Coral bleaching | EPFL Graph Search

Coral bleaching is the process when corals become white due to various stressors, such as changes in temperature, light, or nutrients. Bleaching occurs when coral polyps expel the zooxanthellae (dinoflagellates that are commonly referred to as algae) that live inside their tissue, causing the coral to turn white. The zooxanthellae are photosynthetic, and as the water temperature rises, they begin to produce reactive oxygen species. This is toxic to the coral, so the coral expels the zooxanthellae. Since the zooxanthellae produce the majority of coral colouration, the coral tissue becomes transparent, revealing the coral skeleton made of calcium carbonate. Most bleached corals appear bright white, but some are blue, yellow, or pink due to pigment proteins in the coral. The leading cause of coral bleaching is rising ocean temperatures due to climate change. A temperature about 1&n

Photic stress on coral reefs in the Maldives: The Amphistegina bleaching index

Arthur Adams, Aurélie Angéloz, Anaël Lehmann

The Amphistegina Bleaching Index (ABI) was applied to three Maldivian reefs in the Rasdhoo and North Ari Atolls in 2018, during normal sea surface temperature conditions. This dataset was then compared with a 2015, pre-coral bleaching study. The results provide a context for the verification and application of the ABI in outlining the photoinhibitory stress status of coral reefs outside of the Florida Reef Tract where it was originally developed. The sampling periods encompass different seasons and temperature regimes. The 2015 field sampling preceeded the El Nino induced, mass coral-bleaching events of 2015 and 2016. It was carried out in late April and early May, during the dry season, when temperatures exceeded 31.5 degrees C and photosynthetically active radiation (PAR) was high. The 2018 sampling took place near the September equinox, towards the end of the monsoon, when PAR was again high, though water temperatures were similar to 30 degrees C. Although there were slightly higher percentages of bleached Amphistegina in 2018, there were also higher percentages of juveniles, indicating either that (1) the chronic stress was insufficient to impact asexual reproduction or (2) the onset of stress was within the past few weeks; the latter hypothesis was supported by an increase in PAR and temperature coinciding with the time of sampling. From the ABI plots it is possible to distinguish between the 2015 (high data scatter), highly stressed pre-bleaching conditions with elevated photo-oxidative stress levels, and the near-baseline conditions represented by the 2018 dataset (tight data clustering). Overall, this study thus shows the potential of Amphistegina populations and the ABI in forecasting bleaching events, and contributing to the question of the resilience potential of the coral reefs as a whole. It also highlights the usefulness and suitability of the ABI, within Maldivian coral reefs, as an indicator of photo-inhibition through photo-oxidative stress that can increase susceptibility to coral bleaching as water temperatures approach or exceed the bleaching threshold.

ELSEVIER2020

Responses of reef bioindicators to recent temperature anomalies in distinct areas of the North Ari and Rasdhoo atolls (Maldives)

Arthur Adams, Aurélie Angéloz, Anaël Lehmann

Assessments of reef sediments in the North Ari Atoll (Maldives) were conducted in 2015 and 2018 on reefs of three islands with different management strategies: community, resort, and uninhabited. Indices applied were the Foraminifera in Reef Assessment and Monitoring Index (FI) and the Sediment Constituents Index (SI). Both indices are based on shells or fragments of functional groups, which for the FI are foraminiferal shells and for the SI are sediment components. The FI is considered to be an indicator of water quality and the SI an indicator of water quality, community structure, and processes such as grazing and bioerosion. Both indices indicated that environmental deterioration occurred between 2015 and 2018, likely related to the intense temperature anomaly in March–June 2016 that caused widespread coral bleaching and mortality. Median FI declined from 5.1 to 4.0 overall, indicating that water quality still supports reef accretion, though the replacement of coral cover by algae and sponges likely provides more food sources for smaller, faster-growing foraminiferal species. The median SI values similarly declined from 3.8 to 3.0, reflecting a decrease in identifiable coral fragments and an increase in unidentifiable clasts, likely indicative of increased bioerosion. Although a minor component, molluscan fragments also increased by 25%, likely in response to more algal cover for grazers. In 2015, the FI and SI data indicated that the island management regime contributed to the reef health status. Uninhabited islands were associated with higher indices compared to resort and community islands. A clear distinction between management regimes was not observed in 2018, because a major decrease in FI (median: 4.9 in 2015, 2.9 in 2018) was recorded offshore from an agricultural settlement on the previously “uninhabited” island surveyed. These observations support the usefulness of these indices in reef assessment, and provide additional understanding that the FI can respond to a coral-mortality event that alters food sources in the benthic community.

2020

Bio-optical properties and radiative energy budgets in fed and unfed scleractinian corals (Pocillopora sp.) during thermal bleaching

Niclas Heidelberg Lyndby

Corals live in symbiosis with algal dinoflagellates, which can achieve outstanding photosynthetic energy efficiencies in hospite approaching theoretical limits. However, how such photosynthetic efficiency varies with environmental stress remains poorly known. Using fiber-optic and electrochemical microsensors in combination with variable chlorophyll fluorescence imaging, we investigated the combined effects of thermal stress and active feeding on the radiative energy budget and photosynthetic efficiency of the symbiotic coral Pocillopora sp. At ambient temperature (25°C), the percentage of absorbed light energy used for photosynthesis under low irradiance was higher for fed (~5-6%) compared to unfed corals (4%). Corals from both feeding treatments responded equally to stress from high light exposure (2400 µmol photons m-2 s-1), exhibiting a decrease in photosynthetic efficiency, down to 0.5-0.6%. Fed corals showed increased resilience to thermal-induced bleaching (loss of symbionts) compared to unfed corals. In addition, while unfed corals decreased their photosynthetic efficiency almost immediately when exposed to thermal stress, fed corals maintained a constant and high photosynthetic efficiency for 5 more days after onset of thermal stress. We conclude that active feeding is beneficial to corals by prolonging coral health and resilience during thermal stress as a result of an overall healthier symbiont population.

2019

Photic stress on coral reefs in the Maldives: The Amphistegina bleaching index

Arthur Adams, Aurélie Angéloz, Anaël Lehmann

ELSEVIER2020

Responses of reef bioindicators to recent temperature anomalies in distinct areas of the North Ari and Rasdhoo atolls (Maldives)

Arthur Adams, Aurélie Angéloz, Anaël Lehmann

2020