Bleaching

The Impact of 2005 and 2010 Coral Bleaching Events

Recent episodes of coral bleaching are the greatest modern threat to coral reefs of the USVI and, although global forces are the cause, there are local management actions that can limit degradation of vital reef resources. Unprecedented high seawater temperatures affected the USVI and the wider northeastern Antilles from August to October of 2005 (Eakin et al. 2010). Climate modeling suggests that human‐induced global warming from the emission of greenhouse gases caused this event (Donner et al. 2007). The year 2010 started warmer than 2005 and experts predicted another mass bleaching event, however when Hurricane Earl passed the USVI sea surface temperatures dropped rapidly in an example of tropical storm cooling (Manzello et al. 2007) averting more significant bleaching and mortality.

Thermal bleaching is caused by exposure of corals to high temperatures and light. This initiates a breakdown of the microscopic brown algae (Symbiodinium) that live in coral tissue and a loss of the food they supply to the coral animal. Eventually these symbiotic algae are expelled from the coral, turning the tissues pale to stark white (Fig. 1). Bleached corals are at increased risk of disease (Brandt and McManus 2009) and death (Baker et al. 2008).

The 2005 coral bleaching event had major, negative impacts to coral communities in the USVI. About 60% of shallow corals in water less than 25 m depth were approximately 60% bleached to stark white. This led to unprecedented levels of white disease and severely affected the foundational reef building boulder star coral (Montastraea annularis spp. complex), leading to an approximate 50% loss of coral cover. Boulder star corals are responsible for much of the habitat creation that supports USVI fisheries and other coral species. Deeper, mesophotic reefs in waters greater than 25m depth were less affected by bleaching, but subsequently experienced high and enduring levels of white disease that led to a loss of coral cover of about 20%.

There was high site‐to‐site variability in the mortality suffered following bleaching. Most monitored mesophotic sites fared better than shallow water sites. However, mesophotic wall environments showed extensive bleaching and, although cover was not monitored in 2005, they appear to have lost large amounts of cover. In general, low coral cover reefs showed less dramatic or no loss of coral cover. This may reflect a coral community composed of species that are more resistant to coral bleaching related mortality. Offshore shallow sites with higher abundance of boulder star coral showed higher losses of coral cover, which ranged from 22% (Flat Cay) to an extreme 87% (Mutton Snapper).

The USVI shallow water coral communities’ response suggests that Caribbean reefs under seawater warming conditions will shift to higher dominance of more resistant species. The boulder star corals and the giant brain coral (Colpophyllia natans) were shown to be very susceptible to disease after bleaching and this led to large losses of coral cover. On the other hand, moderate bleaching, no to low disease, and no to low cover loss was seen in the resistant small massive species Diploria strigosa, Montastraea cavernosa, Porites astreoides, and Siderastrea siderea. The species Agaricia agaricites and branching Porites were very susceptible to bleaching and mortality, but are showing signs of fast‐regrowth and resilience.

Reefs that saw losses of coral cover also may show differing capabilities for recovery. On many St. Croix reefs areas exposed due to loses in coral cover were colonized by an abundance of macroalgae and filamentous cyanobacteria, indicating that fishes and invertebrates that normally control algae were not present in sufficient numbers. St. John and St. Thomas, with more robust fish communities, were less likely to gain macroalgae and cyanobacteria. This might indicate that reefs with higher protection of critical ecological fish species, such as parrotfish, will rebound faster (Mumby and Harborne 2010). The specific mechanisms for this in the USVI need further research.

Recommendations

There are local management actions that work to protect coral reefs in the face of a changing climate (Marshall and Schuttenberg 2006) and ensure they continue to provide economic and social benefits for the USVI. These strategies involve understanding how corals are responding across the seascape and applying best management practices to support reef health. Data from the TCRMP can help determine the most effective actions with the least direct and indirect costs to the USVI economy.

Strategy #1 Reduce local stressors, such as removal of ecologically important species (e.g., large‐bodied parrotfish), land‐based sources of pollution, and physical damage to reefs. Bleached reefs are most fragile during bleaching and the following year. Special restrictions might be considered during this period to reduce stress and disease.

Strategy #2 Identify reefs resistant to climate change. The TCRMP has identified deeper mesophotic reef systems as resistant to bleaching. Mesophotic reefs likely exceed shallow water reef area in the USVI and the may have the ability to reseed degraded shallow systems. Special management could be applied, including reducing fish traps that remove ecologically important fishes and damage coral, and restricting anchoring and cable laying. These reefs should be fully researched to understand their extent, their future persistence, and their ability to re‐seed young corals to degraded shallow reefs.

Strategy #3 Identify reefs and species susceptible to climate change. A shift away from reef dominance by boulder star corals to small corals and algae would degrade the fisheries and tourist potential of USVI reefs, yet shallow boulder star coral reefs are the most susceptible to bleaching. These reefs should be fully catalogued and set aside as special areas. Many USVI protected areas with the power to assist coral reef recovery (e.g., no take reserves) are sited outside the densest areas of star corals. For example, the East End Marine Park’s no‐take area inside a barrier reef contains relatively little boulder coral, whereas the outer barrier reef has large populations of these important species.

Bleaching Resistant Species in the Caribbean and Implications for Coral Reef Management

TCRMP research has uncovered new information on the response of Caribbean corals to thermal stress events that has high relevance for management and was recently published in the journal Ecosphere (Smith et al. 2013). Shallow reefs of the USVI were affected by thermal stress in 2005 and 2010 when temperatures exceeded a stress point for many corals known as the bleaching threshold. The longer corals spend above the bleaching threshold and the greater the temperature surpasses the bleaching threshold the more likely corals are to bleach, or lose the pigmented microalgae that live in their cells and give corals the lion’s share of their nutrition. Weakened corals are susceptible to disease and direct mortality. In 2005 the temperatures were very high for very long and the majority of shallow water corals bleached, with a loss of 50-60% of the coverage of reef corals in the USVI (Miller et al. 2009). In 2010 the temperatures were on their way to surpass the heat stress in 2005, but temperatures were cooled by the upwelling of deep water stimulated by the passage of Hurricane Earl (August 30th). Heat stress was less than half that experienced in 2010, with limited bleaching, disease and mortality, often confined to specific areas (Brandt et al. 2013).

The TCRMP was able to use its resources to provide one of the most comprehensive data sets of coral reef response to thermal stress and bleaching in the Caribbean. One central question to ask of the data was were the responses of very different coral species similar over the extreme and mild thermal events? We used data from 18 of our shallow water (<25m/83’ depth) monitoring sites to look at the response of nine coral species for which we had hundreds to thousands of observations. The response was divided in to bleaching (the proportion of corals that bleached and the extent of bleaching on the colonies surface), disease (the proportion of corals that showed white disease signs), and mortality (the proportion of colonies that had partial or total mortality and the change in the reef coverage of the species).

We found that there were three distinct groupings of species that we labeled “types”. Type I had high bleaching and initial mortality, no subsequent white disease, and severe losses of cover (exhibited by Agaricia agaricites and branching Porites species); Type II had moderate bleaching and initial mortality, high subsequent white disease prevalence, and severe losses of cover (exhibited by Colpophyllia natans, and Orbicella spp.); Type III had moderate to low bleaching and paling, low to no subsequent white disease, and low to no loss of cover (exhibited by Diploria strigosa, Montastraea cavernosa, Porites astreoides, and Siderastrea siderea). The biggest surprise was that a group of species (Type III) was almost entirely resistant to bleaching induced mortality. These species may become progressively more dominant on coral reefs of the Caribbean with increasing frequency and severity of coral bleaching events. In contrast, the Type II species, including main reef building species in the Caribbean, Orbicella spp., was only moderately affected by bleaching across our sites, but was highly susceptible to disease after the bleaching event. However, for thermally sensitive Type I and II species partial mortality was much more common than whole colony mortality and the surviving tissues may aid in recovery.

Recommendations

The findings of our research have very important implications for localized management with regards to thermal stress that includes which coral species do not need a lot of active management and which corals should be managed and where management might be most effective.

Strategy #1 Do not expend a lot of effort to protect Type III species, as they will likely weather future thermal stress events quite well. These species also tend to be ones that are quite resistant to other localized stressors, such as sedimentation. Type III species will persist through at least the first half of the 21st century and will likely become more dominant in reef systems. However, they may not increase in abundance sufficiently to replace the species lost to thermal stress. Hence, even with persistence, ecological roles played by reefs, such as the provision of nursery and adult habitat for fishes will likely degrade.

Strategy #2 Stress local management actions that promote the regrowth of surviving coral tissues. These include the usual actions, such as limiting land-based sources of pollution and sediments and reducing fishing that affects populations of herbivorous fishes and urchins that consume seaweeds that compete with coral for space. As an example, the highly thermally sensitive Agaricia agaricites is recovering after 2005 from tissue fragments in reefs with low land based sources of pollution and low seaweed abundance (e.g., Flat Cay), but not at reefs that are clearly still being impacted by local stressors (e.g., Fish Bay).

Strategy #3 Large Orbicella spp., some many hundreds of years old, were mostly, but not completely, killed in 2005. These corals are akin to the giant sequoia trees of central California that are long-lived and virtually irreplaceable. The fact that the centuries old genotypes are hanging on in USVI reefs is cause for hope and action. Every effort should be made to protect these recovering ancients, including education and active restoration. As for the latter, restoration of Orbicella spp. has not yet been attempted in the USVI to our knowledge. However, the fact that un-eroded large skeletons still remain intact provides an opportunity for replantation of fragments that could re-sheet old skeletons, short-circuiting the recovery process and protecting essential habitat. This area is very ripe for research into effective restoration strategies.

Impacts of the 2019 Coral Bleaching Event and Comparison with Previous Events

In 2019 the USVI suffered its third mass coral bleaching event in the last 15 years. Sea surface temperatures of the USVI have been increasing at a rate of about 0.2°C per decade since the 1980’s, increasing the incidence of coral thermal stress, measured as degree heating weeks (DHW), and coral bleaching. Major bleaching events, with heat stress values above 8 DHW, have occurred in 2005, 2010, and 2019. Weekly average sea surface temperatures in 2019 peaked at 29.8°C during the week of Sep. 8., compared with 30.4°C in 2005 and 30.1°C in 2010. Combined with the duration of heat stress this caused max DHW values of 15.5, 9.8, and 8.9 for 2005, 2010, and 2019, respectively.

The bleaching response of corals at TCRMP sites followed a trajectory across years that reflected the different relative severity of the events. The year 2005 showed the highest prevalence of bleaching (75% of colonies across sites) and also the most dramatic mean extent (82% of the colony surfaces affected). The peak of beaching was sometimes missed at a given site in 2005 and, thus, sites-specific values are underestimated in some cases.

The year 2010 had a moderate prevalence of bleaching (47%) with low extent on colonies (14%). In 2019 the prevalence of bleaching was similar to 2010 (49%), but the extent on colonies was higher (44%). This is interesting as the years had similar peak DHW values. With the exception of Cane Bay and Cane Bay-Deep, all St. Croix sites were completed in late August to early September 2010, before peak heat stress and, potentially, maximum bleaching response. However, even at sites that were sampled at or after peak heat stress, such as St. Thomas and St. John sites, the extent was low. More research is needed to understand why 2010 was milder when colonies did show bleaching.

The coral community response to the 2019 thermal stress is not yet known. St. Thomas and St. John sites were resurveyed between mid-February and early March 2020 to assess impacts from bleaching. Sites unaffected by stony coral tissue loss disease (SCTLD) appeared to show little change in coral cover up to March 2020, suggesting that there was limited mortality from bleaching alone. All sites will be resurveyed in 2020 and the territory-wide impacts will be assessed as a research highlight in the 2020 report.