Recent and rapid reef recovery around Koh Phangan Island, Gulf of Thailand, driven by plate-like hard corals

Abstract

Mass bleaching events and local anthropogenic influences have changed the benthic communities of many coral reefs with pronounced spatial differences that are linked to resilience patterns. The Gulf of Thailand is an under-investigated region with only few existing datasets containing long-term developments of coral reef communities using the same method at fixed sites. We thus analyzed benthic community data from seven reefs surrounding the island of Koh Phangan collected between 2014 and 2022. Findings revealed that the average live hard coral cover around Koh Phangan increased from 37% to 55% over the observation period, while turf algae cover decreased from 52% to 29%, indicating some recovery of local reefs. This corresponds to a mean increased rate of coral cover by 2.2% per year. The increase in live hard coral cover was mainly driven by plate-like corals, which quadrupled in proportion over the last decade from 7% to 28% while branching corals decreased in proportion from 9% to 2%. Furthermore, the hard coral genus richness increased, indicating an increased hard coral diversity. While in other reefs, increasing live hard coral cover is often attributed to fast-growing, branching coral species, considered more susceptible to bleaching and other disturbances, the reefs around Koh Phangan recovered mainly via growth of plate-like corals, particularly of the genus Montipora. Although plate-like morphologies are not necessarily more bleaching tolerant, they are important for supporting reef fish abundance and structural complexity on reefs, aiding reef recovery and sturdiness. Hence, our findings indicate that the intensity of local stressors around Kho Phangan allows reef recovery driven by some hard coral species.

Keywords: Bleaching; Coral growth morphologies; Ecosystem recovery; Habitat complexity; Long term monitoring; Phase shifts; Reef community.

Figure 1. Location of Koh Phangan in the Gulf of Thailand (small map) and locations of the seven surveyed reefs around the North-west coast of Koh Phangan (large map).

Basemap was provided by QGIS, which is under the Creative Commons Attribution-ShareAlike 3.0 license (CC BY-SA: https://creativecommons.org/licenses/by-sa/3.0/). The shapefile of Thailand was taken from https://data.humdata.org/dataset/cod-ab-tha, available under the Creative Commons Attribution 3.0 IGO license (CC BY 3.0 IGO: https://creativecommons.org/licenses/by/3.0/igo/).

Figure 2. Development of the benthos cover from 2014 to 2022.

(A) Live coral cover, (B) turf algae, (C) sand cover, and (D) other invertebrates cover. Bars represent the mean and error bars the standard error of the cover in the study area, while black lines represent the linear regression lines.

Figure 3. Percent coverage of growth forms from 2014 to 2022.

(A) Massive growth forms, (B) branching growth forms, (C) plate-like growth forms, and (D) solitary growth forms. Bars represent the mean and error bars the standard error of the cover in the study area, while black lines represent the linear regression lines.

Figure 4. Development of hard coral genus richness of Koh Phangan’s coral reefs from 2014 to 2022.

Bars represent the mean and error bars the standard error in the study area, while the black line represents the linear regression line.