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In situ high-frequency measured turbidity can potentially be used as a surrogate for riverine phosphorus (P) concentrations to better justify the effectiveness of nutrient loss mitigation measures at agricultural sites. We explore the possibilities of using turbidity as a surrogate for total phosphorus ( TP ) and particulate phosphorus ( PP ) in four snowmelt-driven rivers draining agricultural clayey catchments. Our results suggest slightly stronger relationship between in situ measured turbidity and  PP  than between turbidity and  TP . Overall, linear  TP  and  PP  regressions showed better error statistics in the larger catchments compared with their sub-catchments. Local calibration of the in situ sensors was sensitive to the number of high P concentration discrete water samples. Two optional calibration curves, one with and one without influential data, resulted in a 17% difference in the estimated mean  TP  concentrations of a snowmelt storm contributing 18% of the annual discharge volume. Accordingly, the error related to monthly mean  TP  estimates was the largest in spring months at all sites. The addition of total dissolved phosphorus ( TDP ) improved the model performance, especially for sites where the  TDP/TP  ratio is large and highly variable over time. We demonstrate how long-term discrete samples beyond sensor deployment can be utilized in the evaluation of the applicability range of the local calibration. We recommend analysing the validity of P concentration estimates, especially during high discharge episodes that contribute substantially to annual riverine nutrient fluxes, since the use of surrogates may introduce large differences into the P concentration estimates based on selected local calibration curves.   Electronic supplementary material  The online version of this article (10.1007/s10661-020-08335-w) contains supplementary material, which is available to authorized users.

High-frequency measured turbidity as a surrogate for phosphorus in boreal zone rivers: appropriate options and critical situations