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Characterizing regional temperature regimes of NW Mediterranean coastal waters: a step forward to better understand the responses of benthic communities to current climate change

Bensoussan Nathaniel

The North-Western Mediterranean area has been affected by two unprecedented severe large scale mortality outbreaks of benthic invertebrate species during the last decade (summers 1999 and 2003). Anomalous high temperature conditions were evoked as the main triggering factor of the mortality outbreaks. Bearing in mind the recent climate change scenarios and the warming trend in the NW Mediterranean, new temperature anomalies may likely occur in the next decades. This context raises concerns about the conservation of the rich Mediterranean biodiversity. To better understand the potential effects of climate change in the sublittoral benthic communities, high resolution temperature series are necessary to obtain baseline data and characterize temperature anomalies along regional and depth gradients.
Here, we present pluri annual (since 1999 for the oldest, till 2006) hourly temperature time series recorded every 5m from 5 to 40 m depth in four NW Mediterranean littoral regions (from W to E): Medes islands (N Spain), Riou island (Marseilles, France), Port-Cros island (National Park, Var, France) and Scandola natural reserve (Galeria, Corsica, France). These regions suffered different degree of impact during the mass mortality events. Data analysis revealed clear-cut inter regional thermal patterns both in magnitude and variability, stressing the importance of regional meteorological conditions. From July to September, differences up to 4°C in mean temperatures were noted in the upper 15 m between Scandola (warmest) and Riou (coldest) while Medes exhibited the warmest conditions below 25 m. During the same period, variability (CV) was the highest in Riou in the whole water column while it was particularly high in Scandola and Port-Cros around the position of the thermocline (20-25 m) and below 30 m in Medes. Inter-annual variations never resulted in a significant shift of the detected patterns except during the 1999 and 2003 temperature anomalies.