The Big Bleach

Quantifying a Coral Catastrophe in the Maldives

Schooling bannerfish (Heniochus diphreutes) swimming past dead table corals

Schooling bannerfish (Heniochus diphreutes) swimming past dead table corals

2016 was a rough year for the world’s coral reefs. Higher than normal ocean temperatures caused coral bleaching all around the planet, from East Africa across the Indian Ocean and throughout Indonesia, among Pacific islands like Hawaii and Kiribati, in Florida and the Caribbean, from Japan and the Philippines to the South China Sea, with an especially dramatic hit to Australia’s Great Barrier Reef. Bleaching events of this scale have happened only a few times in recorded history and are becoming more frequent as oceans warm. Such catastrophic occurrences threaten the survival of remarkable coral ecosystems as well as hundreds of millions of people who rely on these habitats for food and jobs.

A dead table coral after the recent mass bleaching

A dead table coral after the recent mass bleaching

The recent Hydrous expedition to the Maldives began in late November, about eight months after the worst of this bleaching, so we had a chance to view the aftermath. We had heard reports that ~60% of Maldivian reefs were bleached but, keeping in mind that a bleached reef is not a dead reef and knowing that corals can recover from events like this, we hoped for the best. We did not expect to see much bleaching, if any, because water temperatures had cooled by the time we arrived. Rather, we were interested in assessing post-bleaching mortality and looking for signs of recovery.

Before we unpacked our dive slates, underwater paper, and sampling quadrats, we glimpsed the damage. On our first checkout dive, we saw the skeletons of enormous table-top Acropora colonies that were gray as stone, overturned, swamped by sediment, or overgrown with green algae. It was like viewing the ruins of a once great city. Living corals are colorful; a healthy reef glows with golden browns, blues, and pinks, whereas this reef was dominated by muted grays and greens.

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Still hopeful, we knew that systematic, quantitative data is far superior to personal impressions. We can’t draw conclusions from one dive and a lazy color assessment. Reef monitoring is a central component to Hydrous expeditions and most people on board learned a standardized rapid protocol to assess coral bleaching. This method was developed by Wildlife Conservation Society (WCS) scientist Dr. Tim McClanahan and has been used around the world to monitor bleaching events since 1998, including over 250 sites in 13 countries during the 2016 El Niño. Citing the importance of long-term monitoring and “many eyes on the reef,” WCS scientists developed these methods to also be accessible to local community members, divers, and snorkelers who may not have ecology backgrounds. The Hydrous expedition taught these methods to our unique team of designers, engineers, tech professionals, and Maldivian environmental stewards to monitor the frequency and severity of bleaching on the reefs that we were visiting.

Dr. Woolsey using a quadrat, a clipboard, underwater paper, and a pencil to survey the reef with the WCS protocol  

Dr. Woolsey using a quadrat, a clipboard, underwater paper, and a pencil to survey the reef with the WCS protocol  

The Hydrous team conducted surveys at five sites in the central Maldives: Bandos, Bodufolhudhoo, Rasdhoo, Reethi Beach, and Villingilli (Fig. 1). Three of these sites (Bandos, Reethi Beach, and Villingilli) were monitored with the same protocol during the height of the bleaching in April/May 2016 by a scientist from the International Union for the Conservation of Nature (IUCN). This scientist observed substantial bleaching at these sites: 24% of all coral colonies were bleached at Bandos, 78% at Reethi Beach, and 64% at Villingilli (Fig 2). Surveys were conducted at 2 meters depth at Reethi Beach and 10 meters at both Bandos and Villingilli. [Note: Bandos was monitored at the onset of bleaching in April. When the IUCN scientist viewed this reef at the height of the bleaching, closer to 85% of the reef was bleached.]

Figure 1: Map of the central Maldives, showing 5 sites surveyed by the Hydrous in December 2016. Sites that were also surveyed in April/May 2016 using the WCS protocol are shown with white diamonds. Sites surveyed only in December 2016 are represented by black squares (these data will be used for future comparisons).

Figure 1: Map of the central Maldives, showing 5 sites surveyed by the Hydrous in December 2016. Sites that were also surveyed in April/May 2016 using the WCS protocol are shown with white diamonds. Sites surveyed only in December 2016 are represented by black squares (these data will be used for future comparisons).

Figure 2. Percentage of bleached colonies observed during the mass bleaching event in April/May of 2016. Percentages were calculated by dividing the total number of bleached coral colonies by total number of colonies. Note: error bars are absent in this figure because the protocol uses a running tally of colonies rather than counts per quadrat.

Figure 2. Percentage of bleached colonies observed during the mass bleaching event in April/May of 2016. Percentages were calculated by dividing the total number of bleached coral colonies by total number of colonies. Note: error bars are absent in this figure because the protocol uses a running tally of colonies rather than counts per quadrat.

Using the April/May survey data that the IUCN scientist generously shared with Hydrous, we could compare the state of the same three sites (shown in white diamonds in Fig.1) during and after the bleaching. By the time we arrived in the Maldives in late November, water temperatures had dropped to pre-bleaching levels. We recorded 28-29ºC at each site compared to 30-31ºC measured in April/May 2016 (it’s amazing how a sustained rise of only 1-2ºC degrees can inflict such widespread damage). Though we didn’t observe bleaching, we could still use the WCS protocol to understand the impacts of bleaching by measuring indicators for reef health including live cover of hard, reef-building corals, presence of macroalgae (i.e. seaweed, which competes for space with coral), colony density, and coral mortality.

We found that the reefs had changed considerably after the bleaching (Figs 3-7). Hard coral cover dropped after the bleaching in both Bandos (from 25 ± 3% to 13 ± 2%) and Reethi Beach (from 26 ± 5% to 17 ± 3%), whereas it remained low in Villingilli (~10%; Fig 3). Macroalgae cover increased at both Bandos and Reethi Beach, but remained under 10% (Fig 4). Unfortunately, such reductions in live coral cover and increases in macroalgae cover are signs of a degrading reef. At Villingilli, macroalgae cover was high both during and after the bleaching (19 ± 4% and 18 ± 3%, respectively; Fig 4). 

Figure 3: Mean percentage hard coral cover at each site during (light blue) and after (dark blue) the 2016 bleaching event. Error bars show standard error. Means were calculated by estimating the percentage of hard corals (i.e. reef building corals or scleractinians) in each square meter quadrat then dividing the sum by the number of total quadrats.