Porewater as a matrix in toxicity bioassays with sea urchins and bivalves: evaluation of applicability to the Venice lagoon (Italy). Porewater plays an important role in sediment toxicity assessment using bioassays, but the most reliable extracting method and the potential contribution of confounding factors to the real toxicity need to be studied. The applicability of bioassays with the early life stages of Paracentrotus lividus, Mytilus galloprovincialis and Crassostrea gigas on porewaters extracted by centrifugation from the Venice Lagoon (Italy) is evaluated and demonstrated: toxicity tests can discriminate the toxicity of porewaters from sites with different kinds and levels of pollution and, using toxicity scores, data are classified in five toxicity classes. Sulphides do not represent a confounding factor in porewater toxicity; in contrast ammonia exhibited some concentrations above the toxicity threshold for sea urchin embryos.