During February at Landaa Giraavaru, 24 new coral frames were transplanted (13 sponsored by guests, 11 sponsored by Four Seasons) plus a further 434 frames were monitored, mainly at the Blu site (cleaning, re-transplanting and photographing them).
At Kuda Huraa this month, we transplanted 14 new coral frames, and monitored a further 45 frames. Most coral fragments used for transplanting were harvested from our mature frames at the Channel site. In anticipation of the seasonally warmer ocean temperatures, we have started to move frames to deeper waters and to set up shading for vulnerable frames.
Prospective Coral Bleaching Predictions
The data projections issued by American agency NOAA aim to predict the global thermal bleaching stress for the next few months, based on the best available science.
Projections for the Maldives during March (weeks 1-4) indicated low stress for the corals, but weeks 5-12 (April-May) are at “Alert Level 2”, meaning severe bleaching and mortality is likely throughout much of the country.
The prediction for ocean temperatures during 2020 is higher than for 2018 and 2019, so we can expect the coral bleaching to be more severe this year.
The severity of the outlook will likely change as time progresses, and the NOAA projections will be closely monitored for shifts in date and severity.
We are ramping up our efforts in preparation for the higher Sea Surface Temperatures (SST) expected during April-May.
This month, we retrieved 4 of our HOBO ocean temperature loggers from the waters around Kuda Huraa. The loggers have been measuring the ocean temperatures continually every hour, from 4 July 2019 through to 6 February 2020. We downloaded and collated the data for plotting and analysis of the average and maximum temperatures, at each site during this time period.
Using 30.8°C as a bleaching threshold (taken from NOAA’s coral reef watch benchmarks), all the average recorded temperatures from July 2019 to February 2020 are more than one degree Celsius below the bleaching threshold. All the sites had similar average temperatures (~29.3°C), and the average temperature has decreased overall since July 2019. Although average temperatures were safely below the bleaching threshold, the hottest maximum temperatures per site occasionally spiked above the bleaching threshold by an average of 2°C (the corals can tolerate this in short bursts).
Recently, the Water Villas site experienced the highest logged temperature of 33.9°C, thought to be a combination of low tide, low winds and full sun. Moving forward, the biggest risk to coral health will be if the average temperatures stay above the bleaching threshold for days or weeks at a time. These weeks are referred to as ‘Degree Heating Weeks’ by NOAA, and are categorised by duration as follows:
- 1-4 weeks: bleaching is possible,
- 4-8 weeks: bleaching is very likely,
- >8 weeks: significant coral bleaching is certain, followed by death of the corals.
Our temperature loggers will be redeployed and checked regularly over the next few months, as we monitor the sea surface temperatures and prepare for NOAA’s heat stress predictions.
At the Water Villas site, we have started to clear the areas under the boardwalks. We are relocating the more established frames so we can move our newer frames to the cooler partial-shade. We have seen that “partially-shaded” corals fare better than both “fully-shaded” and “unshaded” corals, because they receive some sunlight for nutrients but avoid full UV radiation. However, due to the shallow depth at low tide (as low as 1m), the Water Villas site can still be vulnerable to thermal stress (and coral bleaching) compared to deeper sites around Kuda Huraa.
We have relocated 42 mature frames to the ‘Blue Hole’ refuge site (depth 5-6m, total 440m2). A temperature logger was deployed here, but we were surprised to discover the temperatures were not as low as we’d hoped, averaging 29.6°C for the week (with 32.5°C spikes). It will be interesting to see if this site will be able to act as a refuge in the upcoming warmer months.
At the Channel, the southern stretch of the site is shallower and warmer, so we plan to deploy ‘shade sails’ here to protect existing frames (and we will not locate any new frames here for several months). At the House Reef site, we repositioned some frames to create a new ‘snake’ shape, along with 3 new heart shapes for Valentine’s Day.
In order to better understand coral bleaching at the local level, we started CoralWatch monitoring on 12 February. When bleached coral colonies are encountered, we will take photographs and record the relevant parameters (date, location, water temperature, depth, species affected).
We are working closely with the Dive Team to monitor the reefs and collect as much data as possible at every opportunity, throughout February to May. During the last 2 weeks of February, bleaching has been recorded at the Funadhoo dive site and around Landaa itself (Blu, House Reef, Parrot Reef). The two main coral genera present around Landaa (Acropora and Pocillopora) were found to be affected by this early bleaching. The water temperatures are averaging 29-30°C, which is generally below the bleaching threshold, so perhaps:
- the bleached corals are less resistant to elevated temperatures (especially for longer durations);
- there are extra stresses such as sedimentation or localised warm ocean currents;
- there have been temperature spikes and increased UV light experienced during low tides;
- some of the paling colonies could be fighting disease (unrelated to water temperatures).
To mitigate against the effects of increased ocean temperatures, work is in progress to decrease the bleaching sensitivity of our coral frames. Over the past few months, we have been choosing deeper locations for our newly transplanted coral frames, placed 7m-15m depth, at Anchor Point and Deep Blu sites. During our follow-up monitoring tasks, we can see that deeper sites are showing healthy initial growth with good encrusting onto the frame.
We are also transplanting more resilient Pocillopora species for our new frames, that are known to be more resistant to heat stress and to general changes in environmental conditions.
Acropora coral colonies starting to bleach (left) and fully bleached (right) [February 2020, Landaa Giraavaru, Maldives]
Location of our deeper coral refuge sites for mitigation against coral bleaching (Landaa Giraavaru, Maldives)
Coral Plates in Aquarium One (plates KH01, KH02, KH05) and Aquarium Two (plates KH03, KH04)
KH01 – 37 fragments remaining, 31 of which are encrusting onto the plate. Interestingly, one of the Acropora hyacinthus (variant 2) fragments lost half its encrusted skeleton from the plate; under the microscope we observed the corallites surrounded by the interlinked canals of the coenosarc. Additionally, it is possible to see the individual zooxanthellae cells in areas of lower concentration.
The observable interaction between A. cytherea and A. hyacinthus (variant 1) suggests that despite the higher growth rate of A. hyacinthus, A. cytherea is more likely to win the competition for space.
KH02 – All 37 remaining fragments of Galaxea fascicularis are encrusting, and the fragments on the top of the plate are growing along the edge and back of the plate. Lifting is still visible on some of the polyps, however, this may be due to the secretion of calcium carbonate over the basal plate, rather than due to tissue loss as we originally thought (Al-Horani et al., 2005).
KH03 – 31 fragments remaining, of which 25 are encrusting to some extent, but this represents a serious decline in coral health (15 fragments lost in 2 months) perhaps due to the tissue loss in January. This affected all species, particularly A. millepora and A. digitifera (where all fragments died) except the A. valida fragments, which continue to grow well, extending branches on both sides of the plate.
KH04 – 26 fragments remaining, 22 encrusting, with good recovery after the January tissue loss. The observable interaction between A. hyacinthus variants 2 and 3 suggests that different colonies might not affect each other’s health (we will continue to monitor the interaction).
KH05 – The new Galaxea fascicularis has been growing well so far, with 7 of the 28 fragments starting to grow and encrust (transparent skeleton with new polyps, and coloured areas with zooxanthellae). Taller fragments with live polyps have been extending their skeletons downwards and developing new corallites through budding. All polyps are also extending their tentacles and are exhibiting a brown colour morph.
‘Coral Core’ Experiment
Overall, the coral plugs and the parent colony are showing improved health after we relocated them to increase water flow and reduce direct sunlight. Encrusting has started on the edges of the plugs and the donor holes, and we expect the addition of Epoxy will close the larger gaps and further encourage encrusting.
We have been on several scoping dives to look for new sites. Site#1 consists of 2 large colonies of Porites lobata (2m diameter x 1m high, at depths of 1-5m). They both have areas of healthy living tissue (for donor plugs) interspersed with areas of bare rock (as transplant sites) that will ensure that plugs are subjected to very similar environmental conditions, which should improve the chances of survival.
Upon starting the experiment, we realised the drilling stages were tricky and very time-consuming, so we kept the total number of plugs to a minimum. Further practise and experience with underwater drilling and handling the Epoxy will improve our protocols and decrease the overall time taken.
Drilling and removal of Medium and Large plugs were found to be easier than the Small size, however the Small size will initiate faster encrusting (Medium/Large plugs take 2 months to encrust). Photos will be taken at weekly intervals and added to PhotoQuad software to calculate exact growth rates for each size class at each site. The exposed holes will also be monitored by the photographs for: encrustation, disease, predation, competition, algal growth.
Yaniu Rauf, our current marine biology apprentice, is continuing the coral sexual reproduction experiment for his personal scientific project.
After three months, we have achieved a 75% success rate (76 surviving coral colonies, from an initial 101). Several colonies have now grown too large to continue individual polyp counts, so we will switch to recording dimensions and colour as indicators of health.