organic matter (DOM) exported during both high‐flow and extreme‐rain event periods were higher in O/C and lower in H/CDOM exported during the extreme‐rain event was more diverse, and enriched in P‐containing formulae [ABSTRACT FROM AUTHOR]