The risk of chemical mixtures to ecosystems is often assessed by applying the model of concentration addition or response addition combined with species sensitivity distribution (SSD) curves. Mixture effect predictions have been shown to be consistent only when these models are applied for a single species, however, and not with several species simultaneously aggregated to SSDs. The more stringent procedure for mixture risk assessment would hence be to apply first the concentration addition or response addition models to each species separately and, in a second step, to combine the results to construct an SSD for a mixture. Unfortunately, this methodology is not applicable in most cases because the large data sets it requires are usually unavailable. Based on theoretical data sets generated, the authors aimed to characterize the difference that can exist between these 2 methodologies. Results show that the use of concentration addition on SSD directly may lead to underestimations of the mixture concentration affecting 5% or 50% of species, especially when substances present a large standard deviation in ecotoxicity data constructing their SSD. The application of response addition can lead to over- or underestimations, depending mainly on the slope of the dose–response curves of the individual species. When assessing the risk of mixtures, one must therefore keep in mind this source of error when applying concentration addition or response addition to SSDs directly.

Mixture risk assessment predictions have rarely been confronted with biological changes observed in the environment. In this study, long-term monitoring of a European great lake, Lake Geneva, provides the opportunity to assess to what extent the predicted toxicity of herbicide mixtures explains the changes in the composition of the phytoplankton community next to other classical limnology parameters such as nutrients. To reach this goal, the gradient of the mixture toxicity of 14 herbicides regularly detected in the lake was calculated using concentration addition and response addition models. A temporal gradient of toxicity was observed which decreased from 2004 to 2009. Redundancy analysis and partial redundancy analysis showed that this gradient explains a significant portion of the variation in phytoplankton community composition with and without having removed the effect of all other co-variables. Moreover, species that are significantly influenced, positively or negatively, by the decrease of toxicity in the lake over time are highlighted. It can be concluded that the herbicide mixture toxicity is one of the key parameters to explain phytoplankton changes in Lake Geneva.

Aquatic organisms are typically exposed simultaneously to several organic compounds released from human activities like agriculture, industries, or simply from people living in cities. The ecological risk assessment of mixtures of such compounds has therefore to be addressed by scientists. The aims of this paper are (1) to describe the current mixture risk assessment procedures, (2) to apply such approach to a specific case study, Lake Geneva and the River Rhône in Switzerland, and (3) to discuss the outcomes of such an application. Two models, called concentration addition and independent action, are recognized to be robust enough to predict the mixture effect of substances on a given species. They are classically used also to assess the risk of mixtures for the ecosystem, but their use is often limited by the lack of available ecotoxicity data. Adopting a first level assessment, we describe the evolution of the mixture risk for several years of Lake Geneva, and for 2010 for the River Rhône. These first assessments allow identification of the most problematic substances demanding risk reduction measures. Furthermore, again for the two cases studies, we show that the risk levels associated with mixtures of compounds can rapidly exceed critical aquatic thresholds, and therefore, it is the sum of the substances that is problematic, which is more challenging in term of risk management. Further analysis of effects in compound mixtures as well as a better characterization of the overall ecological risk are necessary for the thousands substances co-occuring at very low concentrations.

Species sensitivity distributions (SSDs) are an important predictive tool in risk assessment. Usually, literature data are used to build SSDs that are mostly based on planktonic species. But, to get adequate protective thresholds for environmental communities, one could argue that SSD should be built on ecotoxicological data obtained from species found in the ecosystem that should be protected. This is particularly true when benthic algae are of concern. Due to the lack of literature data, building SSD on benthic microalgae is difficult. This paper aims in comparing SSDs, and thus protective thresholds (hazardous concentration that affects 5% of the species of a community, HC5), built on ecotoxicological data obtained (1) from literature and (2) with specific bioassays on benthic diatoms from a lake. Thresholds were derived for protection against four herbicides separately and for a mixture of them. Sensitivity data obtained from literature were statistically lower than the specific data for all herbicides: Species tested in the literature were usually more sensitive (mainly chlorophytes), leading to more protective lower HC5. The HC5 thresholds (literature or specific) derived for protection against the mixture were also compared to the observed sensitivity of an assemblage of benthic diatom species exposed to an increasing range of herbicide mixture concentrations. We observed that one species within the assemblage (Fragilaria rumpens) was affected at a concentration below both the literature and the specific HC5 thresholds.

Amphipod (or amphipoda) An order of malacostracan crustaceans. General characteristics include no carapace, laterally compressed body, different forms of appendages, size range from 1 to 340 mm in length (most are less than 10 mm), and most are aquatic detritivores (scavengers). Areas of concern Areas in the (Laurentian) Great Lakes that were identified in the 1970s by the International Joint Commission as having a severely degraded aquatic environment (Grapentine 2009). Bioavailable The portion of a chemical that is available for uptake by an aquatic organism and reaches the site(s) of toxic action, where it exerts a toxic effect; tissue concentrations of a chemical are generally used as a surrogate measure of bioavailable chemical, as it is not usually feasible to measure the concentration of the chemical at the actual site of toxic action. Bioaccumulation The process by which chemicals are taken up by aquatic organisms directly from water as well as through exposure through other routes, such as consumption of food and sediment containing the chemicals (Rand 1995). Critical body concentrations Body concentrations of a contaminant (or contaminants) measured in test organisms that are associated with observed toxicity. Direct toxicity Toxicity that results from the toxic agent(s) acting directly at the site(s) of toxic action in and/or on the exposed organisms that are exhibiting the adverse biological response in question (Rand 1995). Dissolved oxygen The amount of oxygen (O 2 ) dissolved in water, commonly measured as milligrams of O 2 per liter (mg/L), millimoles of O 2 per liter (mmol/L), or percent saturation. Ecology A branch of biology dealing with the relations between organisms and their environment (Random House College dictionary). Ecosystem A biological community and its chemical/physical environment. Ecotoxicology The study of the impact of toxic chemicals on biological organisms (populations, communities, and ecosystems). Flow-through An exposure system for aquatic toxicity tests in which the test material solutions and control water flow into and out of test chambers on a oncethrough basis either intermittently or continuously (Rand 1995). In situ (exposures) Exposure of a defined population of test organisms in confined chambers in the field, under natural or near-natural conditions, followed by measurement of typical toxicity or bioaccumulation test end points. In situ exposures possess more realism than laboratory tests but more control than field studies (Chappie and Burton 2000). In vivo (tests) Tests using whole, living organisms (as opposed to in vitro tests), which are conducted on organs, tissues, cells, etc. Indirect toxicity Adverse effects or toxicity that results from the toxic agent(s) acting on and producing changes in the chemical, physical, and/or biological M 672 Macroinvertebrate Ecotoxicity Testing (MET) Abbreviations AOCs Areas of concern ASTM American Society for Testing and Materials CCME Canadian Council of Ministers of the Environment EC Environment Canada EEM Environmenta...