Effect of food concentration on the chronic toxicity of sodium dodecyl sulphate to Daphnia magna

Food concentration supplied during subchronic and chronic toxicity tests, is one of the most important factors that might influence the response of test organisms to toxicants. The green microalga Scenedesmus incrassatulus was used as food for the cladoceran Daphnia magna, in a chronic toxicity test with the toxicant sodium dodecyl sulphate (SDS). Test concentrations were 0.625, 1.25, and 2.5 mg l^-1 of SDS, equivalent to 1/40, 1/20, and 1/10 of the average 48-h LC50 previously determined. Food concentrations were 9.5, 19, and 38 mg l^-1 (dry wt.). Survival and reproduction were recorded, and the data were analyzed using a life-table approach. After 55 days, the main findings were as follows:

1.	Average clutch size decreased as food concentration increased and there was a negative interaction between the toxicant and the algal concentration.
2.	The Net Reproductive Rate (R 0) of the controls fed the lowest food concentration was approximately 10 times higher than the controls fed the highest food concentration.
3.	R 0's for the treatments with SDS fed the lowest food concentration, were 2.5 to 5 times higher than those fed the highest food concentration.
4.	Reproduction in controls and treatments with the same SDS concentrations, became similar as food level increased.

According to these results, the food concentration to be supplied in chronic toxicity tests should be accurate, as an incorrect food concentration might negatively affect the survival and reproduction of test organisms, thus masking or confounding the effects of the toxicant being examined. A concentration around 10 mg l^-1 (dry wt.) of S. incrassatulus seems to be appropriate for D. magna economic toxicity tests.