Determining patterns in the composition of dissolved organic matter in fresh waters according to land use and management

Abstract

In fresh waters, the origins of dissolved organic matter (DOM) have been found to exert a fundamental control on its reactivity, and ultimately, its ecosystem functional role. A detailed understanding of landscape scale factors that control the export of DOM to aquatic ecosystems is, therefore, pivotal if the effects of DOM flux to fresh waters are to be fully understood. In this study we present data from a national sampling campaign across the United Kingdom in which we explore the variability in DOM composition in three broad landscape types defined by similar precipitation, geology, land use and management, hydrology, and nutrient enrichment status. We characterised samples from fifty-one sites, grouping them into one of three major underlying classifications: circumneutral streams underlain by clay and mudstone (referred to as ‘clay’), alkaline streams underlain by Cretaceous Chalk or by Carboniferous or Jurassic Limestone (‘limestone’), and acidic streams in peatland catchments underlain by a range of low permeability lithologies (‘peat’). DOM composition was assessed through organic matter stoichiometry (organic carbon: organic nitrogen; organic carbon: organic phosphorus; C/N(P)DOM) and metrics derived from ultra-violet (UV)/visible spectroscopic analysis of DOM such as specific UV absorption (a254 nm; SUVA254). We found similar SUVA254, C/NDOM and DOM/a254 relationships within classifications, demonstrating that despite a large degree of heterogeneity within environments, catchments with shared environmental character and anthropogenic disturbance export DOM with a similar composition and character. Improving our understanding of DOM characterisation is important to help predict shifts in stream ecosystem function, and ecological responses to enrichment or mitigation efforts and how these may result in species composition shifts and biodiversity loss in freshwater ecosystems.

Exploring the nature, origins and ecological significance of dissolved organic matter in freshwaters: state of the science and new directions

“…We then move on to a suite of papers in which the authors apply these techniques at scale, often over multiple catchments with varying environmental character, to reveal the origins of DOM in freshwaters as this varies in space in time. The multi-catchment research of Williamson et al (2021) 2021) tackles a major river basin, the Mississippi, USA, using optical and molecular-scale approaches to reveal the impacts of anthropogenic land cover on DOC flux and the molecular composition of the stream DOM pool, with many parallels to be drawn between this study and the DOC findings reported by Yates et al (2022) and Williamson et al (2021). Voss et al (2022) then use a suite of molecular and isotope chemistry approaches to infer the likely sources of DOC and the role of organic matter respiration on stream DOM pool composition in another large river basin: the Fraser River, in SW Canada.…”

Chemodiversity of Dissolved Soil Organic Matter from Amazon Rainforest as Influenced by Deforestation

“…Additionally, it has been reported that in deforested sites, there is an increase in the relative contents of recalcitrant compounds (e.g., condensed hydrocarbons, lignin, and tannins) [ 10 ]. Meanwhile, deforestation can also affect the composition of DOM by altering soil microbial activities responsible for its decomposition [ 11 ]. However, the relationship between the chemodiversity of DOM and deforestation in the Amazon basin, as well as the underlying mechanisms that control the diversity of labile and recalcitrant compounds in tropical soils, is still largely unknown.…”