Physiological recovery from episodic acid stress does not mean population recovery of Gammarus fossarum

The physiological responses of the acid-sensitive amphipod Gammarus fossarum exposed in situ to acid stress (pH 4.5 and 5.5) and then transferred back to neutral water were investigated. Survival rate and haemolymph [Cl−] and [Na+] were assessed after 24, 48 and 72 h of exposure in acidic streams and after a recovery period of 12, 24, 36, 48 and 60 h. After 24 h, exposure to slightly acidic (pH 5.5) and strongly acidic water (pH 4.5) led to a severe and significant depletion in haemolymph [Na+] and [Cl−] compared to organisms exposed in circumneutral water (pH 7.3). However, after only a 12 h-period of transfer back in neutral water and whatever the previous exposure time (24, 48 and 72 h) in both slightly and strongly acidic water, haemolymph [Na+] and [Cl−] were equal or superior to the control level without associated mortality. In spite of this fast physiological recovery capacity, populations of G. fossarum living in streams undergoing episodic acid stresses were drastically affected thus, demonstrating the high acid-sensitivity of this species. We discuss the possible reasons of population regression and the absence of population recovery.