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

Area of Science:

• Environmental Science
• Hydrology
• Biogeochemistry

Background:

• Accurate quantification of fluvial dissolved organic carbon (DOC) is essential for carbon load estimation.
• Laboratory measurements at low frequencies can yield unreliable DOC estimates.
• High-frequency DOC proxy sensors (fluorescence, absorbance) offer a potential solution but require validation in diverse environments.

Purpose of the Study:

• To evaluate the suitability of fluorescence and absorbance sensors for high-frequency DOC monitoring in upland peatland environments.
• To assess sensor performance across varying environmental gradients and hydrological continua.
• To identify factors influencing sensor accuracy in high-DOC waters.

Main Methods:

• Deployed fluorescence and absorbance sensors across eleven sites, from first-order streams to a 126 km² catchment, over seven campaigns (August 2023-August 2024).
• Measured DOC concentrations along environmental gradients and a hydrological continuum under varying flow conditions.
• Conducted post hoc experiments to identify causes of sensor signal interference, including humification levels (E4:E6 ratio).

Main Results:

• The absorbance-based sensor demonstrated a linear relationship with DOC concentrations across the entire hydrological continuum and diverse flow conditions.
• The fluorescence-based sensor exhibited signal issues in upland areas with DOC > ~15 mg L⁻¹, particularly during high flow events.
• Signal quenching in fluorescence sensors was linked to increased water humification, confirmed by the E4:E6 ratio (r = 0.282, p = 0.024).

Conclusions:

• Absorbance-based sensors are recommended over fluorescence-based sensors for high-frequency DOC monitoring in peatland fluvial systems with high DOC concentrations (>15 mg L⁻¹).
• Absorbance sensors offer greater reliability and transferability across different fluvial environments and flow conditions.
• Future research should consider humification levels when selecting proxy sensors for DOC monitoring in peat-rich catchments.