Contribution of climate variability to occurrences of anoxic crises ‘malaïgues’ in the Thau lagoon (southern France)

The Thau lagoon, located in southern France, suffers episodically in summer from anoxic crises known as ‘malaïgues’. Such crises mostly occur under warm conditions and low winds. In this paper we investigated effects of local weather conditions (air temperature, wind speed and precipitation over southern France), and two climate oscillations (the North Atlantic Oscillation and the El Niño-Southern Oscillation) on malaïgue occurrences based on 33-year data series and dates when malaïgues appeared. The study shows that the probability of occurrence of malaïgues increases with increasing temperature and decreasing winds in August both mostly associated to the high phase of an index of North Atlantic Oscillation. Malaïgues are found to be frequently preceded by increased precipitation in July which contributes to a larger nutrient input into the lagoon and to its eutrophication. It is also found that malaïgues coincide with the warm phase of an El Niño-Southern Oscillation-like oscillation and are most significantly associated to values of this oscillation during the preceding May. This result is used to propose a simple method for an early warning of appearance of malaïgues with a high degree of confidence.