Coral Reefs: A Reef Resilience Toolkit Module

Recovery From Bleaching

If a coral reef is exposed to stressful conditions known to cause bleaching, its fate is influenced by three key attributes: the extent to which corals can withstand elevated stress without bleaching (resistance); the ability of corals to survive bleaching (tolerance); and the ability of coral communities to be replenished (recovery) should significant coral mortality occur. An understanding of these attributes and the factors that influence them can inform management responses that aim to maximize the resilience of reefs to climate change.

Four conditions determine the outcome of stressful temperature for coral reefs: bleaching resistance, coral tolerance, reef recovery and human adaptive capacity. Each of these is influenced by a suite of factors that, in combination, determine the resilience or vulnerability of the system. Factors that can be influenced by local management actions are highlighted in green. Factors shown in black cannot be changed through local management interventions, but can be incorporated in the design and placement of management initiatives to enhance ecosystem resilience. Source: Marshall and Schuttenberg 2006, adapted from Obura 2005

Resistance

The variability that characterizes bleaching events points to an important fact: individual corals vary in their bleaching responses to light and heat stress. Variability in bleaching response has been observed within individual coral colonies, among colonies of the same species. These taxonomic variations are further compounded by spatial patterns, with corals of the same species often showing different bleaching responses at different locations. These patterns have been observed at scales ranging from meters to thousands of kilometers.1 Some of the factors that influence the coral response include:

• Sea surface temperature patterns, especially at large scales
• Regional and local differences in weather
• Proximity to upwelling of cooler water.
• Water currents and flow regimes
• Genetic identity of corals
• Genetic variation in zooxanthellae

For more discussion of some of these factors, see Identifying Resilience in this module.

Tolerance

Long term recovery from bleaching will depend on larvae recruitment from source reefs. Photo © S. Wear

For those that survive a bleaching event, characteristics that influence a return to a healthy coral reef community include:

• Event magnitude: The magnitude of bleaching, for example, some corals only lose some of their zooxanthellae.
• Immune system response: A weakened immune system less capable of fighting off disease. Because coral reefs are weakened by a bleaching event, their ability to reproduce and fight diseases may be impaired.
• Cryptic survival: The number of coral colonies that initially appear dead, but then regenerate from deep tissues. These are called “phoenix corals.”
• Return interval of bleaching: If another bleaching event occurs before all colonies have returned to pre-bleaching event health.
• Grazing: Healthy herbivore population which retards potential invasion on bleach damaged coral tissue.
• Metabolic adaptations: Persistence of these coral reefs may depend, partly, on the differential ability of some corals to acclimatize their physiology to warmer temperatures. Note this is different than selection, this is phenotypic plasticity in physiological capabilities in response to increasing temperatures.

Recovery

Reefs that suffer substantial mortality face different challenges than those where the majority of corals manage to survive the bleaching event. The biggest difference is a much longer time lag until returning to pre-bleaching structure, for those that have recovered vs. those that have survived. How long it takes a coral community to recover from bleaching related mortality depends on a variety of factors, including the supply of larval recruits, the recruitment conditions at the site, and controls on post-recruitment survival, and growth rates. Some of these factors are further outlined below.

• Favorable recruitment conditions: These include good water quality, open hard substrate for settlement, presence of coralline algae (good settlement substrate), and healthy herbivore populations.
• Connectivity: Reefs with high mortality after bleaching depend on connectivity to other sources of live corals for re-seeding. For example, it is possible for reefs receiving great numbers of larvae from other source reefs to recover in a relatively short time span (~10 years), provided that recruitment conditions are favorable.
• Larval supply: Regardless of how good recruitment conditions are (e.g., availability of substrate, presences of important herbivores) reefs that do not receive a robust supply of larvae from source reefs will be slow to recover (decades or longer).
• Grazing: If important herbivores are missing, overgrowth by algae can slow reef recovery by taking up space that could otherwise be available to coral recruits.
• Natural selection: The recovery of coral reefs may be facilitated by settlement of larvae from nearby, more heat-resistant corals that survived the temperature-driven bleaching event. Over time, this could lead to heat-tolerant species increasing their distribution range into habitats previously dominated by other species.
• Synergistic effects: factors not previously recognized as important to resilience, such as robust tissue regeneration, high competitive ability of the corals, seasonal dieback in seaweed bloom, a backdrop of an effective marine protected area system, and moderate water quality, can result in rapid coral recovery, as was the case in the Keppel Islands, Australia2