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TECHNICAL REPORT
Ecological Risk Assessment

Combination Effect of Light and Toxicity in Algal Tests

^a Department of General Biology, Aachen University of Technology, Kopernikusstraße 16, D-52056 Aachen, Germany
^b Department of Chemical Ecotoxicology, UFZ Centre for Environmental Research, Permoserstraße 15, D-04318 Leipzig, Germany
^c Aachen University of Technology, Worringerweg 1, D-52056 Aachen, Germany

* Corresponding author (cleuvers{at}bio2.rwth-aachen.de)

Received for publication April 2, 2001. The sensitivity of Scenedesmus subspicatus against potassium dichromate is positively correlated to the photon flux density during the algal growth inhibition test. Low photon flux densities led to significantly reduced maximum effects and higher EC₅₀ levels. To improve the testing of colored substances, we distinguished between the toxic effect (chemical part, represented by potassium dichromate) and the shading effect (physical part, simulated by reduced light intensities during the test) of a hypothetical light absorbing substance. The contribution of these single effects to the total inhibition varied greatly. At high concentrations of potassium dichromate (1.6 and 3.2 mg L^-1) the physical part never exceeded 25% of the total inhibition, not even at strongest light reduction, while at low concentrations (0.2 and 0.4 mg L^-1) the physical effect became more prominent when halving the amount of available light. Further, the combination effect of the chemical and the physical effect could be calculated well only by using the concept of independent action. Thus, if chemical and physical effects are measured in combination, as is the case in tests with dyestuffs, the current test protocol for the algal growth inhibition test may lead to incorrect estimations of the toxic potential.

Abbreviations: EC, effect concentration • ETAD, Ecological and Toxicological Association of Dyes and Organic Pigments Manufactures • IPQ, index of prediction quality