Genus-specific bleaching at Con Dao Islands, Southern Vietnam, June 2019

Abstract

Coral communities in the shallow waters of Con Dao islands, located on the continental shelf of Southern Vietnam, were monitored in June 2019 when bleaching alert level 1 was announced by NOAA Coral Reef Watch (https:// coralreefwatch.noaa.gov/satellite/index.php) and the surface seawater temperature was recorded between 30-32℃ in the field. We used the ReefCheck transect method (Hodgson et al. 2006) to record cover of bleached corals and non-bleached corals at two depths (shallow: 3-5 m and deep: 6-8 m) at 5 sites (EMS Fig. 1). The study sites were characterised by the dominance of hard corals with their cover more than 50% at 4 sites and around 25% at Cua Ga. Overall, 32.5±7.5% (mean±SE) of hard corals (mean cover＝51.8%, SD＝23.7%) and 100% of soft corals (mean cover＝2.6%, SD＝4.5%) were observed to bleach. Bleaching at genus level was considered for common coral genera with their total cover (%) as follows respectively: Acropora (17.3), Montipora (9.9) Porites (4.2), Diploastrea (2.4), Pachyseris (1.7), Pavona (1.6), Fungia (1.1), Favites (1.0), Pectinia (0.8) and Galaxea (0.6). The hard coral genera with most severe bleaching (all at 100% bleached) included: Fungia (Fig. 1A), Pachyseris (Fig. 1B), Pavona (Fig. 1C), followed by Pectinia (93.8±10.2% bleached; Fig. 1B) and massive Porites (69.9±15.2% bleached; Fig. 1D). Other coral genera commonly found in Con Dao Islands were moderately bleached such as Favites (30.0±26.7%), Goniastrea (23.8±26.1%), Montipora (10.1± 5.7%; Fig. 1A). Two genera Galaxea and Diploastrea did not suffer bleaching. Acropora with most branch and tabulate colonies were less impacted both in deep and shalow waters (0.9±1.3% bleached; Fig. 1D-F). Hard corals in deeper waters exhibited a higher susceptibility to bleaching than their shallow counterparts (48% bleached in deeper transects compared with 15% in shallow transects). The findings were consistent with the higher abundance of the five most susceptible genera mentioned above in deeper (total mean cover＝17.5%) than in shallow (2.2%) transects. An earlier bleaching event at Con Dao islands in October 1998 resulted in 37.8% of hard coral colonies bleached (Vo 2000). The 2019 event had both similarities and differences in impacts. Soft corals were the most susceptible in both events (100% bleached) and Galaxea remained consistently unbleached. In 2019, Pachyseris and Fungia had much higher levels of bleaching than in 1998, when these genera were little affected (7.7% and 8.3% bleached). In contrast, Diploastrea had no bleaching compared with 14% bleached in 1998. Porites was severely impacted at both events with 57% bleached and many dead massive corals covered by filamentous algae were recorded in October 1998. Acropora were among the susceptible genera with 19% bleached colonies together with many dead colonies observed in October 1998 (Vo 2000).

Coral Oasis on Con Dao Islands: A Potential Refuge of Healthy Corals in the Offshore Waters of Vietnam?

“…An assessment of a coral bleaching event in the CDNP in June 2019 at five study sites using the ReefCheck protocol revealed high cover of live stony corals (mean 65.6 ± 18%), of which only one-quarter (25 ± 11.1%) were bleached to a different extent [32]. Moreover, out of all the dominant coral taxa in the CDNP, thermally susceptible Acropora and Montipora exhibited only 2.6% and 11% bleached coverage, respectively, whereas thermally resistant massive Porites showed 50% bleached coverage [32,33]. The previous severe thermal anomaly in 2010 was equivalent to that in 2019 (the DHWs of 13 • C-week in 2010 reached NOAA's Second Bleaching Alert Level), and coral bleaching in the CDNP was more pronounced (mean cover of bleached coral was 43.3 ± 21.1%), although the pattern of coral bleaching was similar to that in 2019 with the largest and smallest proportions of bleached corals being Poritidae and Acroporidae, respectively [34].…”

“…To date, only four publications have highlighted coral bleaching events related to the ENSO in the CDNP in 1998 and 2005 [30,31] and in 2019 during one of the most severe thermal anomalies [32,33]. Results of the coral surveys conducted by CDNP staff yearly at permanent locations using the rapid ReefCheck protocol remain inaccessible to the public.…”

Analysis of composition and abundance of viral communities associated with coral \(\textit{Acropora formosa}\)

“…The bleaching event in Con Dao Island had continued since 1998 (Vo, 2000), to which Acropora is the most susceptible coral genus after 20 years (Vo et al, 2020). In this study, we will analyze the composition of the viral community living in the mucus layer of coral Acropora formosa (Dana, 1846) in healthy and bleached samples by classifying and assigning taxonomy to them went inferring obtained metagenomic sequences from the public database.…”