Exposure to multiple stressors in the stony coral Acropora cervicornis: Effects on photosynthesis and respiration rates
Type of DegreeMaster's Thesis
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Recreational diving is an increasingly popular ecotourism activity that has allowed unprecedented amounts of direct human interaction with coral reef communities. Inadvertent contact by divers with benthic invertebrates such as delicate branching corals causes tissue abrasion that is known to impair coral growth and survival, but impacts on the underlying coral physiological processes are not well understood. In addition, global ocean temperatures are increasing due to anthropogenic climate change, but the interactive effects on reef corals of temperature stress combined with diver-caused abrasion stress remain unknown. The present study investigated impacts on the metabolic physiology of endangered branching staghorn corals Acropora cervicornis from both temperature and abrasion stress treatments under laboratory conditions. Using flow through respirometry, I measured net photosynthesis and respiration rates of cultured branch fragments of A. cervicornis. Two types of treatments were applied: (1) temperature stress, as measured at 22, 25, 28, and 30°C, and (2) abrasion stress, applied as 15% of tissue removed from fragments in contrast to non-abraded control fragments, creating 8 possible classes of the 2 treatments combined. A total of 26 coral fragments belonging to 6 genotypes were examined, with repeated measurements on some fragments, and a mixed effects linear model applied to account for effects of individual fragments. Net photosynthesis of the coral fragments under light conditions peaked at 25°C and declined at extremely low or high temperatures, while respiration increased linearly with temperature in all fragments, leading to an overall decline in the ratio of photosynthesis to respiration (P:R, a measure of energy balance) with temperature. There was a significant interaction effect between thermal and abrasion stress, in that tissue abrasion caused significant impairment of photosynthesis rates and augmentation of respiration, especially at high temperatures. P:R was highest for both types of fragments (abraded and non-abraded) at the relatively cool temperature of 22°C. Non-abraded corals were unable to meet their metabolic energy needs (i.e.: P:R < 1) at temperatures at or above 28°C. In contrast, P:R in the abraded fragments declined to 1 (i.e.: barely meeting energy needs) at the moderate coral reef temperature of 25°C, and was substantially < 1 at all higher temperatures. We conclude that some coral colonies exposed to impacts of frequent diver visitation in the form of tissue abrasion may be unable to meet their metabolic energy demands even at moderate coral reef temperatures. Their depressed photosynthetic rates and elevated respiration at higher temperatures leads to a more highly negative energy balance than for non-abraded corals, when exposed to heat stress. Diver damage in the form of tissue abrasion thus appears to reduce coral resilience to temperature-induced bleaching events. Our results also indicate that from an energetic standpoint, warming temperatures may diminish the viability of coral asexual reproduction via fragmentation, because natural fragmentation involves tissue damage and repair processes that impair the coral’s energy balance at high temperature. We recommend that reef managers mitigate the interactive effects of these two types of reef stressors by working with dive operators to incentivize recreational dive trips to corals reefs during the relatively cool winter months each year, rather than during warmer months. Altered seasonal timing of diver stress on reef corals may allow them to better withstand the temperature stress that often occurs during the relatively warm summer season.