Effects of ultraviolet radiation and nutrient level on the physiological response and organic matter release of the scleractinian coral Pocillopora damicornis following thermal stress.

Lucile Courtial,Victor Planas Bielsa,Christine Ferrier-Pagès,Fanny Houlbrèque,Frank Melzner

doi:10.1371/journal.pone.0205261

Abstract

Understanding which factors enhance or mitigate the impact of high temperatures on corals is crucial to predict the severity of coral bleaching worldwide. On the one hand, global warming is usually associated with high ultraviolet radiation levels (UVR), and surface water nutrient depletion due to stratification. On the other hand, eutrophication of coastal reefs increases levels of inorganic nutrients and decreases UVR, so that the effect of different combinations of these stressors on corals is unknown. In this study, we assessed the individual and crossed effects of high temperature, UVR and nutrient level on the key performance variables of the reef building coral Pocillopora damicornis. We found that seawater warming was the major stressor, which induced bleaching and impaired coral photosynthesis and calcification in all nutrient and UVR conditions. The strength of this effect however, was nutrient dependent. Corals maintained in nutrient-depleted conditions experienced the highest decrease in net photosynthesis under thermal stress, while nutrient enrichment (3 μM NO3- and 1 μM PO4+) slightly limited the negative impact of temperature through enhanced protein content, photosynthesis and respiration rates. UVR exposure had only an effect on total nitrogen release rates, which significantly decreased under normal growth conditions and tended to decrease also under thermal stress. This result suggests that increased level of UVR will lead to significant changes in the nutrient biogeochemistry of surface reef waters. Overall, our results show that environmental factors have different and interactive effects on each of the coral’s physiological parameters, requiring multifactorial approaches to predict the future of coral reefs.

Highlights

Anthropogenic climate change has emerged as a serious global-scale threat to the structure, functions and viability of coral reef ecosystems [1, 2, 3]
No clear effect of ultraviolet radiation (UVR) or nutrient level is detected by the Principal Component Analysis (PCA) (Fig 1B and 1D), suggesting that temperature was the main factor controlling coral physiology
We have examined here the response of the common coral species Pocillopora damicornis to the combined effects of three stressors

Summary

Introduction

Anthropogenic climate change has emerged as a serious global-scale threat to the structure, functions and viability of coral reef ecosystems [1, 2, 3]. Ultraviolet radiation, nutrient level and thermal stress effect on the physiology of Pocillopora damicornis are facing increases in seawater temperature, ultraviolet radiation (UVR) levels and storm events frequency [4, 5]. Beyond their direct effects on reefs, such environmental changes are responsible for variations in nutrient concentrations. Heat waves enhance nutrient depletion through water column stratification [6], while storms can induce water-column mixing and nutrient upwelling [7] On top of these global changes, coastal reefs suffer from local pressure, such as overfishing, sedimentation and eutrophication (increased supply in organic and inorganic nutrients) [8]. We observed an increase in the frequency and the intensity of coral bleaching event (i.e. massive loss of symbionts or photosynthetic pigments from the host tissue), responsible for high coral mortality [11, 12]

Methods

Results

Conclusion