() The role of field-based effects measures in the ecological risk assessment of complex effluents in rivers.

Culp, J1, Baird, D1, Cash, K2, Glozier, N2, 1 National Water Research Institute (Environment Canada), Fredericton, NB, Canada2 Prairie and Northern Wildlife Research Centre, Saskatoon, SK, Canada

ABSTRACT- Ecological assessment of pollution in large rivers is complicated because these ecosystems receive multiple, interacting effluent discharges. Confounding factors, such as complicated mixing hydraulics, can result in equivocal field data that lend weak inference to ecological risk assessments. The use of field-based assessment measures such as in situ bioassays and streamside mesocosms can help tease apart these confounding factors and contribute to multiple-stressor assessment in a weight-of-evidence approach. Although it is considered that these approaches may be too expensive and time demanding for inclusion in regulatory assessment regimes, their use can add considerable insight into the mechanisms underlying adverse changes within aquatic food webs. By linking field observations at reference and impact sites through weight-of-evidence postulates it is possible to describe a conceptual model of community changes across stressor gradients. We illustrate the use of these approaches with case studies of anthropogenic discharges in which the use of such field-based effects measures has been crucial in identifying and separating causal factors to permit appropriate environmental management actions.

Key words: mesocosm, In situ bioassay, multiple stressor

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