[EN] Shellfish toxin data from 11 years and Alexandrium tamarense abundance during 6 of those years are analysed. Comparison of PSP toxicity in Mytilus edulis with PSP toxicity in Mya arenaria shows a significant correlation (r2=0.61), with M. edulis being five times more toxic. The results support using M. edulis as a sentinel species for shellfish toxicity in the St. Lawrence region. High interannual variability was found in the PSP and A. tamarense data, but no trends were manifest. Correlation analysis revealed clearly defined geographical station groups. These groups were characterised by seasonal distributions and outbreak times, and were associated with the surface seawater circulation. The results indicate blooms spreading from the open Gulf, upstream toward the Estuary. Since the spring bloom also spreads upstream, we hypothesise that the A. tamarense bloom is just one of the steps in the traditional phytoplankton succession diatoms-dinoflagellates-small flagellates, which is controlled by the classic oceanographic processes of nutrient depletion and water column stratification. A. tamarense distribution over the entire sampled area was similar to the PSP toxicity in M. edulis. In 1993, 59% of PSP variability was explained by A. tamarense. Furthermore, mussel toxin rises with increasing A. tamarense concentration. Shellfish contamination starts as soon as A. tamarense is present in the water. In addition, detoxification starts when A. tamarense decreases below the level of detection. However, the most important finding of this study is that only 1000 A. tamarense cells per litre are sufficient to raise the toxin in the mussels to the level (80 µg STX eq/100 g tissue) at which the closure of shellfish harvesting activities is obligatory.[ES] Se han analizado once años de datos de toxinas de moluscos y seis años de abundancia de Alexandrium tamarense. Se ha comparado la toxicidad pos PSO entre Mytilus edulis y Mya arenaria. El mejillón es cinco veces más tóxico.Peer reviewed