Total phosphorus reference condition for subalpine lakes: A comparison among traditional methods and a new process-based watershed approach Franco Salerno a, * , Gaetano Viviano a , Elisa Carraro a , Emanuela Chiara Manfredi a , Andrea Lami b , Simona Musazzi b , Aldo Marchetto b , Nicolas Guyennon b , Gianni Tartari a , Diego Copetti a a CNR e Water Research Institute (IRSA), Via del Mulino 19, Brugherio, MB, 20861, Italy b CNR e Institute of Ecosystem Study (ISE), Largo Tonolli 50, Verbania Pallanza, VB, 28922, Italy article info Article history: Received 10 February 2014 Received in revised form 28 May 2014 Accepted 11 June 2014 Available online Keywords: Export coefficient models Phosphorus Reference conditions Morpho Edaphic Index MEI Hydrological transport model Diatom inferred TP Chlorophyll inferred TP abstract Different methods for estimating the total phosphorus (TP) reference conditions of lakes have rarely been compared. This work tests the uncertainty and accuracy of the most frequently used approaches (Morpho-edaphic index -MEI-, export coefficient, diatoms and pigment-inferred TP models) for 35 subalpine lakes. Furthermore, we propose a new process-based watershed approach that was tested on a subalpine environment and consists of combining a space for time substitution with a space for space substitution. The possible presence of uncontaminated or less contaminated environments inside or next to the watershed can be exploited by training a hydrological transport watershed model according to the uncontaminated conditions and then applying the calibration to the entire watershed, which re- constructs a natural or semi-natural TP load scenario. We found that the root mean square error (RMSE) for the MEI is 4 mgL 1 . However, its application is limited for lakes that present with an alkalinity 1 meq L 1 . For lakes with a higher alkalinity, we observed a loss of predictive capability that results from the lower solubility of phosphorus under conditions of high calcium content. The export coefficient model was applied with a mean export coefficient and presents similar prediction capabilities as the MEI. The chlorophyll-inferred TP model shows a higher uncertainty (RMSE ¼ 8 mgL 1 ); however, it produced fewer underestimations and overestimations. With regards to the diatom-inferred TP model, we are only able to evaluate an uncertainty of 5 mgL 1 at the European level. Finally, the proposed process-based watershed approach adequately predicted the reference condition of the selected lake and had an un- certainty lower than the other methods (2 mgL 1 ). We conclude by revealing the potential and limita- tions of this approach in the field of ecological lake modelling more and more attracted by TP pristine load inputs in studies on the effects of climate change and eutrophication of lakes. © 2014 Elsevier Ltd. All rights reserved. 1. Introduction Significant efforts have been taken to establish quality criteria and thresholds for classifying lakes according to their trophic status (e.g., Carlson, 1977; OECD, 1982; Buraschi et al., 2005). These criteria include nutrient (e.g., phosphorus and nitrogen) concen- trations and physico-chemical (e.g., transparency and dissolved oxygen) and biological (e.g., chlorophyll) features. However, the role of phosphorus is generally recognised as a limiting factor in the process of eutrophication; therefore, it is widely used as a reference target to summarise the trophic condition of lakes (OECD, 1982; Schindler et al., 2008). The level of divergence between the cur- rent state and the natural or semi-natural reference condition Acronyms: HYDRO, Hydrological Transport model; MEI, Morpho Edaphic Index; TP, Total Phosphorus; ChlI-TP, Chlorophyll inferred TP model; DI-TP, Diatom inferred TP model; EXP-coeff-TP, Export coefficient TP model; MEI-alk-TP, Morpho-Edaphic Index; alkalinity, MEI-cond-TP; Morpho-Edaphic Index, conductivity; RMSE, Root Mean Squared Error; RMSEP, Root mean squared error of prediction; HA, High Alkalinity group; LM-A, Low-Medium Alkalinity group. * Corresponding author. Tel.: þ39 039 21694221; fax: þ39 039 2004692. E-mail address: salerno@irsa.cnr.it (F. Salerno). Contents lists available at ScienceDirect Journal of Environmental Management journal homepage: www.elsevier.com/locate/jenvman http://dx.doi.org/10.1016/j.jenvman.2014.06.011 0301-4797/© 2014 Elsevier Ltd. All rights reserved. Journal of Environmental Management 145 (2014) 94e105