Understanding dissolved organic matter dynamics in urban catchments: insights from in-situ fluorescence sensor technology.

Dissolved organic matter (DOM) is critically important for catchment biogeochemical cycling, yet the DOM dynamics of many river systems remain poorly characterized. Recently, DOM mobilization and transport in forested and agricultural catchments has received increased attention; however, for urban catchments, our understanding of spatio-temporal variability in DOM concentration and composition is very limited. This is a particular concern as urbanization can increase and alter labile DOM fluxes leading to a shift from downstream transport of stream carbon to increased microbial production and respiration of stream carbon in headwaters. Furthermore, the anthropogenic modification of the water cycle and the flashy hydrology of urban rivers has constrained attempts to characterize intra- and inter-seasonal variability in DOM across the spectrum from low to storm flows. In this focus article, we synthesise the contemporary literature on urban DOM sources, flow paths and spatio-temporal variability and present a conceptual model to unravel system dynamics and inform future monitoring efforts. The potential of field deployable fluorescence sensor technology to overcome monitoring challenges in urban rivers is highlighted. We use a case study of a relatively well-studied UK urban river to illustrate the potential of in-situ fluorescence to reveal DOM dynamics in a system with marked inter-event variability in DOM sources and pathways. Finally, we outline future directions for this research, particular the need to standardize field and laboratory protocols and advance new sensor development.