Reefscapers Acropora digitifera bleaching Maldives

Acropora digitifera colony – coral bleaching over 10 days (lab conditions)

Coral Propagation – Monthly Progress

At Landaa during May, a total of 18 new coral frames were transplanted, sponsored by guests (6), online (1) and the Resort (11). We also monitored (cleaned, repaired, photographed) a total of 669 coral frames.

At Kuda Huraa, we transplanted 280 coral fragments on five frames, and we maintained the frames around the Water Villas. During the middle of May, we experienced a period of bad weather caused by the unusually powerful Cyclone Tauktae, which increased the movement of sand around the island. We had to relocate 140 Water Villas frames for protection from the shifting sands (four small frames were lost), delaying some of our Reefscapers coral propagation and monitoring work.

At the Blue Hole site, many of the coral colonies were paling and fluorescing, and a few colonies had died. At the Channel site, there was some coral bleaching and mortality; Montipora digitata continues to proliferate, outcompeting the other species. We also observed some areas of poor water circulation, which leads to higher water temperatures and lower oxygen levels, and results in algal overgrowth and the presence of cyanobacteria.

Reefscapers coral frames healthy corals

Healthy corals growing on our reefscapers coral frames (May 2021)

Bleaching Monitoring Surveys (Coral Watch)

On 15 May, we started our Coral Watch surveys at the Water Villas site, and will expand to other sites throughout June. At each site, 24 selected healthy coral colonies of four different species (both robust and less resilient species) will be monitored weekly (ideally). We will photograph each colony and assign colour codes.

From our initial results, we can see that colonies of Acropora tenuis and A. humilis remain healthy, with no signs of paling or bleaching (photos, below).

However, aside from our official survey sites, we have observed some coral colonies of various species start to show signs of stress. During the second week of May, colonies of A. valida were observed fluorescing, which increased during the month in other Acropora species, and was seen alongside some paling colonies (severe bleaching has not been yet observed.)

Reefscapers coral frames Acropora humilis (May 15th & 26th)

Acropora humilis (May 15th & 26th)

Reefscapers coral frames Acropora tenuis (May 15th & 26th)

Acropora tenuis (May 15th & 26th)

Bleaching Alerts

According to the NOAA Bleaching Alerts, North Malé atoll changed from “Watch” to “Warning” (on 10 May) before returning to “Watch” status (20 May). For the first days of May, NOAA reported sea surface temperature (SST) values above 30°C, with an increase of 0.2 to 0.5°C compared to April. The lower air temperatures and sunlight levels have lowered the bleaching risks, although a few coral colonies have been observed fluorescing and paling.

Reefscapers coral bleaching Acropora valida

Coral bleaching – Acropora valida

Reefscapers coral frame Acropora digitifera colony

Coral paling – Acropora digitifera

Coral Plates in Aquarium One (plates KH01, KH02, KH05) and Aquarium Two (plates KH03, KH04, KH06)

Overall, the coral plates in both aquaria remain healthy. Plates in Aquarium-Two continue to collect more algae, and weekly cleaning is necessary to prevent coral mortality.

  • KH01 (Acropora digitifera, millepora, Galaxea fascicularis) – healthy, growing steadily and fusing.
  • KH02 (Galaxea fascicularis) – healthy and encrusting successfully.
  • KH03 (Acropora valida) – 26% of the fragments present old mortality. The other fragments continue to calcify and grow steadily.
  • KH04 (replenished December, digitifera, A. millepora) – although this plate is the most overgrown with algae, the fragments are healthy and encrusting.
  • KH05 (Galaxea fascicularis) – continues to grow steadily (we rotate regularly for even light distribution).
  • HK06 (Galaxea fascicularis, new Feb-21) – remains healthy (we rotate regularly for even light distribution).

Reefscapers Coral-Cooling Experiment

Due to the increased threat of climate change to coral reefs exacerbated by warming oceans, we are conducting ongoing experiments to study the overnight cooling of a water mass. The key aim was to identify how long it takes a mass of water to then reach ambient seawater temperature again, post-cooling.

Newton’s Law of Cooling’: the temperature of a body changes at a rate proportional to the difference in temperature between the body and its surroundings; i.e., the rate at which a body loses heat is directly proportional to the difference between the temperature of the body and its surroundings. (This is highly dependent on the nature of the material and the surface area of the body).

Method

We used two outdoor water tanks (white in colour), connected to a constant ocean water flow, and fitted with HOBO temperature loggers. A pump was used to continually stir the water, to distribute the heat evenly. We used an additional temperature logger to record the ambient air temperature.

  • Tank 1: ‘Open Flow’ system with constant flow of seawater.
  • Tank 2: ‘Closed Flow’ system, which was open during the day and closed overnight (with a stirring pump).

Five fragments of Acropora tenuis and five fragments of Galaxea fascicularis were placed in each Tank, a total of 10 fragments per species.

  • The surface area (SA) for Galaxea was calculated by analysing photographs using FIJI by Image J.
  • Growth rates for Acropora were monitored every two weeks using a series of Height, Width and Length to determine ecological volume using the equation: 3.14*POWER(R,2)*Height

Week 1 – The Tank 2 ‘Closed Flow’ cooled overnight, and took 2 to 3 hours to warm to ambient ocean temperature once the flow was opened in the morning. As expected, when the air temperature was low, the water temperature remained lower as well. (The temperature spikes are caused by the morning inflow of warmer water, before it became mixed into the water mass.)

Week 2 revised methodology – Tank 2: now completely closed, to identify the warming rate of the water mass. As expected, this time the water remained cool for much longer, so we will study this in upcoming weeks.

Results of Our Growth Rate Analysis

Following a period of rain, we used a refractometer to test the salinity (as expected, salinity was slightly reduced in the closed-system Tank 2).

  • Tank 1: Galaxea +7%; Acropora +1%; salinity 32 PPT.
  • Tank 2: Galaxea +1%; Acropora +8%; salinity 31 PPT.
  • Galaxea – our results correspond to published studies, showing decreased growth rates in the absence of water flow (Schutter et al., 2010), perhaps due to decreased feeding ability (Wijgerde et al., 2012).
  • Acropora – by contrast, our results showed that Acropora preferred the closed flow system. This is contrary to published literature that shows a consistent flow of water does increase growth rates. (However, our experiments ran for a short time period of just 2 weeks, so are likely unrepresentative of long-term growth rates.)
  • See our results (below), plotting the temperatures for week 1 and week 2, and analysing the growth of both coral species in both tank configurations.
Reefscapers Maldives coral cooling expt wk1

Temperature plots, week 1

Reefscapers Maldives coral cooling expt wk2

Temperature plots, week 2

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