It is a well known fact that several biological mechanisms are controlled by a combination of both temperature value and exposure time. As a consequence, ecotoxicologists for instance are using LC48 or LC96. Similarly, a Degree Heating Weeks Index measured from satellite-derived surface temperatures has been recently defined to point out bleaching risk areas in the coral reefs of the Pacific Ocean. During Summer 1999, a mass mortality event of large sessile invertebrates living on rocky substrates and coralligenous assemblage has been recorded on both Ligurian and Provence coasts of Italy and France respectively. In the Marseilles-Port-Cros area, available data have shown that a significant increase of temperature (up to 5°C) has occurred between 0 and 40 meters during periods of at least one month (August or September). These abnormal high temperatures during such a long period could well cause physiological stresses in sessile invertebrates and led them to death, associated with opportunistic microorganisms (protozoans, fungi, etc.) proliferation. Ecophysiological experiments are strongly needed to determine the thermal preferendum of the impacted invertebrates and confirm this hypothesis. These experiments should preferably be performed in the field, using temperature-controlled chambers. Parallel to that, there is a need to define relevant parameters that need to be monitored in the coastal zone at an appropriate time-space scale (hours and tens/hundreds of meters) for better prediction of similar events.