This benchmark is the lowest test EC20 (20% effects concentration) values for daphnids. It represents the highest tested concentration not causing a reduction of as much as 20% in the reproductive output of female test organisms.
Suter, G.W. II. 1996. Toxicological benchmarks for screening contaminants of potential concern for effects on freshwater biota. Environ. Toxic. Chem. 15:1232-1241.