For fish, daphnids, and algae, acute to chronic ratios (ACRs) have been determined from experimental data regarding new and existing chemicals. Only test results in accord with the European Union Technical Guidance Document (TGD) and validated by authorities were considered. Whereas the median ACRs of 10.5 (fish), 7.0 (daphnids), and 5.4 (algae) are well below the ACR safety factor of 100 as implied by the TGD, individual ACRs vary considerably and go up to 4400. The results suggest that a safety factor of 100 is not protective for all chemicals and trophic levels. Neither a correlation between ACR and baseline toxicity as modeled through the logarithmic octanol-water partition coefficient nor an ACR correlation across trophic levels exists. Narcosis is associated with a preference for a low ACR; nevertheless, low ACRs are frequently obtained for nonnarcotics. Analysis of chemical structures led to the derivation of structural alerts to identify compounds with a significantly increased potential for a high ACR, which may prove to be useful in setting test priorities. At present, however, life-cycle tests are the only way to conservatively predict long-term toxicity.

Purpose Standardized sediment toxicity assays often employ periodic additions of uncontaminated food to sustain energy and growth requirements of the test organisms. Consequently, selective feeding on this uncontaminated food may reduce exposure to sediment particles containing the test substance. To address this issue, some standard guidelines propose to add food to the sediment before spiking with the test substance to account for multiple exposure routes, including ingestion of contaminated food. The present study focused on the influence of different feeding regimens and compositions of the aqueous medium on water quality (ammonia concentrations) and test organism development. Materials and methods Lumbriculus variegatus, Hyalella azteca, and Chironomus riparius larvae were used as test organisms. Ammonia production was investigated under different feeding regimens and test conditions as well as under the presence of a potential inhibitor of nitrification, 4,4'-methylenedianiline. Results and discussion Ammonia concentrations strongly depended on the feeding regimen and on the type of food. An influence of 4,4'-methylenedianiline on test organisms or ammonia concentrations was not found at the tested level. Independently of ammonia concentrations, L. variegatus were more sensitive to food type and less sensitive to medium composition than H. azteca. Ammonia levels, emergence ratio, and development rate of C. riparius were not different under periodic feeding and single addition of Urtica. Findings suggest that in case of a "sediment-incorporated" feeding regimen, the time point of food addition to the sediment and careful pH control appear critical. For H. azteca and L. variegatus, an Urtica/cellulose mixture provides a balance between ammonia production and organism development. Conclusions This article gives recommendations and caveats for conducting spiked-sediment tests with a single addition of sediment-incorporated food. The presented work also contributed to the development of two recently adopted OECD test guidelines related to sediment toxicity and bioaccumulation testing with endobenthic oligochaetes.

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