Dynamics of dissolved organic matter composition in Scottish rivers and headwater streams – resolving environmental and biogeochemical process interactions

Abstract

Dissolved organic matter (DOM) has a wide range of chemical structures that give it a multifunctional role in the natural environment. Although the role of DOM in aquatic ecosystems has been the focus of previous work, a comprehensive understanding of the compositional behaviour of DOM under different environmental processes is still incomplete. New field-based geochemistry data is presented from a two-year study (03/2017- 03/2019) in Scottish headwaters and a 9-month study in large Scottish rivers. This research shows that the DOM mobilisation follows seasonality with enhanced exports of DOM during winter months compared to the summer. At a larger spatial scale, the seasonal trend is overprinted by the catchments soil type. Size-Exclusion Chromatography combined with high-resolution time series of DOM variables reveal that precipitation events preferentially mobilise humics from the surrounding soils, while humics concentration decline during low flow conditions. Furthermore, the data show that non-UV absorbing (“invisible”) low molecular weight (LMW) neutrals (iDOM) contribute up to 50 % to the total DOM pool in headwaters, especially during low flow conditions, and on average 13 % to the DOM in larger river systems. The source of iDOM was found to be the topsoil of peatland and peaty podzols. Consequently, more labile OM can be leached from soils into the aquatic environment in the future through disturbed soils promoting instream microbial growth and act as a nutrient source for aquatic plants.