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In the present study, the ecological status of four major South Korean rivers flowing near industrial complexes is assessed using physico-chemical analyses and various descriptors based on periphytic diatom assemblages. Principal component analysis conducted with physical and chemical variables singled-out four sites as more severely impacted (higher concentration of metals and total nitrogen, higher conductivity and biological oxygen demand), while the remaining 12 sites indicated less impacted conditions, although showing nutrient enrichment. Less-contaminated sites based on physico-chemical properties showed higher cell densities, higher richness and diversity, a larger proportion of live and healthy-looking diatoms, lower production of lipid bodies (in terms of number and size) and fewer teratologies than more-contaminated sites. Non-taxonomical metrics (cell heath status, cell size, lipid bodies and valve deformities) were in good agreement with traditional taxonomical metrics (assemblage structure, richness, diversity). Overall, water quality assessment based on diatom assemblages and diatom-based metrics had a good fit with the available physico-chemical data and agreed on the most impacted sites. However, additional sites showed signs of degradation based on the diatom metrics used. This suggests that the use of biotic indicators provides useful complementary information on the health status at the selected sites.

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Highlights 30-An ecotoxicological assessment of natural waters based on a multi-organism trial was conducted 31 in four South Korean rivers 32 -The tested organisms showed distinct levels of performance in their response to natural waters 33 -A scoring system is proposed to integrate biological responses into an overall toxicity category 34 -This bioassay approach identified more sites as potentially degraded as did water chemistry 35 measurements alone 36 37Abstract 38 Despite their proven reliability for revealing "acceptable" degrees of toxicity in waste-39 and reclaimed waters, bioassays are rarely used to assess the toxicity of hazardous 40 contaminants present in natural waters. In this study, we used organisms from different 41 trophic levels to assess the toxicity of water samples collected from four different South 42 Korean rivers. The main objective was to develop a multi-descriptor index of toxicity for 43 undiluted river water. The responses of six test organisms 44 (Aliivibrio fischeri, Pseudokirchneriella subcapitata, Heterocypris incongruens, Moina m 45 acrocopa, Danio rerio, and Lemna minor) after laboratory exposure to water samples 46 were considered for this index, as well as the frequency of teratologies in diatom 47 assemblages. Each individual test was attributed a toxicity class and score (three levels; 48 no toxicity = 0, low toxicity = 1, confirmed toxicity = 2) based on the organism's 49 response after exposure and a total score was calculated. The proposed index also 50 considers the number of test organisms that received the highest toxicity score (value =2). 51An overall toxicity category was then attributed to the water sample based on those two 52 metrics: A = no toxicity, B = slight toxicity, C = moderate toxicity; D = toxicity, E = high 53 toxicity. The susceptibility of the test organisms varied greatly and the sensitivity of their 54 response also differed among bioassays. The combined responses of organisms from 55 different trophic levels and with different life strategies provided multi-level diagnostic 56 information about the intensity and the nature of contamination. 57 58