Effects of Temperature, pH, Ammonia Nitrogen Content and Salinity on Coral （Acropora millepora ＆ Palythoa sp. ） Bleaching

摘要

This study was conducted to investigate the effects of temperature,pH,ammonia nitrogen content and salinity on the bleaching of Acropora millepora and Palythoa sp. This study included two experiments: Experiment 1 was designed to test the effects of different gradients of temperature,pH,ammonia nitrogen content and salinity at a time point( 6 h) on Palythoa sp. breeding; and Experiment 2 tested the effects of different levels of temperature,pH,ammonia nitrogen content and salinity at three time points( 24,48 and 72 h) on A. millepora bleaching. Before the experiments,A. millepora and Palythoa sp. were washed with filtered seawater( 0. 45 μm microporous membrane filtration) and then reared in interior seawater aquarium cylinder. The experiments were carried out when the coral tentacles extended and the growth condition were good. The results showed that under high temperature( 32-40 ℃),both A. millepora and Palythoa sp. showed drastic reactions in the course of time,reflecting considerably significant stress responses. The number of releasing zooxanthellae increased with increase or decreaseof the pH value( pH 0,4,6,10,12) over time. Meanwhile,a relatively thick layer of mucus was found on the upper zone of the seawater and the corals started to bleach. As the concentration of ammonia nitrogen( NH_4^+,0 mmol/L) increased in the course of time,the number of zooxanthellae floating on A. millepora and Palythoa sp. had the tendency to rise. When the salinity level were at 15‰ and 40‰ for A. millepora and below 34‰ for Palythoa sp.,a considerable number of zooxanthellae floated out and had rising trend in their number over time at 24-72 h and 6 h,respectively. In this occasion,the corals stopped growing and started to bleach. In conclusion,it revealed that temperature,pH,ammonia nitrogen content and salinity are the potential impact factors on coral breeding. It is suggested that,as temperature,pH value and salinity decrease or increase,as well as ammonia nitrogen content rises,the quantity of phycobiont would be raised,and the bleaching would be more severe.