Compounds with different modes of action may affect life cycles of biota differently. The aim of the present study was therefore
to investigate the impact of four chemicals with different modes of action, including the essential metal copper, the nonessential
metal cadmium, the organometal tributyltin, and the polycyclic aromatic compound phenanthrene, on chronic lethal and sublethal
life-cycle effect parameters of the nonbiting midge Chironomus riparius, applying a 28-day sediment toxicity test. Tributyltin
and cadmium delayed emergence significantly over a wide range of sublethal concentrations, while this range was narrow for
copper and almost absent for phenanthrene. The chronic LC50/LOECEmT50 ratio, expressing these differences, amounted to 1.5,
3.5, 12.0, and 18.2 for respectively phenanthrene, copper, cadmium, and tributyltin. Thus the more specific the compounds
mode of action, the higher the chronic LC50/LOECEmT50 ratio, as previously observed for acute-to-chronic ratios (ACRs). Comparison
of our results with literature derived LC50/LOEC ratios showed a comparable trend and a lower variability compared to ACRs.
We therefore conclude that the presently proposed chronic ratio is indicative for the specificity of a chemicals mode of action
and that it is less variable than the ACR.