Ecological Systems and Devices Annotation << Back Two-Species Test System in Assessing the Toxicity of Substances Ipatova V.I. The infl uence of light aluminum metal in the form of aluminum chloride was studied both on monocultures and on a twospecies test system consisting of representatives of the same trophic level – chlorococcal microalgae Scenedesmus quadricauda and Monoraphidium arcuatum. Their interactions were studied in a mixed culture at different levels of exposure to aluminum chloride in the concentration range of 0.1–100 mg/l in chronic experiments lasting 30 days. A two-species toxicity test showed that the growth rates of the laboratory population of each species were different from those in tests on individual species, both in control variants (without toxicants) and during intoxication. In a mixed culture, at low to moderate levels of aluminum chloride exposure (0.1–50 mg/l), M. arcuatum is more susceptible to competitive displacement by S. quadricauda, and at high levels of exposure (100 mg/l), M. arcuatum becomes more resistant to competitive displacement over a long period of time. The fi ndings suggest that single-species bioassays in standard biological assays may underestimate the toxicity of metals in natural waters. A new two-species test system with an equal seed titer of cells has been developed based on two species of microalgae sensitive to toxicants, similar in cell surface area and meeting all the necessary requirements for test objects, for a more adequate assessment of the toxicity of substances polluting fresh water bodies. Keywords: mixed culture, Scenedesmus quadricauda, Monoraphidium arcuatum, biotic interactions, bioassay, aluminum chloride. Pp. 48-60.