Life history responses of Daphnia similoides simultaneously exposed to microcystin-LR and ammonia and their postexposure recovery

Aquatic organisms often suffer episodic stress from cyanobacterial bloom-derived concomitants and sometimes interactive stressors, and they may recover when stressors terminate. To assess whether exposed Daphnia can quickly recover from combined toxicity of bloom-derived stressors, Daphnia similoides were exposed to mixtures of ammonia (0mgL^-1, 0.37mgL^-1, and 0.58mgL^-1) and dissolved microcystin-LR (0μgL^-1, 10μgL^-1, 30μgL^-1, and 100μgL^-1) in a full factorial design for 14 d and subsequently allowed to recover for 7 d. During the exposure, the mixtures of ammonia and microcystin-LR showed different effects on the selected variables. Ammonia delayed the time to maturation, but microcystin-LR was not negative to the development of the tested daphnids. Furthermore, microcystin-LR reduced the prolonged developmental time to first eggs and first clutch caused by high ammonia. The total reproduction of D. similoides was dramatically reduced by 64% to 79% by both toxicants and their combinations, but there were no interactive effects. After the recovery period, organisms under most treatments did not recover completely from the combined stress but showed positive signs of recovery, based on the size of the last clutch and the mean number of clutches during recovery; thus the affected D. similoides have the potential to quickly recover from the combined toxic stressors.