Many fish were found dead along the edge of the Berre pond in Saint-Chamas, Bouches-du-Rhône, after being exposed to water with very low oxygen levels. The Gipreb mixed syndicate, which manages the Berre pond, explained that the fish were affected by oxygen-depleted water and sought to provide a scientific explanation to reassure the public. The event, while alarming, is part of a natural cycle that occurs in the pond during the summer months. During the summer, high temperatures and sunny conditions create ideal conditions for phytoplankton—tiny floating plants—to flourish. These organisms grow rapidly in warm, nutrient-rich water. However, once the phytoplankton die, they sink to the bottom of the pond. As bacteria break down this organic matter, they consume the available oxygen, creating a low-oxygen environment. In this oxygen-deprived setting, other types of bacteria that do not need oxygen take over. These bacteria release acids into the water, making it more acidic and less hospitable for aquatic life, especially bottom-dwelling species like worms, shellfish, and fish. Wind plays a crucial role in reversing this process. Strong winds can help move organic-laden water toward the sea and mix the pond’s waters, restoring oxygen levels and cooling the water. A recent windy period on Wednesday and Thursday did stir the pond, but it also triggered an upwelling—a phenomenon where deeper, acidic water rises to the surface. This sudden change in water conditions caught the fish off guard. They were unable to escape to more suitable areas in time and died due to the sudden increase in water acidity. While the fish mortality was significant, the syndicate noted that the event had a positive effect on the pond. It helped remove a large amount of accumulated organic material from the summer, transporting it out to the sea. The overall water quality remains unaffected, though authorities have advised against swimming in areas where dead fish are washing up on the beach. Additionally, EDF, the energy company operating turbines in the area, has postponed resuming operations due to the pond’s current critical condition.