High frequency DOC proxy sensor assessment for peatland streams and across hydrological continua

Fluvial dissolved organic carbon (DOC) can only be quantified through laboratory measurements which, at low frequency intervals, may lead to unreliable carbon load estimates. As a solution, studies have implemented high frequency DOC proxy sensors, which are generally fluorescence or absorbance based. These sensors, however, have rarely been tested in an upland peatland environment with very high DOC concentrations or across hydrological continua. Therefore, to test their suitability and fill this knowledge gap, fluorescence and absorbance sensors were used to take measurements along varying environmental gradients and a hydrological continuum at eleven sites. These ranged from a first-order stream in an open moor peatland, down the main channel, and up to 126 km² during seven separate campaigns (August 2023-August 2024). Results showed that the absorbance-based sensor provided a linear relationship with DOC concentrations across the hydrological continuum and during different flow conditions. However, the fluorescence-based sensor experienced signal issues when taking measurements in the upland portion of the catchment, when DOC concentrations exceeded ∼15 mg L^-1, and struggled during high flow events. Through a series of post hoc experiments, uncorrectable signal quenching was identified to be caused by a higher level of humification present in the water, measured through the E4:E6 ratio (r = 0.282, p = 0.024). Therefore, it is recommended that future peatland fluvial carbon studies implementing high frequency proxy monitoring (with higher DOC concentrations >15 mg L^-1) should use absorbance based sensors rather than fluorescence based, and which offer transferability to other fluvial environments and flow conditions.