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

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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 have 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 have constrained attempts to characterize intra‐ and inter‐seasonal variability in DOM across the spectrum from low to storm flows. In this focus article, we synthesize 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.

Conceptual representation of dissolved organic matter (DOM) and water fluxes to headwater urban river system during: (a) low flow and (b) high flow. CSO, combined sewerage overflow; GW, ground water; HMW, high molecular weight; MW, molecular weight; PAH, polycyclic aromatic hydrocarbon; WwTW, waste water treatment works; ?, DOM lability is dependent on waste water treatment process. Size of arrows are proportional to the magnitude of the flux.
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Event discharge and hysteresis loops for tryptophan‐like fluorescence (TLF) and turbidity. (a) Low magnitude event with first flush indicative of in‐channel organic matter mobilization. (b) Delayed flush event with dissolved organic matter (DOM) preceding discharge and likely associated with combined sewer overflow (CSO) and storm drain sources. (c) High magnitude event with significant depletion/exhaustion of DOM sources apparent. AR, 14‐day antecedent; ER, total event rainfall; rainfall; RI, maximum rainfall intensity.
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Relationship between in situ fluorescence and laboratory measurements of biochemical oxygen demand (BOD) and dissolved organic carbon (DOC) for the Bourn Brook. For (a) and (b), tryptophan‐like fluorescence (TLF) was measured using a Cyclops 7 corrected for turbidity and temperature following Khamis et al. For (c) and (d), humic‐like fluorescence (HLF) and TLF were both measured using an FL30 flow cell sensor (Albillia Co), and corrected for turbidity and temperature following Khamis et al. Note that the FL30 was more sensitive to turbidity effects than the Cyclops 7 and turbidity compensation was ineffective above 200 NTU, hence these data points have been omitted.
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Time series at 15 min resolution for the Bourn Brook, Birmingham, UK, autumn 2014. (a) Discharge and precipitation, (b) specific electrical conductivity (EC) and turbidity, (c) tryptophan‐like fluorescence intensity (TLF), in situ readings are lines and laboratory measurements are dots. In (a) the number of stars denotes the event number used in Figure (a) and (b). The TLF sensor was a Cyclops 7 (Turner Designs, EX 285 ± 10 nm EM 350 ± 55 nm) and was integrated with a Manta 2 multi‐parameter sonde (Water Probes, Austin, TX, USA) that also logged turbidity, pH, EC, and temperature (15 min resolution). CI, confidence interval.
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(a) Map of UK with Birmingham highlighted, (b) land use of the Bourn Brook catchment, (c) image of the Bourn Brook directly downstream of the monitoring site. (d) Cyclops 7 (Turner Designs, San Jose, CA, USA) open face tryptophan‐like fluorescence sensor, and (e) FL30 multiwavelength, flowcell fluorometer (Albillia Co, Neuchatel, Switzerland).
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Conceptual representation of labile dissolved organic matter (DOM) source contributions to stream to total DOM flux over storm events. CSOs, combined sewer overflows; WwTW, waste water treatment works.
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