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

Abstract

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 TM 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 organicrich 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 organicrich systems such as peatlands.

Self‐diagnosis of model suitability for continuous measurements of stream‐dissolved organic carbon derived from in situ UV–visible spectroscopy

“…Beam attenuation coefficient data from the in situ spectrophotometer (spectro::lyser) is path length-normalized and has units of m À1 . To better approximate in situ absorption, all data were corrected for particle light-scattering effects by subtracting the average absorption, estimated from the measured attenuance, across 700-730 nm from the rest of the spectra (Grayson and Holden 2016). It should be noted this correction, performed with the data available, does not account for the full effect of particles such as wavelength-dependent scattering or even absorption by particles.…”

Diurnal Patterns in Solute Concentrations Measured with In Situ UV-Vis Sensors: Natural Fluctuations or Artefacts?

“…In the field, sensors are subjected to seasonal and diurnal temperature variations. In addition, the fouling of the sensor measurement windows can occur through the oxidation of iron or manganese by UV light [23] or biofouling [21,25,47,53]. Additionally, other substances in the water can interfere with the measurements, such as nitrite or bicarbonate [45].…”

“…These sensors use algorithms to calculate solute concentrations based on absorbance at a specific wavelength or multiple wavelengths. Although the use of in situ UV-Vis sensors presents several challenges, such as biofouling [23], local calibration [24,25], and power supply [24,26], numerous studies have used such sensors in the field [20,21,[27][28][29][30]. In most of these studies, grab samples analyzed in the laboratory were used to validate data recorded by the sensor [24,31].…”

High frequency UV–Vis sensors estimate error in riverine dissolved organic carbon load estimates from grab sampling

“…High frequency ultraviolet–visible (UV–Vis) sensors are becoming more common for estimating DOC fluxes as they offer a method of estimating DOC concentration at a resolution that can overcome the issues of manual and automatic sampling (Grayson & Holden, 2016 ; O’Driscoll et al, 2018 ; Ruhala & Zarnetske, 2016 ). These higher resolution data, collected at intervals of minutes or hours, offer insight into, for example, the hysteresis between DOC concentration and flow.…”