Back from the bleach – how isolation helps coral reefs recover

Coral bleaching is a serious issue, but we’re learning how reefs can best recover. AFP/Great Barrier Reef Marine Park Authority

Coral reefs around the world are under pressure from multiple threats. A burgeoning gas industry – such as that near Gladstone – is one of the newest of these. Pollution, sedimentation, declining water quality and overharvesting are among other ongoing problems.

In the long-term, global climate change is regarded as an additional ominous threat to the viability of tropical coral reefs.

To learn more about how climate change will affect coral reefs, my colleagues and I spent several years studying the resilience of coral communities at Ashmore Reef in the north west of Australia.

Our research suggests that isolation from coastal influences has enhanced corals’ ability to recover after climatic disturbances. Put simply, coral that lives further offshore may recover better after disturbances to climate than coral living closer to the coast.

Bleaching is a clear sign of stress in corals, and occurs when the relationship between the coral and unicellular algae (called dinoflagellate symbionts) breaks down. Without the nutrients provided by the algae (via photosynthesis) the coral host starves.

This breakdown is typically prompted by extreme environmental conditions such as:

  • excessive warming or cooling
  • excessive levels of light/UV
  • excessive levels of wind exposure (such as at low tide).

Bleached corals are less “fit” and far more likely to become diseased or perish.

Coral bleaching has already caused widespread death of corals around the world. The bleaching of corals is a natural response to environmental fluctuations. But it is a catastrophic problem when entire communities of corals become stressed at the same time in a mass-bleaching event.

In the past decade, it has become clear that some species and some reefs are more susceptible to mass-bleaching events than others. More often than not, it is the reefs closest to urban centres that are hardest hit.

Zoe Richards. Copyright Australian Museum

For example, after the 1998 mass bleaching event – believed to be linked to unusually high ocean temperatures – 67% of inshore reefs on the Great Barrier Reef were affected.

Many of these reefs never fully recovered. The damage has been compounded more recently by inundation from fresh water, pollutants, sediment and the two severe cyclones that tracked down the reef last summer.

For all these reasons, reefs isolated from urban centres could provide critical refuges for the survival of coral.

On the other hand, isolated offshore coral reefs may be much more susceptible to climatic disturbances (compared to coastal reefs). Because there are no other reefs nearby, there is no source of coral “larvae” to replenish damaged reefs.

To test these theories we studied the change in coral cover and relative abundance of coral genera over a four-year period (2005 to 2009) at Ashmore Reef National Nature Reserve. Ashmore Reef was seriously affected by the mass bleaching of 1998.

We saw the cover of hard coral increase threefold in all protected and exposed parts of the reef from approximately 10% in 2005 to roughly 30% in 2009. We also saw the cover of soft corals double, from roughly 4% in 2005 to approximately 7% in 2009. We detected significant shifts in the composition of hard corals.

Our results imply that, contrary to expectations, coral recovery can be rapid in isolated locations. This is presumably through self-recruitment (where surviving corals re-seed the population rather than new coral larvae been recruited to the reef from neighbouring locations) and reduced exposure to pressures such as coastal pollution.

Our study has important implications for ecosystem resilience.

AFP/Lucy Trippett

While many reef systems appear to be experiencing systematic declines in live coral cover and unprecedented rates of biodiversity loss, this study has revealed sustained increases in the live coral abundance at Ashmore Reef.

The types of coral that increased in cover at Ashmore Reef include staghorn corals (corals with a branch-like structure). This change added substantially to the complexity of the reef, and is likely to lead to positive flow-on effects to coral-associated biodiversity in the future.

It’s important to note that isolated reefs are not immune from increasing effects of global climate change. Furthermore, the long-term fate of isolated reefs (such as Ashmore Reef) will depend on the recurrence and severity of major disturbances relative to expected rates of recovery.

We can reduce the severity and geographic extent of anthropogenic disturbances to coral by protecting catchments to prevent soil erosion and by reducing the amount of pollutants entering waterways, for example.

This will make a significant contribution to helping coral communities recover from extreme climate events.

This article was co-authored by Dr Daniela Ceccarelli, a marine consultant based at Magnetic Island.