Genus and size-specific susceptibility of soft corals to 2020 bleaching event in the Philippines

ABSTRACT

Soft corals are zooxanthellate sessile animals supporting various organisms in coral reefs. However, their populations are threatened by the impacts of ocean warming. Under thermal stress conditions, soft corals may experience mild to severe bleaching which may lead to death. Understanding soft coral bleaching responses highlights the importance in predicting how populations and diversity may be affected by changing climate scenarios. In this study, we examined the bleaching responses of the three dominant soft coral genera (Lobophytum, n = 1318; Sarcophyton, n = 116; Sinularia, n = 639 colonies) in the Bolinao-Anda Reef Complex (BARC), Pangasinan, north-western Philippines during the 2020 thermal stress event in terms of genus and colony size susceptibility, and zooxanthellae density. Degree heating week (DHW) data from 1986–2020 were obtained using remotely sensed data to determine thermal anomalies in the study sites. The maximum DHW (6.3) in 2020 occurred between July–August while bleaching surveys were done during October of the same year. The percentage of bleached portions in each colony was used to determine bleaching category: no bleaching (0%), moderately bleached (1–50%) and heavily bleached (>50%). Quantification of bleaching prevalence and susceptibility of colony sizes were determined by colony count and mean diameter measurements taken from quadrat photographs in October 2020. Haphazard tissue collection (∼3 cm) in each colony of three soft coral genera per bleaching category was done to quantify zooxanthellae density. Results showed that Lobophytum colonies had the lowest bleaching prevalence (41%), followed by Sinularia (66%) and Sarcophyton (78%). All colony size classes of the three genera were susceptible to bleaching. However, smaller colonies of Lobophytum (<15 cm), Sarcophyton (<5 cm) and Sinularia (<5 cm) showed less susceptibility than large colonies. Zooxanthellae density was significantly reduced in moderately and heavily bleached colonies. The results of this study highlight that bleaching susceptibility is genus specific, with Sarcophyton and Sinularia being more susceptible to bleaching than Lobophytum. Smaller colonies seemed to be less susceptible to bleaching than large-sized soft corals suggesting a differential thermal stress response. Spatial variations in bleaching prevalence were also found among reef sites with varying environmental conditions and thermal stress histories. This work provided initial observations on how bleaching affects soft corals. Further studies on soft coral community recovery are recommended to fully understand how these organisms perform after thermal stress events.

KEYWORDS:

• Octocorals
• colony size
• Sinularia
• Lobophytum
• Sarcophyton

Acknowledgements

We acknowledge the Bolinao Marine Laboratory of the University of the Philippines for logistics and fieldwork assistance. Thanks to Kevin Yatco and Socorro Rodrigo for providing technical assistance in obtaining remotely sensed temperature data. Thank to Kevin Yatco and Socorro Rodrigo, and Kevin Labrador for providing technical assistance in obtaining remotely sensed temperature data and assistance in statistical analysis, respectively.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Author’s contribution

CCB, RMAL, CSS and MVBR conceptualized and designed the study. CCB, RML and CSS performed the field and laboratory work. All authors analysed the field and laboratory data and generated figures and tables. DAMV analysed and generated the temperature figures and conducted statistical analyses. MVBR secured the funding of this work. All authors wrote the manuscript with CCB as the lead. All authors read and approved the manuscript.

Data availability statement

The datasets derived from this work are available from the corresponding author on reasonable request.

Consent for publication

All authors give their consent to the publication of this manuscript.

Ethics approval consent to participate

This article does not contain experiments with animal testing conducted by any of the authors. All necessary permits for field studies have been obtained by the authors from the competent authorities.

Additional information

Funding

This study was funded by OVCRD (Office of the Vice Chancellor for Research and Development) Outright Research Grant (Project no. 202037) and the Marine Science Institute In-house Research Grant of the University of the Philippines awarded to MVBR and partially supported by the Soft Coral Diversity as Indicator of Reef's Health project funded by the Department of Science and Technology (DOST) of the Republic of the Philippines.