Application of postexposure feeding depression bioassays with Daphnia magna for assessment of toxic effluents in rivers.

A bioassay that used postexposure feeding depression in Daphnia magna Straus as an endpoint previously had been developed under laboratory conditions. Laboratory studies revealed that this was a sensitive, robust endpoint, which could potentially be used in an in situ bioassay. This study adapted the laboratory bioassay for use in the field and deployed D. magna in situ at four known or suspected contaminated and reference sites. The bioassay was demonstrated to be reliable for use in the field because more than 90% of test organisms were recovered live from the test chambers after exposure allowing feeding rates to be measured after exposure. At each of the contaminated sites, significant depressions in postexposure feeding rates were recorded. Although depressions in postexposure feeding rates were apparent at all contaminated sites, with the exception of Langholm, no impacts were detected on the benthic macroinvertebrate community, when using the Biological Monitoring Working Party scoring system. This demonstrated that during this study, post-exposure feeding depression was a more reliable and sensitive endpoint to use to detect toxicity than were changes in community structure. Therefore, the postexposure feeding depression bioassay can offer a sensitive, robust, ecologically relevant diagnostic endpoint for use in water-quality assessment schemes.