Clearing the waters: Evaluating the need for site‐specific field fluorescence corrections based on turbidity measurements

Abstract

In situ fluorescent dissolved organic matter (fDOM) measurements have gained increasing popularity as a proxy for dissolved organic carbon (DOC) concentrations in streams. One challenge to accurate fDOM measurements in many streams is light attenuation due to suspended particles. Downing et al. (2012) evaluated the need for corrections to compensate for particle interference on fDOM measurements using a single sediment standard in a laboratory study. The application of those results to a large river improved unfiltered field fDOM accuracy. We tested the same correction equation in a headwater tropical stream and found that it overcompensated fDOM when turbidity exceeded 300 formazin nephelometric units (FNU). Therefore, we developed a sitespecific, field-based fDOM correction equation through paired in situ fDOM measurements of filtered and unfiltered streamwater. The site-specific correction increased fDOM accuracy up to a turbidity as high as 700 FNU, the maximum observed in this study. The difference in performance between the laboratory-based correction equation of Downing et al. (2012) and our site-specific, field-based correction equation likely arises from differences in particle size distribution between the sediment standard used in the lab (silt) and that observed in our study (fine to medium sand), particularly during high flows. Therefore, a particle interference correction equation based on a single sediment type may not be ideal when field sediment size is significantly different. Given that field fDOM corrections for particle interference under turbid conditions are a critical component in generating accurate DOC estimates, we describe a way to develop site-specific corrections.

Biased measurements by stationary turbidity‐fluorescence instruments due to phototactic zooplankton behavior

“…Turbidity has several units, such as Nephelometric Turbidity Units (e.g., Doxaran et al 2009) and Formazin Nephelometric Units (e.g., Saraceno et al 2017). The US Geological Survey offers the definitions for different turbidity units.…”

Analysis of the Mixing Processes in a Shallow Subtropical Reservoir and Their Effects on Dissolved Organic Matter

“…When monitoring the fDOM concentration, the optical signal of the fDOM probe is affected and distorted by temperature, turbidity, pH, salinity and inner filter effects. The type of turbidity is also important, with the magnitude of fDOM signal bias being related to the shape and size of suspended particles [29,30]. Sequential compensation models were recently developed to investigate such environmental interferences on a fDOM probe for its calibration in Tingalpa Reservoir [30].…”

Multi-Parameter Compensation Method for Accurate In Situ Fluorescent Dissolved Organic Matter Monitoring and Properties Characterization

“…Previous researchers (e.g., [12,19]) have also tried to simultaneously account for multiple interferences. Downing, Pellerin, Bergamaschi, Saraceno and Kraus [12] developed compensation models for different probes, however the turbidity and inner filtering effects were analyzed using standard material rather than site-specific concentrated turbidity or DOM, with laboratory-based corrections unlikely to account, for example, for different particle size distribution of the site-specific turbidity [11]. While Shutova, Baker, Bridgeman, and Henderson [19] developed site-specific compensation in this study, a turbidity compensation model was not considered due to the low historical turbidity values recorded, thus implying that potential future extreme events leading to high turbidity might lead to the inability to compensate the fDOM readings with their model.…”

“…Optical measurements are also affected by the presence of suspended particles in the sample, which causes light scattering and light absorption. The assessment of fDOM-turbidity relationships should be site-specific, because changes in the particle size distribution (PSD) of suspended sediments, chemical composition, shape, and its concentration determine the degree of fDOM signal bias, and these vary locally, over time [11]. As turbidity increases in the water, an increased amount of the excitation light emitted by the fDOM sensor scatters, thus reducing the light available in the sampling volume to excite fDOM; this eventually results in nonlinear fDOM signal beyond a certain turbidity level [12].…”