When corals are stressed by changes in conditions such as temperature, light, or nutrients, they expel the symbiotic algae living in their tissues, causing them to turn completely white or bleached.
During the annual fish and coral monitoring in Tubbataha on May 26 to June 2, 2020, TMO researchers observed some coral bleaching in the permanent monitoring sites in the vicinity of Kook, Delsan Wreck, and Jessie Beazley Reef. Fire corals (Millepora) were the most affected in the said sites. The coral colonies observed at the time were not completely white in color but were only partially bleached. However, two massive corals (Porites) near the dive site Terraces were completely bleached.
(Figure 1. Bleached massive coral (Porites) observed in Seafan Alley (left). Parts of this massive coral (right) are already covered in algae indicating that it was bleached at least a month before this photo was taken on the last week of May 2020.)
After several weeks, on June 23 and 24, 2020, other kinds of corals were observed to have turned from florescent to completely white in color. These include encrusting (Isopora), branching (Stylopora, Poccilopora, Millepora), foliose (Montipora), massive (Goniopora), solitary corals (Fungia), and even soft corals in most sites in Tubbataha. Coral bleaching was observed in patches. Based on the 5-point scale of the Philippine Coral Bleaching Watch, the severity of bleaching in North and South Atolls may be described as mild to moderate (Category 1-2). In the east side of Jessie Beazley Reef, bleaching can be described as moderate (Category 3).
(Figure 2. Patches of bleached corals in the monitoring station near Delsan Wreck dive site in South Atoll.)
(Figure 3. Partial bleaching of foliose and branching corals was observed in the east portion of Jessie Beazley Reef on June 24, 2020 (top and middle). Soft corals (bottom) showing signs of stress turn florescent yellow. In the foreground is the same kind of soft corals in normal conditions.)
Behind the ranger station inside the lagoon, varying degrees of bleaching amongst branching and tabular (Acropora), massive (Porites), solitary (Fungia), and other kinds of coral were observed. Some started to show signs of stress, turning florescent, while others were completely white. The severity of bleaching in the lagoon may be described as moderate (Category 3).
Coral bleaching was observed between the depths of two to 15 meters, with water temperature between 30° to 31° Celsius.
(Figure 4. Behind the ranger station inside the lagoon, a florescent branching coral next to dead corals with algae (top). Bleaching in progress: colonies of branching and massive corals (middle) in various stages of bleaching. Branching corals (bottom) covered in algae in the foreground usually look like the blue corals behind.)