Coral monitoring finds annual coral bleaching on Bonaire

The beautiful corals of Bonaire are loved for their stunning array of colors. However, many don’t realize that these colors are not the coral themselves, but small microscopic algae, referred to as zooxanthellae or symbiodinium, living within them.  This algae and coral have a symbiotic relationship, where the zooxanthellae provide nutrients to the coral in exchange for protection and habitat within the coral’s skeletal structure. Under normal conditions, this relationship is mutually beneficially. However, if the zooxanthellae become toxic, the coral can evict their partner, leaving behind its colorless abode.

Climate change

One of the many negative effects of climate change is a slow but steady increase in average Sea Surface Temperatures (SST).  While the exact causes and mechanisms of coral bleaching are still being investigated, one theory that has strong support hypothesizes that bleaching is triggered by the production of excessive abnormal oxygen molecules. As SSTs rise above normal (even if just for just a few weeks), the zooxanthellae are unable to effectively photosynthesize and begin to produce reactive oxygen which can damage coral tissue. As a defensive response, the coral sometimes ejects the algae, leaving its white skeletal structure empty giving it the appearance of being 'bleached'.

If enough corals eject enough zooxanthellae, this becomes known as a mass coral bleaching event. These events can last anywhere from days to months and, in extreme events, even years. Unfortunately, the coupling of worsening water conditions due to human activity (pollution, overfishing and uncontrolled land development) and stressors due to climate change have led to an increase in the frequency and duration of mass coral bleaching events. Without the zooxanthellae producing energy, corals are forced to rely on stored energy reserves and feeding directly on zooplankton. Bleaching events can be dangerous for corals even if they do not result in direct mortality as this can leave them more susceptible to disease, decreases coral spawning success and can lead to long term changes within the community composition.

Building Resilience

Luckily, not all coral, or zooxanthellae, are the same. In fact, new research has uncovered differences between corals which host a single type of zooxanthellae versus those with a more diverse array, where some may be more tolerable to temperature shifts than others. A new theory, known as the Adaptive Bleaching Hypotheses, even states that following bleaching events, the make up of zooxanthellae may shift within corals, allowing new, more resilient combinations of zooxanthellae to move in. This creates the opportunity for coral communities to build resilience after particularly destructive years.

Bonaire

Although global bleaching events have been happening regularly since the late 1990s, Bonaire suffered its first significant coral lost due to bleaching in 2010. During this episode, Bonaire registered nearly ten percent coral mortality among populations at ten meter depth. Since 2016, some degree of coral damage, ranging from paling to full bleaching, has occurred on Bonaire’s reefs every year.  Already, even without the official survey for 2021 being completed, divers have reported bleaching at depths of 35 meter and deeper.

A new report, published by STINAPA, highlights the impact coral bleaching has had within the Bonaire National Marine Park between 2016 and 2020. Each year, after SST began to drop (usually between November and December), STINAPA surveys ten sites within the park, noting signs of bleaching. These sites included eight locations along the leeward side of the island and two off the coast of Klein Bonaire (see map below).  At each location, quadrants were photographed at depths of 10 and 25 meter, with additional photographs taken at 5 meter for four sites starting in 2017.

Trouble in the Deep

Over this four-year study, coral bleaching was detected within the photographed quadrants every year, affecting 26 percent of corals in 2016, 55 percent in 2017, 9 percent in 2018, 24 percent in 2019 and 61 percentin 2020. It should be noted that methodology changes in 2018 may have contributed to an underrepresentation of coral bleaching.

STINAPA found that the corals most susceptible to bleaching are those found at deeper depths. Interestingly, when comparing the three depths, there were significant bleaching differences between 25 and 10 meter, but no significant differences between 10 and 5 meter.

STINAPA also found that bleaching trends from 2020 indicate that certain species of coral are at higher risk of bleaching than others. For example, corals such as Orbicella and Agaricia (Boulder, Mountainous star and Lettuce corals) were more often bleached, yet Madracis species (Yellow pencil and Ten-rayed star corals) appear to be more resilient.

The future

Protecting these corals will require action at all levels. Locally, the government can help build resilience through more effective fishery management, wastewater treatment and promote responsible coastal development and sustainable tourism. Individually, we can all help by minimizing our contribution to pollution, avoiding direct contact with the reef while swimming or diving and wearing reef safe sunscreens in the water.

Together, by promoting a nature first attitude towards conservation, we can help building stronger, more resilient environments to combat the threats of climate change moving forward.

More information

• Find more information and read the full report on the Dutch Caribbean Biodiversity Database.

Text: DCNA
Photos: Kai Wulf
Map: STINAPA