Coral bleaching

A recent report released by STINAPA notes the occurrence of coral bleaching on Bonaire between 2016 and 2020.  During this study, coral bleaching was detected every year, highlighting the need for continuous monitoring and rigorous conservation measures to build resilience moving forward.

The beautiful corals of Bonaire are loved for their stunning array of colors, but what many don’t realize is that these colors are not from the coral themselves, but small microscopic alga, referred to as zooxanthellae or symbiodinium, living within them.  This alga 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.

Photo credit: Kai Wulf

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 alga, 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.

Photo credit: Kai Wulf

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 10% coral mortality among populations at 10m 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 35m 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, Figure 1.  At each location, quadrants were photographed at depths of 10 and 25m, with additional photographs taken at 5m 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% of corals in 2016, 55% in 2017, 9% in 2018, 24% in 2019 and 61% in 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 10m, but no significant differences between 10 and 5m.

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.

Map of the 10 coral bleaching survey sites on the leeward coast of Bonaire and Klein Bonaire. (STINAPA, 2021)

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 build stronger more resilient environments to combat the threats of climate change moving forward.

https://www.dcbd.nl/document/coral-bleaching-bonaire-national-marine-par...

Article published in BioNews 44

STINAPA report

Mass coral bleaching is becoming more frequent and widespread and poses a major threat to coral reefs worldwide. Mass coral bleaching is a response to thermal stress triggered by high Sea Surface Temperatures (SSTs) or ultraviolet radiation attributed to changing regional and global climate patterns. Since 2016, STINAPA Bonaire has surveyed the severity of coral bleaching in the Bonaire National Marine Park at 10 sites on the leeward coast. Each year, corals exhibited signs of thermal stress including paling, partial bleaching, and fully bleaching, but no mortality. Since 2016, the year with the lowest percentage of corals affected was 2018 (9%) and the year with the highest percent of corals affected was 2020 (61%). Corals deeper in the water column were more susceptible to thermal stress in all years, but susceptibility trends by site were not consistent throughout the study. While addressing the global-scale causes of coral bleaching is daunting, STINAPA Bonaire monitors the severity of coral bleaching and helps develop local management strategies that may improve the resistance and resilience of coral reefs in the Bonaire National Marine Park to climate change.

At the end of 2016 / beginning of 2017 a coral bleaching event was reported on Bonaire. Directly after the event quadrant photographs were taken to inventory the impact of the bleaching event, at:

• Playa funchi
• Rei Willem Alexander, no-dive reserve
• Karpata
• Oil slick leap
• Reef scientifico
• Playa lechi
• Invisibles
• Vista blue
• Mi Dushi
• Ebo's special

The photographs have been analysed to report on impacted species and magnitude. 

Please contact the DCBD administrator or STINAPA for access to the raw digital photographs.

What is coral bleaching?

When corals are stressed by changes in conditions such as temperature, light, or nutrients, they expel the symbiotic algae living in their tissues, causing them to turn completely white.

Can coral survive a bleaching event? If the stress-caused bleaching is not severe, coral have been known to recover. If the algae loss is prolonged and the stress continues, coral eventually dies.

Warmer water temperatures can result in coral bleaching. When water is too warm, corals will expel the algae (zooxanthellae) living in their tissues causing the coral to turn completely white. This is called coral bleaching. When a coral bleaches, it is not dead. Corals can survive a bleaching event, but they are under more stress and are subject to mortality.

In 2005, the U.S. lost half of its coral reefs in the Caribbean in one year due to a massive bleaching event. The warm waters centered around the northern Antilles near the Virgin Islands and Puerto Rico expanded southward. Comparison of satellite data from the previous 20 years confirmed that thermal stress from the 2005 event was greater than the previous 20 years combined.

Not all bleaching events are due to warm water.

In January 2010, cold water temperatures in the Florida Keys caused a coral bleaching event that resulted in some coral death. Water temperatures dropped 12.06 degrees Fahrenheit lower than the typical temperatures observed at this time of year. Researchers will evaluate if this cold-stress event will make corals more susceptible to disease in the same way that warmer waters impact corals.

[NOAA, 10-10-2017]

Coral bleaching occurs when corals expel their symbiotic algae, called zooxanthellae, or when zooxanthellae expel their photosynthetic pigments during times of high environmental stress. The exact reason why corals bleach has not yet been determined, but it is theorized that a combination of multiple environmental stress factors is the cause. It is also possible that coral bleaching serves as an adaptive mechanism by allowing different types of zooxanthellae, which may be more stress-resistant than the original zooxanthellae, to colonize the coral. Temperature, salinity, over-sedimentation, anoxia, presence of pollutants, and high amounts of UV irradiation are all factors thought to contribute to bleaching. Extensive coral bleaching research has been conducted since the mass bleaching event of 1998, but there is no data on the frequency of coral bleaching on Bonaire, Netherlands Antilles. This paper proposes a monitoring program that may be implemented to collect coral bleaching and recovery data on Bonaire’s reefs.

This student research was retrieved from Physis: Journal of Marine Science I (Fall 2006)19: 45-48 from CIEE Bonaire.

Bleaching is caused by the combination of light and temperature stress on corals, and can result in coral mortality. On the island of Bonaire, a monitoring program continuously records light and temperature data at 13 sites along the leeward shore of the island. In order to predict where bleaching would occur a light and temperature index was developed. Natural rages of temperature and light were determined from the monitoring program. Ranges were coded and codes were multiplied to calculate the index. Four sites were chosen. Two that were assumed to be impacted by anthropogenic modifications and two sites where there is little modification of the shoreline. Corals at the four sites were surveyed for bleaching three times during October, and then compared to the index, which was a mean of six values from the two weeks prior to sampling. Impacted sites were hypothesized to have the highest index numbers due to anthropogenic influences. However, a higher index corresponded to a lower percentage of bleaching. Impacted sites did not seem to have a significantly higher amount of bleaching then at low-impacted site. An interesting trend is noted in light levels that may be related to runoff or nutrient addition. The prevailing current along the west coast of Bonaire is from south to north. Light levels at 12 m and 20 m decreased from a high at the southern most site.

This student research was retrieved from Physis: Journal of Marine Science IV (Fall 2008)19: 40-44 from CIEE Bonaire.

Coral reefs worldwide are currently jeopardized by anthropogenic factors such as land-based pollution, coastal development, and sediment erosion. In the Caribbean alone, nearly two-thirds of coral reefs have been deemed as threatened. This study investigated the potential negative effects of water quality and eutrophication, Enterococci bacteria (found in human gut), and sedimentation on coral disease, bleaching, and macroalgal growth on the near shore reefs of Bonaire, N.A. Monitoring sites were defined according to their proximity to anthropogenic activity: “more impacted” or “less impacted” (< 200 m and > 200 m from coastal development, respectively). Water samples at 5 m were collected weekly and at 12 m biweekly from each site and tested for nutrient concentrations (NO3, NO2 - , NH4-N, PO4), Most Probable Number of Enterococci bacteria, sedimentation rates, and particle size distributions. Video transects (100 m) were also taken at defined depths and analyzed for live coral cover and diversity, percent disease and bleaching, and macroalgal cover. Data showed elevated NH4-N levels at all sites, Enterococci bacteria present at 3 of the 4 sites (mainly at 5 m), and sediment particle counts showed significant differences among sizes at both depths and between the interaction of size and impact at 12 m. There was also a strong trend of finer grained sediments at high impact sites and coarser grained sediments at low impact sites. Very little overall coral disease (1.105 ± 1.563 % at more impacted sites and 0.400 ± 0.566 % at less impacted sites in 12 m) and bleaching (3.245 ± 0.615 % at more impacted sites and 1.390 ± 1.966 % at less impacted sites in 12 m) was found on the reefs however, neither were present at 5 m. There was significantly more macroalgae at 12 m and a strong trend of more macroalgae at the deeper, more impacted sites. This study suggests that increased anthropogenic activity on Bonaire is contributing to the increased NH4- N levels, Enterococci bacteria presence, and finer particle sediments, which future studies may correlate significant interactions between these parameters and coral disease, bleaching, and macroalgal growth.

This student research was retrieved from Physis: Journal of Marine Science VI (Fall 2009)19: 35-43 from CIEE Bonaire.

Certain goby species, including the Peppermint Goby (Coryphopterus lipemes), Sharknose Goby (Elacatinus evelynae), Glass Goby (Coryphopterus hyalinus), and Bridled Goby (Coryphopterus glaucofraenum) are known to dwell on brain coral species Colpophyllia natans, Diploria labyrinthiformis, and Diploria strigosa. Coral degradation (eg. bleaching and disease) can have adverse effects on coral-dwelling fishes, such as these goby species. The purpose of this study was to display how coral bleaching and disease affect the goby populations that live on brain corals. Goby abundance was compared between healthy and bleached specimens for each observed species and specimens in total. Amongst all the coral species, healthy or bleached, the greatest number of gobies was observed on healthy C. natans individuals (64 gobies). In total, there was a greater number of gobies dwelling on the bleached corals than healthy corals (71 and 67 gobies, respectively). Goby density was calculated by dividing the number of gobies dwelling on a brain coral by the surface area of each coral head. Average goby density on bleached coral heads (0.0038 ± 0.0040) was found to be significantly greater than average goby density on healthy coral heads (0.0011 ± 0.0006) (t-test; d.f.=12; p=0.0178). Although statistically significant, this result may not be biologically significant. The results imply that gobies can persist on moderately degraded brain corals. This suggests that gobies are resistant to early stages of degradation due to bleaching.

This student research was retrieved from Physis: Journal of Marine Science XVIII (Fall 2015)19: 70-76 from CIEE Bonaire.

El Niño Southern Oscillation (ENSO) events are known to bring high sea surface temperatures and in turn cause coral bleaching. The Fall 2015 ENSO event has had record breaking temperatures and has been severely detrimental to Pacific coral reef ecosystems. To gauge the effect this ENSO event would have on the Caribbean, this study looked at the frequency and severity of bleaching and paling, during this ENSO event. The bleaching was measured along a 2 m wide by 10 m long transect. Coral colonies along the transect were observed once a week for four weeks and the water temperature was recorded hourly. At the end of data collection, the overall number of corals experiencing bleaching was recorded and the percent difference in paling and bleaching from week to week was measured. At the end of the four weeks it was found that 60 out of 192 coral colonies were experiencing some form of bleaching. By the fourth week there was no significant increase in bleaching, and paling had significantly increased until week four. This trend followed the decrease of water temperature from week one to week three with signs of coral recovery, but there was also evidence of water temperature starting to increase again by week four.. This study shows the resilience of Bonairean reefs and that this ENSO event may have a lesser affect on the Caribbean coral reefs compared to the Pacific.

This student research was retrieved from Physis: Journal of Marine Science XVIII (Fall 2015)19: 47-52 from CIEE Bonaire.

Unusually warm ocean temperatures surrounding Bonaire during the late summer and fall of 2010 caused 10 to 20 % of corals to bleach (Fig. 1). Bleaching persisted long enough to kill about 10 % of the corals within six months of the event (Steneck, Phillips and Jekielek Chapters 2A – C). That mortality event resulted in the first significant decline in live coral at sites monitored since 1999 (Fig. 2). Live coral declined from a consistent average of 48 % (from 1999 to 2009) to 38 % in 2011 (Steneck Chapter 1). This increase in non-coral substrate increased the area algae can colonize and the area parrotfish must keep cropped short (Mumby and Steneck 2008). For there to be no change in seaweed abundance would require herbivorous fish biomass and population densities to increase, but they have been steadily declining in recent years. This decline in parrotfish continues despite the establishment of no-take areas (called Fish Protection Areas – FPAs) and the recent law that completely bans the harvesting of parrotfish. The other major herbivore throughout the Caribbean is the black spined sea urchin, Diadema antillarum. However, since 2005 Diadema abundance has steadily declined. Damselfishes continue to increase in abundance (except in FPAs) and their aggressive territoriality reduces herbivory where they are present. These declines in herbivory resulted in a marked increase in macroalgae (Steneck Chapter 1). Although patchily distributed, algae on some of Bonaire’s reefs are approaching the Caribbean average (Kramer 2003). All research to date indicates that coral health and recruitment declines directly with increases in algal abundance (e.g., Arnold et al 2010).
On the bright side, predatory fishes are increasing in abundance in general but increasing most strongly in FPAs. Typically, responses to closed areas take 3 - 5 years to begin to manifest themselves. Predators of damselfishes have increased significantly in FPA sites and there, damselfish abundances are trending downward. These trends are the first signs of changes in the FPAs, and they are encouraging.
Overall, Bonaire’s coral reefs today are more seriously threatened with collapse than at any time since monitoring began in 1999.
 
Monitoring Results
The abundance of live coral at the monitoring sites has been remarkably constant since 1999. However, the bleaching related mortality event (Fig. 1) resulted in the first marked decline in live coral.
Seaweed abundance (“macroalgae”) increased sharply in 2011. While the greatest increase in algae occurred at the 18th Palm site where effluent could have increased nutrient levels, most of the other sites showed marked increases in algal abundance (see Steneck Chapter 1). Coralline algae, which has been shown to facilitate coral recruitment, remains at or near unprecedentedly low levels (Fig 2). Herbivory from parrotfishes and the grazing sea urchin Diadema antillarum remains at or near the lowest levels recorded since monitoring began in 1999 (Fig. 3 and see Cleaver Chapter 5). Herbivory from parrotfish is widely thought to be most important (e.g., Steneck and Mumby 2008) but territorial damselfishes can negate parrotfishes’ positive effects by attacking grazing herbivores and preventing them from effectively grazing (Arnold et al 2010). Damselfish abundances have trended upward in recent years (Fig. 3). However, there is a hint of a reversal to this trend in the FPAs (see Arnold Chapter 3). This reversal is consistent with the possibility that areas without fishing have elevated abundances of damselfish predators such as species of groupers and snappers (Randall 1965)  
Predatory fishes including snappers, groupers, barracuda, grunts and others increased in abundance at our monitored sites (Fig. 4 and see DeBey Chapter 6a). Specific predators known to eat damselfishes (see Preziosi Chapter 6b) show variable population densities with only a hint of an increase in 2011.   
Predatory fishes increased in abundance in both biomass (most striking) and population densities (Fig. 5). While biomass of predators in FPA and control sites is identical, the population density of predators is slightly greater at FPA sites
Coral recruitment remained lower than recorded in 2003 and 2005 (Fig. 6). However, the abundance of juvenile corals was higher in 2011 than was quantified in 2009