How can I know the difference between healthy and dead reefs?

In general, a healthy Caribbean reef typically has relatively high coral cover (at least 20% of the seafloor is living coral), and low levels of fleshy macroalgae (plants that are longer than a few centimeters and are edible to fish and crustaceans). Healthy reefs also contain a high abundance of fish and invertebrates of of various species (i.e., high biodiversity). You will know when a reef is healthy because it is colorful, vibrant and filled with life.

A reef is generally considered “functionally dead” when it no longer provides adequate ecosystem services to animals and humans. A dead reef typically has less than 5% coral coverage and is predominantly characterized by coral skeletons covered in macroalgae. Biodiversity is low, and often the reef matrix has been or is actively being eroded. You will know when a reef is dead or losing function because it will lack vibrant colors and an abundance in marine life.

If all corals in an area of reef have died, can the reef ever recover?

Two key processes are essential to reef recovery: (1) herbivory and (2) coral recruitment. Macroalgaes are incredibly quick to establish themselves on bare reef skeleton and can use various tactics including chemical defense to ward off new corals from establishing themselves. Herbivory is thus imperative in ensuring macroalgae levels are controlled, and is only possible if existing populations of grazers such as urchins, parrotfish and surgeonfish are healthy. The colonization by new coral larvae (known also as coral recruitment) and their success in growing into adult corals is critical to coral reef recovery. Coral recruitment relies on healthy corals to sexually reproduce within the region. Corals only spawn once a year, and must be in proximity to one another to allow gametes (sperm and egg) to fertilize in the water column, therefore pockets of healthy and mature reefs are necessary to support the recovery of neighboring degraded reefs.

What does coral loss mean for fish and other forms of marine life?

Imagine trying to live in the rubble of your home which was recently destroyed by a hurricane or earthquake. This is how we imagine dead coral reefs as they crumble and erode due to wave energy. Dead reefs fail to attract and retain healthy fish populations because they seldom provide the habitat space and necessary food sources to sustain productivity and diversity on the reef.

What is coral bleaching?

Coral bleaching is a response to thermal stress that causes corals to expel the algae within their tissue that give them both their color and their main source of food. While corals can survive in a bleached state for up to several weeks, they are vulnerable and far more susceptible to mortality due to poor water quality and disease.

Should we relocate corals into tanks during heat-stress events?

While temporarily relocating nursery corals to cooler environments such as tanks may increase survival rates, it does not select for corals that are resilient to warm conditions, nor does it increase their heat tolerance. Therefore this is an important strategy when trying to preserve species and genetic diversity, but is not necessarily effective for rearing corals that can be expected to thrive when outplanted into ‘real world’ conditions.

In addition to heat stress, what are the major drivers of degradation on coral reefs?

Fertilizer, chemical and sewage pollution has a negative impact on coral reefs by promoting algal and bacterial growth which leads to macroalgae proliferation, the emergence of disease vectors in corals and the poisoning of coral tissue with synthetic and toxic chemicals. Even products as simple as sunscreen or garden fertilizers can contain various agents that reduce water quality and have adverse impacts on corals. Furthermore, deforestation and coastal development increases the likelihood of land-based sediments, litter and pollution to enter the marine environment.

Does coral reef restoration make sense in a changing climate?

Current projections for a warming climate makes coral restoration even more critical, albeit challenging. With only a portion of existing genotypes capable of enduring warmer conditions, it is highly unlikely that these resilient corals will be close enough to each other to sexually reproduce and disseminate their genetics. Active restoration is essential to propagating these resilient genotypes, facilitating their reproduction, and accelerating the ability of a species to evolve in the face of rapidly changing environmental conditions.