Thermal anomaly in the waters of the Gulf of Marseilles during summer 1999. A partial explanation of the mortality of certain fixed invertebrates?

During summer 1999, a massive mortality of attached benthic invertebrates was reported at several points along the French Mediterranean coast, east of the Rhone River. Such a wide geographical extension suggested the occurrence of a climatic anomaly as a direct or an indirect cause of this phenomenon. The systematic observations of surface rater temperature (1 record/30 min) and vertical thermal structure from 0 to 55 m (semi-monthly frequency) performed by the service of observation of centre d'oceanologie de Marseille since 1994 in the Gulf of Marseilles (northwestern Mediterranean) compared with historical data obtained for the summer periods (Dune till October) indicates a thermal anomaly during summer 1999. Although the surface temperatures do not strictly shore- exceptional values, the stability during long periods of high temperatures (24.1 C plus or minus 0.5 in September, n = 1 018) and the thermal structure of the water column are remarkable. The vertical temperature profiles of 1999 reflect a general warming of 2-3 C in the water layer overlying the thermocline, accompanied by its deepening (Lip to 30-40 m). This situation can be explained by the scarcity of NW winds with a marked drop of their relative frequency during July and October 1999 (13-27 versus 42 %, mean value calculated from the previous 12 years) and the short duration of these windy periods (between 30 min and 5 h as a mean). Parallel to this, a large number of no wind registrations (125-250 h/month as a mean) were observed during this period. A similar thermal stratification, of lesser amplitude, associated with comparable meteorological conditions was also observed in 1997. The hypothesis that this summer 1999 temperature anomaly could constitute a partial explanation for the recorded massive mortality is discussed.Original Abstract: Durant l'ete 1999, une mortalite massive d'invertebres benthiques fixes a ete observee en de nombreux points de la cote mediterraneenne francaise a l'est du Rhone. Une telle extension geographique suggerait l'existence d'une anomalie climatique a l'origine directe ou indirecte du phenomene. Les mesures systematiques de la temperature de l'eau de surface (1 mesure/30 min) et de la structure thermique verticale de 0 a 55 m (a frequence bimensuelle) menees depuis 1994 par le Service d'observation du centre d'oceanologie de Marseille, comparees aux donnees historiques pour les periodes estivales (juin a octobre), mettent en evidence une anomalie thermiq