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Toxicity to aquatic algae and cyanobacteria

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Description of key information

The toxicity of sodium tripolyphosphate to Scenedesmus subspicatus was assessed by Herschke and Cellier (1983). The EC50 after 90 hours of exposure was found to be 160 mg/l and 69.2 mg/l, for the growth rate and the biomass respectively. The criteria of validity for the growth rate, set out in the OECD Draft Guideline 202 (July 2001), were fulfilled: the coefficient of variation for the daily growth rates in the controls was less than 35% and the coefficient of variation for the average growth in replicate controls was less than 15%. Therefore only the EC50 based on the growth rate is considered. No analysis of the substance into the test medium was performed. The study was in accordance with international standard methods (ISO/TC 147/SC 5/WG 5N 84). 
A study with the marine species Skeletonema costatum was carried out by the same authors (Herschke and Cellier, 1983). From 10 mg/l to 320 mg/l of sodium tripolyphosphate and after an exposure time of 6 days, the product stimulated the growth by comparison with the controls, whereas at 900 mg/l it was slightly inhibitory. No analysis of the substance into the test medium was performed. The algae growth was very low in the controls (less than a factor 16 in 144 hours) and the addition of sodium tripolyphosphate at 10 and 32 mg/l led to a steady increase of the algae growth indicating that the test medium is deficient in phosphates. With 100 mg/l and 320 mg/l the growth decreased regularly but at the end of the test it was still higher than in the controls. It is suspected that the positive effect on the algae growth due to the o-phosphate formed by hydrolysis of sodium tripolyphosphate was counterbalanced by the complexation of metals like Mg++, Cu++, Zn++, B3+ … which are essential to the algae growth. Due to the low growth of controls and the opposite direct and indirect effect of sodium tripolyphosphate on the algae medium, this study cannot be used for derivation of a PNEC in the risk assessment.

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Additional information

It is suspected that the algae growth inhibition observed at 32 mg/l and above was probably due to complexing effects between the sodium tripolyphosphate and some essential elements of the medium. This nutrient deficiency may happen on a laboratory scale but will not be relevant in the environment. So this result cannot be used for the PNEC calculation.

Such nutrient depleting effects which may have no relevance in the real environment have been shown for a number of complexing agents like tetrasodium ethylenediaminetetraacetate (Na4EDTA), phosphonates and NTA. Hence for instance, the Authorities decided not to take into account the results obtained on algae for the classification and for the PNEC calculation of Na4EDTA. Accordingly, the results obtained from the algal toxicity test of STPP will not be used for the PNEC derivation.