Effects of storage temperature and duration on toxicity of sediments assessed by <i>Crassostrea gigas</i> oyster embryo bioassay

Abstract The effects of temperature and duration of storage on the toxicity of estuarine sediments were investigated with the Crassostrea gigas oyster embryo bioassay. Sediments ranging from unpolluted (controls) to extremely polluted with heavy metals (>100 ppm Hg, Cu, Zn, and Pb) and total hydrocarbons (>1,000 ppm) were collected from sites in southwest France and northern Spain. Control sediments were toxic only at the highest concentrations tested and after freezing in liquid nitrogen (−196°C). Polluted sediments significantly reduced the success of oyster embryogenesis. Analysis of variance showed that the effect of storage temperature on toxicity increased with the prolongation of storage. Prolonged storage of fresh (4°C) sediments resulted in a loss of toxicity, which was more rapid in the less-polluted sediments. Deep-frozen sediments (−196°C) were highly toxic regardless of origin and storage time, and because deep-freezing causes spurious toxicity in the control samples, it cannot be recommended for toxicological studies. In the context of the assessment of sediment toxicity by embryo–larval bioassays, fresh (4°C) storage is recommended when sediments need to be stored for no longer than a few days. The advisable duration of fresh storage to avoid false-negative results is directly related to the degree of toxicity. Should the sediments require prolonged storage, freezing at −20°C appears to be the best choice.