Dissolved organic matter (DOM) is broadly defined as the fraction of organic matter that passes through a 0.45 µm filter, encompassing compounds with a wide variation in size, solubility, charge and function. Although the composition of DOM in freshwaters is not currently well defined, ca. 20 % of DOM is present as labile, low molecular weight (LMW) DOM which is a key component of in-stream cycling of nutrients, including carbon (C), nitrogen (N), phosphorus (P) and sulphur (S). Presently, C and N export from freshwater to marine environments are increasing globally, due to climate change and agricultural intensification respectively, however, current water quality legislation rarely considers the monitoring, and management, of DOM. The overall aims of this thesis were therefore to: i) gain further insight into DOM processing in rivers across a range of spatial gradients (e.g. land-cover, inorganic/organic nutrient pool size); ii) compare DOM processing to inorganic nutrient processing; and iii) identify how DOM metabolism changes under different nutrient conditions. Radioisotope tracer techniques (14C, 33P, 35S) were used measure the uptake of DOM components (DOC, DON, DOP, DOS) in river waters and sediments. Due to the rapid cycling of LMW DOM compounds by the aquatic microbial biomass, sample preservation methods were investigated. Maintaining samples at a cool temperature, in the dark and commencing experiments within 24 h was the simplest and most efficient method to ensure that DOM within samples was not badly degraded. The use of freezing and acidification were also deemed to be viable options for long-term storage, however, the choice of method depends on their compatibility with subsequent analytical protocols. Landscape-scale analysis of DOM processing found that DOM uptake was faster in inorganic nutrient (N/P) enriched rivers, however the reverse was true for inorganic nutrient uptake. This suggests DOM uptake in nutrient-enriched rivers may not be driven by N/P demand but C limitation. Further work using dual-labelled isotopic methods may provide insight into DOM utilisation following uptake. Experimental work in oligotrophic (peat) and mesotrophic (improved grassland) rivers also found DOC uptake to be elevated in nutrient-enriched river waters and sediments. Microbial growth in sediments was indicated by a lag phase in DOC uptake. Sediments, particularly mesotrophic, had the capacity to process high DOC inputs (5-10 mM) which has implications for water quality management. Nutrient limitation removal by N and/or P addition to oligotrophic sediments led to changes in DOC uptake and metabolism. Metabolome analysis indicated that N addition led to increased DOC processing, while P addition increased amino acid synthesis, attributed to the P-containing enzymes required for the process. Additionally, DOS was found to be preferentially utilised by the microbial biomass, which goes against the tenet that inorganic S is the preferred source of S for most microorganisms. In conclusion, this thesis has provided a basis for exploring the mechanistic basis of DOM processing across physiochemical gradients in river catchments. Further research is now required to ground truth these findings across a wider range of global habitats. The capacity for LMW DOM to be processed by the microbial biomass of river waters and sediments, in addition to preferential uptake compared to inorganic nutrient sources in some contexts, highlight the importance for DOM to be at the forefront of water quality monitoring and management, alongside inorganic nutrients. This information will provide an evidence base from which effective legislation and management strategies can be designed to protect freshwater ecosystems.