In situ bioassays with daphnids currently employ lethality as an endpoint, and although sublethal responses (reproduction and feeding rate) can be measured in the field, such endpoints pose major practical challenges. Previous studies have indicated that Daphnia magna exposed to toxic substances can exhibit delayed recovery in feeding behavior (postexposure feeding depression). This simple, robust response has the potential to be an ecologically relevant and potentially diagnostic endpoint. This study developed and tested the use of postexposure feeding depression as a toxicity endpoint in the laboratory environment. First, replicate numbers were manipulated to produce statistically reliable results. Second, postexposure feeding depression in D. magna was studied under laboratory conditions, by employing toxic substances with differing modes of action. Although most substances caused feeding inhibition during direct exposure, not all substances produced postexposure feeding depression. However, the use of lethality as a supplementary endpoint provided an alternative measure when no feeding depression was apparent after exposure. In combination, these endpoints offer a potentially more sensitive, ecologically relevant alternative to the use of lethality alone for in situ bioassay studies.

No abstract

A fields study was conducted to validateconcentrations of zinc in feshwater sediments that are tolerated by benthic macroinvertebrate communities and to determine whether a relationship exists with the acid volatile sulfide (AVS)–simultaneously extracted metal (SEM) model. In both the lake and riverine systems, one sediment type was high in AVS and one low in AVS, which resulted in zinc‐spiked sediments that ranged from low to high SEM to AVS ratios. The colonization trays were sampled seasonally, ranging from 6 to 37 weeks of exposure, and were evaluated using several appropriate benthic indices. Results of the field evaluations at the four test sites confirmed the validity of the AVS‐SEM model, predicting benthic macroinvertebrate effects correctly 92% of the time. In sediments where the SEM to AVS ratio or the AVS and organic (OC)–normalized fractions exceeded 8 and 583 μmol/g of OC, toxicity was observed from the zinc‐spiked sediments. Conversely, when the SEM to AVS ratio or OC‐normalized AVS fractions were less than 2 or 100 μmol/g of OC, no toxicity was observed. In the range of 148 to 154 μmol/g of OC, toxicity varied in two treatments. Total zinc concentrations in sediments showed no relationship to benthic effects. The most impaired benthic community occurred in the high‐gradient stream sediments, which had low OC and AVS concentrations and SEM to AVS ratios of 33 and 44 in the spiked sediments. Five to six benthic metrics were depressed at SEM to AVS ratios of 8.32 and 9.73. The no‐observed‐effect level appeared to be near a SEM to AVS ratio of 2, with slight to no effects between ratios of 2.34 and 2.94. No sites with ratios of less than 2 showed any adverse effects.

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.

In situ bioassays with daphnids currently employ lethality as an endpoint, and although sublethal responses (reproduction and feeding rate) can be measured in the field, such endpoints pose major practical challenges. Previous studies have indicated that Daphnia magna exposed to toxic substances can exhibit delayed recovery in feeding behavior (postexposure feeding depression). This simple, robust response has the potential to be an ecologically relevant and potentially diagnostic endpoint. This study developed and tested the use of postexposure feeding depression as a toxicity endpoint in the laboratory environment. First, replicate numbers were manipulated to produce statistically reliable results. Second, postexposure feeding depression in D. magna was studied under laboratory conditions, by employing toxic substances with differing modes of action. Although most substances caused feeding inhibition during direct exposure, not all substances produced postexposure feeding depression. However, the use of lethality as a supplementary endpoint provided an alternative measure when no feeding depression was apparent after exposure. In combination, these endpoints offer a potentially more sensitive, ecologically relevant alternative to the use of lethality alone for in situ bioassay studies.

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.