Improved automation of dissolved organic carbon sampling for organic-rich surface waters

•We test an in-situ probe to measure high DOC concentrations in peatland streams.•Linear regression modelling enabled DOC measurement far beyond stated upper limits.•Models combining absorbance for several wavelengths achieved greatest accuracy.•Our approach enables improved in situ long-term fluvial DOC flux measurements.

In-situ UV-Vis spectrophotometers offer the potential for improved estimates of dissolved organic carbon (DOC) fluxes for organic-rich systems such as peatlands because they are able to sample and log DOC proxies automatically through time at low cost. In turn, this could enable improved total carbon budget estimates for peatlands. The ability of such instruments to accurately measure DOC depends on a number of factors, not least of which is how absorbance measurements relate to DOC and the environmental conditions. Here we test the ability of a S::can Spectro::lyser™ for measuring DOC in peatland streams with routinely high DOC concentrations. Through analysis of the spectral response data collected by the instrument we have been able to accurately measure DOC up to 66 mg L− 1, which is more than double the original upper calibration limit for this particular instrument. A linear regression modelling approach resulted in an accuracy > 95%. The greatest accuracy was achieved when absorbance values for several different wavelengths were used at the same time in the model. However, an accuracy > 90% was achieved using absorbance values for a single wavelength to predict DOC concentration. Our calculations indicated that, for organic-rich systems, in-situ measurement with a scanning spectrophotometer can improve fluvial DOC flux estimates by 6 to 8% compared with traditional sampling methods. Thus, our techniques pave the way for improved long-term carbon budget calculations from organic-rich systems such as peatlands.

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