A multi-metal risk assessment strategy for natural freshwater ecosystems based on the additive inhibitory free metal ion concentration index * Cristina M. Alves a , Carlos M.H. Ferreira a , Eduardo V. Soares b, c , Helena M.V.M. Soares a, * a REQUIMTE/LAQV, Chemical Engineering Department, Faculty of Engineering, University of Porto, 4200-465, Porto, Portugal b Bioengineering Laboratory-CIETI, Chemical Engineering Department, ISEP-School of Engineering of Polytechnic Institute of Porto, Rua Dr Ant onio Bernardino de Almeida, 431, 4200-072 Porto, Portugal c CEB-Centre of Biological Engineering, University of Minho, 4710-057 Braga, Portugal article info Article history: Received 5 June 2016 Received in revised form 17 January 2017 Accepted 18 January 2017 Available online 31 January 2017 Keywords: Additive inhibitory metal concentration index (AIMCi) Alga Chronic toxicity assay Metal speciation Metal bioavailability abstract Scientifically sound risk assessment strategies and derivations of environmental quality standards for metals present in freshwater environments are currently hampered by insufficient chronic toxicity data collected from natural ecosystems, as well as inadequate information on metal speciation. Thus, the aim of the present study was to evaluate the impact of freshwater containing multiple metals (Cd, Cr, Cu, Ni, Pb and Zn) on the chronic toxicity (72h) to the alga Pseudokirchneriella subcapitata and compare the observed toxicity results to the total and free metal concentration of the samples. Based on the infor- mation obtained herein, an additive inhibitory free multi-metal ion concentration index, calculated as the sum of the equivalent toxicities to the free metal ion concentration of each sample, was developed. The proposed index was well correlated to the observed chronic toxicity results, indicating that the concentration addition, when expressed as the free-ion activity, can be considered a reliable indicator for the evaluation of ecological risk assessments for natural waters containing multiple metals. © 2017 Elsevier Ltd. All rights reserved. 1. Introduction Metal contamination in aquatic systems has been a subject of great concern for many years due to the abundance, persistence and toxicity of many metals, as well as their ability to bio- accumulate in the food chain and impact human welfare. In natural waters, a small portion of dissolved metals may exist as free metal ions because metals form stable complexes with a large variety of inorganic and organic ligands, which influence its bioavailability, toxicity and mobility. Therefore, the total metal concentration of a water sample is insufficient when interpreting the reactivity of the metal in biological or environmental processes (Mota and Correia dos Santos, 1995). Because metal speciation plays a key role in determining the potential fate and toxicity of a given metal, all available approaches for predicting metal toxicity consistently incorporate geochemical speciation. Organisms in natural aquatic systems are typically exposed to a mixture of different metals. Although aquatic toxicity tests have been conducted using mixtures of various metals for several de- cades (Meyer et al., 2015), clear patterns that could be used to predict toxicity have not yet been identified. In many of these studies, acute exposures were examined; therefore, acclimation of the organism or metal tolerance development was not considered (Campbell and Fortin, 2013). Because few studies have been per- formed under conditions that mimic the complexity of natural waters, these results may not directly or completely translate into natural systems (Campbell and Fortin, 2013). Authorities such as the US Environmental Protection Agency (EPA), World Health Organization (WHO) and European Environ- mental Agency (EEB) have developed and set recommended thresholds for water quality, including those related to metal toxicity guidelines for monitoring metal contamination (European Union, 2008; Fairbrother et al., 2007; US-EPA, 2009; WHO, 2011). Although organisms in natural aquatic systems are usually exposed to a mixture of metals instead of a single metal, most legislation continues to be based on the concentration of only one metal, * This paper has been recommended for acceptance by Dr. Harmon Sarah Michele. * Corresponding author. Chemical Engineering Department, Faculty of Engi- neering, University of Porto, Rua Dr. Roberto Frias, 4200-465, Porto, Portugal. E-mail address: hsoares@fe.up.pt (H.M.V.M. Soares). Contents lists available at ScienceDirect Environmental Pollution journal homepage: www.elsevier.com/locate/envpol http://dx.doi.org/10.1016/j.envpol.2017.01.053 0269-7491/© 2017 Elsevier Ltd. All rights reserved. Environmental Pollution 223 (2017) 517e523