Validating reflectoquant™ method for phosphorus quantification in uruguayan waters

Abstract

This study evaluates the Reflectoquant™ (REFL) method for quantifying soluble reactive phosphorus (SRP) in surface waters, using UV-Vis spectrophotometry (SPEC) as a reference. Statistical analyses—including Spearman correlation, linear regression (simple, logarithmic, square-root, polynomial), and Bland-Altman tests—were conducted to assess method agreement. Spearman correlations were strong (ρ = 0.979 for standards; ρ = 0.890 for river samples). Simple linear regression showed excellent fits (adjusted R² = 0.972, RMSE = 0.010 mg PO₄-P L⁻¹ for standards; adjusted R² = 0.995, RMSE = 0.018 mg L⁻¹ for rivers), outperforming transformed models (e.g., log adjusted R² ≤ 0.856). Residuals from river samples deviated from normality and homoscedasticity, except in the polynomial model for standards. Bland-Altman analysis revealed a small overall bias (+0.009 mg L⁻¹), with REFL overestimating SRP at high concentrations (≥ 0.100 mg L⁻¹, mean difference = +0.015 mg L⁻¹) and underestimating at low concentrations (< 0.025 mg L⁻¹, mean difference = −0.022 mg L⁻¹). Despite limitations at trace levels, Reflectoquant’s portability and low cost make it a valuable field tool. In Uruguay, SRP typically represents ~60% of total phosphorus (TP), a key driver of eutrophication. Since TP thresholds for eutrophication are commonly set at 0.025 mg L⁻¹, SRP values above this level may indicate high eutrophication risk. Reflectoquant enables rapid field screening of potential tipping points in freshwater ecosystems, supporting timely interventions.