Coral bleaching in Keppel Bay

In the shallow waters of Keppel Bay, coral bleaching on a mass scale is infrequent and recovery is usually fast.

Coral bleaching occurs when warmer than usual water combines with high levels of sunlight causing the coral to expel the algae that live in its tissues.

Without the algae, known as ‘zooxanthellae’, the coral loses its energy source and its colour [61]. 

 

A history of bleaching in the Keppels

 

Mass bleaching was recorded in the Keppels in 1998 [62], 2002 [63and 2006 [64], with minor bleaching in between.

There are also unofficial reports by local fishermen of bleaching in 1983 and 1987 [65].

There is no evidence to suggest that there has been an increase in the frequency of bleaching events.

The author surveys a bleached field of Acropora millepora at Miall Island, 2006. © John Dooley.

Bouncing back after bleaching

Keppel reefs can recover rapidly from bleaching because of the excellent growth rates of branching species [35].

After the 2002 bleaching episode, coral cover increased rapidly, reaching the highest coral cover ever documented in 2004 [66].

Bar chart showing coral cover at shallow (2m) and deep (5m) reefs at Middle Island in 2004, following a bleaching episode in 2002.

Source: Sweatman, H., A. Thompson, S. Delean, J. Davidson, and S. Neale, Reef Rescue Marine Monitoring Program Status of Near-Shore Reefs of the Great Barrier Reef 2004: CRC Reef Research Centre Project C1.14. 2004, Australian Institute of Marine Science (AIMS): Queensland, Australia.

Acropora can survive bleaching by sheltering under the seaweed Lobophora variagata, Clam Bay. © Alison Jones.

In 2006, after the loss of 15% of coral cover due to bleaching, some reefs recovered their pre-bleaching coral cover within 12 months [35].

Three things combine to make the Keppel reefs recover fast from bleaching:

  • the prevalence of a single type of seaweed (Lobophora variegata), which shades the base of the branching corals, allowing some polyps to survive and regenerate tissue over the dead branches
  • high rates of coral re-growth
  • good water quality.

Bleached staghorn coral at North Keppel in 2006. © Ray Berkelmans.

The clever coral–algal symbiosis

The pin-cushion-shaped Acropora millepora on the reef flats has another survival mechanism.

It can change its symbiotic algae in response to bleaching, shifting to algae that help the coral survive but slowing down growth and reproduction [42, 44, 67, 68].

In 2006, Acropora millepora with type C algae bleached while colonies with type D algae did not bleach. © Alison Jones.

Brilliant ‘fluorescing’ Acropora millepora on the reef flat are exposed at low tide during the 2006 mass coral bleaching. © Alison Jones.

When coral bleaches, the polyps can still ingest food [25], and can live for some time without their algae. But without the additional food produced by the algae using sunlight, the coral will eventually starve and die.

The relationship between the coral and the algae is a symbiosis i.e. both partners benefit.

Unbleached Turbinaria bifrons against a background of bleached staghorns, North Keppel, 2006. © Ray Berkelmans.

Not all algae are equal

There are many different types of algae, just as there are many different types of corals.

Different types of algae can change how the coral-algae partnership responds to its environment; this ability may have evolved as a way for coral-algae partnerships to adapt to higher water temperatures [69].

Two-timing – a coral survival mechanism

The pin-cushion-shaped Acropora millepora is common on the reef flats in Keppel Bay. On other parts of the Great Barrier Reef it is normally highly sensitive to bleaching, but in 1999 Ray Berkelmans noticed that specimens from Keppel reefs seemed more able to cope with higher water temperatures [70].

This species can partner with two main types of algae at the same time and, in response to changes in its environment, can adjust the numbers of each algae type living in its tissue.

The more common type ‘C’  algae is more susceptible to bleaching by 1–1.5 degrees Celcius than the less common type ‘D’ [71].

In 2006, Acropora millepora colonies with type ‘C’ algae bleached while colonies with type ‘D’ algae did not bleach. © Ray Berkelmans.

During the 2006 summer bleaching, more than 70% of the Acropora millepora colonies on the reef flat at Miall Island were seen changing their algae from type ‘C’ to type ‘D’, and almost all of these corals then survived the bleaching [64].

On the reef flat at Miall Island, most of the Acropora millepora colonies that changed their algae from type C to type D survived the summer 2006 bleaching:

  • figure (a): before bleaching in 2005
  • figures (b)–(d): 3 months after bleaching in 2006
  • figures (e)–(g): 6 months after bleaching in 2006.

Source:  Jones, A.M., R. Berkelmans, J.C. Mieog, M.J.H. van Oppen, and W. Sinclair, A community change in the symbionts of a scleractinian coral following a natural bleaching event: field evidence of acclimatization. Proceedings of the Royal Society of London. Series B, Biological Sciences (1934-1990), 2008. 275: p. 1359-1365.

Bleached Acropora millepora colonies on the reef flat behind the Middle Island Underwater Observatory, 2006. © Ray Berkelmans.

Changing algae reduces vitality

While changing to more of the ‘D’ algae helped these corals cope with warmer water, it also reduced the coral’s vitality:

  • The rate of photosynthesis dropped by about 40%.
  • The growth rate dropped by 38%.
  • Both energy and egg size dropped by about 30% [42, 67].

But, as a temporary measure, what a clever mechanism it is for responding to changes in the environment.

Fluorescent pigments are revealed after the loss of the coral’s microscopic algae. © Ray Berkelmans.