In this study, we assessed the population-level impacts of 4-nonylphenol (NP) on medaka fish based on a population matrix model developed and results of the medaka extended one-generation test (MEOGRT). The concentration-response relationships for the number of fertilized egg and the survival rate were estimated from results of the MEOGRT and were incorporated into the population model. Although the lowest observed effect concentration (LOEC) in the MEOGRT was reported to be 1.27 μg/L NP, the predicted population growth rates at NOEC and even at the maximum test concentration of 89.4 μg/L NP were higher than 1 (358 and 6.2, respectively). We also projected values of mean time to extinction at different concentration levels and corresponding reductions in carrying capacity (K). Based on the comparison of the projected population-level impacts with quantitative criteria used in fishery resource management and IUCN red list categorization, concentrations higher than NOEC can be acceptable. Overall, those results suggest that use of multiple criteria based on individual and population-level impacts rather than a limited number of criteria (such as LOEC and NOEC based on individual-level effects) would be important in ecological risk assessments.

In this study, an OJIP-test of two marine diatom species, Thalassiosira pseudonana (single-celled species) and Skeletonema marinoi-dohrnii complex (chain-forming species), were exposed to EC10 and EC50 levels of diuron and tributyltin (TBT) for 72 h. Increased tendency of relative fluorescence intensity (Ft/Fo value) at 300 μs and J step (2 ms) and a significantly (P < 0.05) increased Mo value (initial slope of fluorescence curve) were observed during 72 of diuron exposure in both species, whereas TBT did not increase those. These results strongly suggest that diuron efficiently blocked photosystem II-catalyzed photosynthetic electron transport at the secondary electron acceptor Q_B, which is original biological activity of diuron in diatoms. However, some parameters (e.g., ϕ_Po and ϕ_Eo, Fv/Fo and PI_ABS,) were significantly (P < 0.05) decreased by TBT treatment. Although the mechanism responsible for the decrease is not clear, relatively severe reductions in levels of Fv/Fo (an indicator of water-splitting activity) by TBT treatment than diuron treatment suggest that TBT inhibits photosynthetic function via inhibition of photosynthetic oxygen evolving systems, a different mechanism than that of diuron. Moreover, present study suggested that PI_ABS derived from OJIP-test is a high sensitive biological marker for detecting the toxic effect of pollutants which inhibit photosynthetic function.