Simultaneous and sequential stress from increased temperature and copper on the metabolism of the hermatypic coral Porites cylindrica
Stressors arising from human activities may interact not only with each other, but also with natural disturbances. However, experimental studies on disturbance complexity and physiological responses of corals to sublethal stresses, especially those due to human activities, are surprisingly few. In this study we investigated the stress response of the scleractinian coral Porites cylindrica after 24 h of exposure to copper (11 µg Cu l–1) and increased temperature (following a 4°C above-ambient curve), separately and in combination. We also investigated the effect of sequential stress where corals pre-exposed to increased temperature for 24 h were exposed to copper (for 24 h) after a 5-day recovery period. Changes in gross primary production (Pg: per milligram chlorophyll a per hour) and respiration (R:per square centimeter per hour) in terms of dissolved oxygen were used as indicators of stress. The results show that heat and the combination of heat and copper significantly reduced production rate. However, corals exposed to elevated temperature displayed a significantly higher production rate following the 5-day recovery period. The combination of the two stressors showed no additive or synergistic effects. Copper alone had no effect on the production rate. However, corals that were pre-exposed to increased temperature and again exposed to copper after 5 days displayed a significant reduction in production rate. The respiration rate was significantly reduced by all treatments, although no significant differences between treatments were detected. The results presented here illustrate how a stressor that does not affect corals when acting in isolation may do so in sequential combination with other stressors.
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