Reefscapers Diary June 2020

Prospective Coral Bleaching Predictions

The NOAA heat stress projections for Maldives are categorised as “Warning/Watch” in June, lowering to “Watch”during July.

Monthly Progress

At Landaa Giraavaru this month, we monitored a total of 122 coral frames, mainly at our Blu and Anchor Point sites.

At Kuda Huraa, we monitored a total of 523 coral frames at the Water Villas site, and spent some time carefully rearranging them into a series of patterns, representing marine life in the Maldives (dolphin, seahorse, turtle, manta).

At the Water Villas site, we recorded a 30% coral mortality rate, with 17% currently recovering from coral bleaching and 53% recorded as healthy. At the Blue Hole site, corals have paled but are recovering. Some predating of Acropora digitifera is evident, thought to be caused by fish hunting for coral crabs.

The Channel site suffered badly due to the seasonally elevated ocean temperatures, with an 82% rate of coral mortality. Our limited shading experiments were able to protect some of our coral frames, which were bleached but are showing signs of recovery. By contrast, wild patches of Montipora digitata have expanded over the last year, and are now growing over some of the frames.

CoralWatch – Monitoring for Signs of Coral Bleaching

At Landaa Giraavaru, some coral bleaching remains evident around the island, but there are increasing signs of healthy recovery, particularly at sites deeper than 15m. Additionally, the Anchor Point site is subjected to strong water movement, and is home to some of the healthiest corals. (If work levels allow, we hope to provide a more quantitative assessment next month.)

At Kuda Huraa, we monitored coral frames at the Water Villas (38 frames), the Blue Hole (31) and the Channel (29), totalling 3759 coral fragments. We recorded 41% “mortality” (plus 13% “recovering”) across all 3 sites. Pocillopora fragments showed consistently higher resistance to coral bleaching when compared to Acropora species (with Acropora digitifera and A. gemmifera suffering most mortality).

Bleaching Acropora digitifera specimen on the reef, surrounded by dead corals covered in algae

Coral Plates in Aquarium One (plates KH01, KH02, KH05) and Aquarium Two (plates KH03, KH04)

• KH01 – all fragments died due to the seasonally elevated water temperatures.
• KH02 – Galaxea fascicularis proved to be most resilient to the high temperatures (some paling persists).
• KH03 – little improvement from last month; some algal overgrowth was cleaned away.
• KH04 – only 2 fragments alive, which are slowly losing calcification; cleaned of algae.
• KH05 – 2 coral fragments now fused; 3 fragments growing but not yet encrusted to the plate.

Coral Reproduction Spawning Experiment

Two additional colonies have transitioned from the polyp-counting to the size-measurement method, with 15 of the 26 colonies now having more than 30 polyps. To prevent colonies from touching the base of the aquarium, we are using sections of ceramic tile. Recently, the algal-grazing fish have been moving and sometimes overturning these, so we have increased the weight by adding extra stones (attached using an aquarium-safe cyanoacrylate).

Time Lapse Experiment to Record Coral Bleaching

A time lapse experiment has been set up to capture the bleaching process in a colony of Acropora coral. Our new dedicated time lapse camera (Brinno TLC200 PRO) uses a High Dynamic Range image sensor and provides a finished time lapse video immediately after the recording ends. Complete with the BCS 24-70mm adapter, this lens is ideal for the macro video we want to capture. (Previous trials utilised a basic camera system with large video files that proved difficult to edit.)

We’ve decided to use Acropora millepora, due to its “fuzzy” appearance caused by excessive polyp activity in all photoperiods. It is hoped that this will allow a greater insight into the polyp behaviour during the bleaching process.

The camera is fixed outside the experiment tank, and is capable of recording continuously for 40 days (32GB storage, capture interval every 10 minutes).  Two aquarium heaters are installed inside the closed system to elevate the water temperature to 29-31°C. The light is supplied by one Radion XR15 PRO, with a 24-hour photoperiod at 100% intensity.

Wang et al. (2008) investigated the prolonged photoperiod and its effects on symbiont relationship. Zooxanthellae maintained a natural progression of reproductive phases for at least the first 11 hours; at hour-17, unnatural populations of phases were noted, lasting the duration of the experiment (72 hours). Zooxanthellae contained an abnormal number of chromosomes and failed to divide in an orderly fashion as would take place under natural conditions.

Junior Marine Savers

We have been developing a new coral module for the ‘Junior Marine Savers’ educational program, covering such topics as:

• basic coral morphology and common Maldivian corals;
• the natural formation of biodiverse coral reefs;
• coral bleaching and other threats to coral reefs.

‘Coral Core’ Experiment

Our in situ coral plug propagation trial has been running for over 4 months now. Some of the corals are under stress due to the recent elevated sea temperatures, and are exhibiting purple tissue surrounding both the extraction and outplant sites. Replicate-2 has seen both medium-sized fragments return to full health this month, whereas 2 of the fragments in Replicate-3 have declined in health.