Sources, Transformation, and Fate of Dissolved Organic Matter in the Gravel Bar of a Prealpine Stream

Abstract

Gravel bars (GBs) are hotspots of biogeochemical activity, likely impacting carbon dynamics in streams and rivers. However, it remains unclear how GBs process dissolved organic matter (DOM) received from stream water and groundwater. Here we investigate the spatial and temporal variability of DOM concentration and composition within a GB using DOM absorbance and fluorescence measurements. We found clear seasonal and diurnal patterns in DOM composition within the GB, indicating changing contributions of DOM sources and transformation processes. While DOM composition was characterized by more protein‐like DOM in summer, with increasing contributions of humic‐like DOM toward winter within the stream water and GB, groundwater DOM exhibited a strong aromatic and humic‐like character year‐round. DOM composition and concentration varied diurnally during autumn and winter within the GB and stream water, while seasonally higher groundwater inputs in summer likely muted the diurnal pattern, pointing to the importance of seasonal hydrological controls on DOM dynamics. Mixing model analysis showed GB DOM characteristics to differ from that predicted by stream water‐groundwater mixing within the GB, particularly in summer. Dissolved organic carbon concentration decreased along flow paths through the GB with increasing residence time, likely pointing to microbial uptake and/or adsorption to sediment surfaces within the GB, with concurrent clear shifts in DOM composition. While freshly produced and more humified DOM increased along GB flow paths, protein‐like fluorescence (C3) was removed, indicating the simultaneous production and removal of DOM. Together, our findings highlight the role of GBs in DOM removal and transformation, influenced by seasonal shifts in temperature and hydrodynamics.

Role of Lakes, Flood, and Low Flow Events in Modifying Catchment‐Scale DOC:TN:TP Stoichiometry and Export

“…Inland waters receive large amounts of C and nutrients from their surrounding terrestrial environment. Extreme events have the potential to disrupt flow patterns, resulting in significant alterations to flow paths, carbon and nutrient inputs, and residence times within aquatic systems (Boodoo et al., 2020; Palmer & Ruhi, 2019; Xenopoulos et al., 2021). Such changes can significantly affect the solubility, reactivity and mobility of solutes, potentially decoupling element fluxes and impacting the ratios of transported solutes in downstream ecosystems (Kaushal et al., 2018; Moatar et al., 2017).…”

Seasonal Freezing Drives Spatiotemporal Dynamics of Dissolved Organic Matter (DOM) and Microbial Communities in Reclaimed Water-Recharged River

“…During the ice-sealed period, the humiclike components of the river significantly increased by over 10%. This phenomenon was likely due to the seasonal control exerted by temperature and light conditions, influencing the composition of DOM [46].…”

How humans alter dissolved organic matter composition in freshwater: relevance for the Earth’s biogeochemistry

“…Similarly, the removal of natural structures, like established beaver and debris dams, and gravel bars may impact DOM concentration and composition. These structures are associated with export of autochthonous and more processed DOM of high biodegradability and range of reactivities (Catala ´n et al 2017), transforming and removing DOM in streams (Boodoo et al 2020;Findlay and Sobczak 1996). Changes in river morphology and connectivity to their surrounding environment (channelization, impervious riverbeds and embankments, and beaver dams) can have consequences for DOM source, export and biodegradation dynamics within streams (Wohl et al 2017;Harvey and Gooseff 2015).…”