Coral have small algae known as a zooxanthellae living within their tissues. These algae provide the food for the corals and have what is known as a symbiotic relationship with the coral, meaning that both the coral and the algae benefit from living together. These algae also give the coral their colour. When the coral becomes stressed the relationship between the coral and the algae breaks down and the algae leave the coral; when this happens the coral become paler in colour or completely white depending on how bad the stress is: this process is called coral bleaching.
We have started to see signs of coral bleaching on the reefs and on our frames this month. As the drier monsoon period comes to a close, the prolonged hot weather has led to an increase in the water surface temperature (up to 30-31°C has been common). The corals have also been exposed to more sunlight due to the lack of cloud cover, causing stress, a reduction in algae and a paling of the coral skeleton.
We perform our Coral Watch every three months, and each time we study the same species at the same locations. We hope to observe how the colour of the corals changes during the course of the annual climate cycle, and we can see if this relates to any environmental events.
Some paling and partial bleaching has been observed on both the natural reef and on our frames, especially in the shallower areas (<2m). The two species showing most bleaching seem to be Acropora robusta (photo, right) and Acropora hemprichii. But with the imminent arrival of the rainy season, ocean temperatures will fall slightly again and the symbiotic algal populations will be restored.
By the end of the dry monsoon in April we were getting very hot and humid days with very little cloud cover in the skies and not much wind, very nice for lazing on the beach and swimming but not so good for the corals. These conditions cause the surface waters to increase. Normally the water temperature is around 30ᵒC but in April we recorded water temperatures of up to 34°C. This is too hot for the corals and causes them to become stressed and bleach and we saw evidence of this in the channel area and in the shallow parts of the House Reef. We have been lucky that these temperatures only lasted for a few days and have now returned to normal. However, when corals bleach it takes time for them to gain their colour once more so we are monitoring one of the frames in the channel to see how long this process takes.
The water temperature on the house reef has decreased drastically after almost two weeks of stormy weather at the start of May, marking the start of the southwest monsoon. Ocean surface temperatures were a steady 31°C at the start of May, but dropped down to 28-29°C by mid-month. Pale and partial bleaching was observed throughout the reef on both the natural reef and on the frames, especially in the shallower areas (<2 m). Only a little ‘total bleaching’ was observed, mainly in Acropora robusta and Acropora hemprichii.
2015 Coral Bleaching
The 2015 coral bleaching event has been followed by a rapid drop in sea temperatures, and increased wave damage at the change in monsoon season has caused some mortality. This year has not been classified globally as a “mass bleaching event” (IUCN definition) as not all coral species have been affected. But certainly a large bleaching event has been seen in the Maldives, mostly affecting the Acropora species of corals.
During a swim over of the worst affected sites, estimations were made of: number of frames with mortalities, percentage of mortalities within frames, and percentage of mortalities between Acropora and non-Acropora species. The following results were recorded:
- Frames with 100% mortality
- Frames with greater than 50% mortality
- Frames with 100% Acropora mortality
The shallower areas seemed most badly affected, in particular the Spa and Green Lagoon sites, which also have slow water currents with water pooling in the lagoon and subsequently heating up. The Water Bungalows, Turtle and parts of the Channel sites are also shallow but with much stronger currents flowing over them, and as such the bleaching was observed to be much less at these sites. We have stationed temperature loggers at various sites around Landaa Giraavaru, and you can see a graph of the readings below. Between mid-March and mid-May 2015, the ocean temperatures were the highest we have seen for several years, peaking at 30.5 °C.
Our new marine biology Intern, Sebastian Szereday, is currently recording the bleaching event data and analysing the impact on the coral frames. The selected site for the study is the Water Villas Arc, which is well representative of the affected sites. For this study, we count the number of live fragments versus the number of dead fragments on each frame, to obtain a mortality rate. We then classify the frames by age, and we have noticed that the younger frames (<2 years) are far more affected by mortality than the older frames (>2 years). These results are displayed in the graph, clearly showing the 6-month frames with 80% mortality, compared to the 2 year frames which largely recovered.
As an extension to this project, we plan to take extra monitoring photographs in the chosen area (Water Villa 400 site). Historically, we already have a full set of healthy frame photographs taken twice-yearly, so the next set of photos will be taken to show the extent of the coral bleaching. We will then take follow-up photos after the necessary maintenance and re-transplantation is complete, to produce 3 photo sets:
Before the bleaching event – After the bleaching event – After re-transplantation
Our new marine biology intern, Sebastian Szereday (University of Vienna), is based at Landaa for 11 weeks, and will be assisting with our coral bleaching research. As part of his busy schedule, he will also be studying coral fluorescence and continuing with our coral frame biomass project.
- Global coral bleaching event: What you need to know – The New Scientist (Oct 2015)
- World’s oceans facing biggest coral die-off in history, scientists warn – The Guardian (Oct 2015)
- NOAA declares third ever global coral bleaching event – NOAA (Oct 2015)