Comparison of methods to obtain a liquid phase in marine sediment toxicity bioassays with Paracentrotus lividus sea urchin embryos.

Sediment toxicity bioassays using planktonic organisms are widespread tools in environmental quality assessment, but they have limitations because the method for extracting and diluting the liquid phase may affect the final toxicity. The present study compares the toxicity on sea urchin embryos and larvae of elutriates and pore water obtained from intertidal sediments by various methods. The effects of mixing method, dilution protocol, aeration, and pH adjustment of the pore water are discussed. For a quantitative assessment of toxicity, a posteriori dilution of the elutriate yielded results comparable to mixing the equivalent proportions of sediment and sea water. The highest sensitivity was found by using pore water. Rotator mixing in air-tight closed flasks was a second-best choice. In both cases the geochemical parameters of the liquid phases obtained were not suitable for the incubation of water-column organisms. To avoid the environmentally irrelevant toxicity caused by these natural conditions typical of highly reduced sediments, gentle aeration or dilution with O2-saturated control sea water is necessary prior to testing, despite the unavoidable changes in redox conditions. Measuring length of 48-h-old larvae improves the sensitivity and the discriminating power of the test.