When laboratory ecotoxicity assays are employed to assess the quality of fluvial environments, the main goal is to verify how healthy the organisms could be and to identify potentially toxic areas. The traditional mechanism used in laboratory ecotoxicity assays is to expose organisms to samples taken from a specific point of the river and to measure a given response. This exposure system assumes that organisms are passive uptakers of contaminants with no ability to escape from contamination as the contact with the sample (and possible contaminants present in it) is forced and mandatory. Under real situation, such condition is not expected to occur for mobile organisms as it is expected that they move to different areas where contamination levels could be distinct. When this occurs, responses at individual (or lower) level fail in predicting potential risk at higher biological organization such as ecosystem. Although the avoidance by organisms of a disturbed area is a protection mechanismthatpreventstoxic effect at individual level, since an ecosystems level the effects can be quite severe as organisms partially or totally disappeared.By the forced exposure approach is possible to obtain a direct concentration-response relationship, but it presentstwo disadvantages: (i) it only provides information of potential toxicity directly related to individual level (or sub-individual: proteins, gens, cell, tissues), therefore statements about the biological effects at ecosystem level is prevented or hard to be achieved and (ii) it does not consider the ability of the organism to detect and avoid contaminants, nor as contaminants affect the spatial distribution of the organisms.If laboratory ecotoxicity assays are performed with mobile organisms, for a better understanding about the biological effects at ecosystems level, organisms should be exposed simultaneously to different areas and not exclusively to a specific sampled sample. Recently, amulti-compartmented, nor-forced exposure system that allows testing many samples taken along the ecosystems to be studiedhas been developed. In that approach organisms are equally distributed in the system containing different samples (concentrations), whereby they can freely move, and at the end it is possible to check the preferred and avoided areas. This non-forced exposure approach provides information about how contamination can affect the habitat selection patterns by organismsand recolonization processes. The non-forced exposuresystem can also be used to assess effects of a contamination gradient (for specific regions of the river) or of heterogeneously disturbed ecosystems (where contamination patches are formed due to the presence of several discharges along the river).By using the first condition (contamination gradient) it is possible to predict the preferential spatial distribution of organisms along a gradient and to hypothesize from which point populations are not expected to be found. However, by simulating a patchy contamination it is possible to identify less habitable areas and the presence of chemical barriers due to contamination.

The present work brings a novel tool for ecotoxicity assays in which organisms can beexposed to different river samplesand can freely move among them, including therefore the concept of habitat selection to ecotoxicological studies.In addition, some results of preliminary studies will be presented and discussed. Finally, a brief discussion about how non-forced system could help predicting the effects of contamination linked to other environmental disturbance factors, such as habitat fragmentation due to presence of reservoirs, will be presented. The recent experience with the non-forced exposure system have shown that this approach can provide important information about the role of the contaminants as habitat disturbers even when individual toxic effects are not detected.

Trabajo presentado en la NET SCARCE International Conference (Rivers under water scarcity: Threats and Challenges), celebrada en Barcelona el 15 y 16 de noviembre de 2016.

CVM Araújo thanks Spanish Ministry of Economy and Competitiveness for the Juan de la Cierva contract (IJCI-2014-19318).

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