Reefs of Keppel Bay

The coral reefs surrounding the Keppel Islands are extraordinary because they survive so close to the mouth of the mighty Fitzroy River.

About once every 10–20 years, the flooding river spews a great plume of fresh water, mud and nutrients into Keppel Bay.

More than anything else, these floodwaters influence which corals grow where, and how long it takes for near-decimated reefs to fully recover.

Pete’s dive boat Saracen surveys flood damage on the reef flat at Clam Bay which was decimated in the 2011 flood. Pin-cushion-shaped colonies of Acropora millepora were eventually covered in turf algae. © Alison Jones.

Despite these floods, the reefs keep functioning, eventually returning to near their original state [23, 33, 3435].

They survive because:

  • they have a rich diversity of well-adapted coral species
  • the coral species are fast-growing and use food and sunlight to grow
  • the coral species can clone themselves
  • there are plenty of surfaces for the corals to attach to.

Acropora and Chromis netida (half and halfs) Middle Island. © Alison Jones.

Adapted to mud and fresh water

To survive so close to the river mouth, a wide array of coral species has evolved, each finding its own niche:

  • The highest densities of hard, branching corals (Acropora) are found around the outlying islands where the water is clearest.
  • Around the islands closest to the river mouth and coast, the density of coral is lower but many more species have adapted to the muddier conditions—soft corals such as sea fans and sea whips, hard corals of the Mussidae and Faviidae families and from the genus Goniastrea

    [36].

Diverse corals

Along the Great Barrier Reef, the number of species decreases as you move further south. So, when Robert van Woesik conducted the first surveys of corals in the Keppel Islands [37, 38], he was surprised to find many more species than expected, compared to those reefs to the north and south.

In the Keppels, 167 hard coral species and many more soft corals have been discovered and recorded [35, 39, 40, 41].

Genetically distinct

The vast distance between the Keppel Islands and other reef systems (the Percy, Capricorn and Bunker groups are the closest, about 100 km away) means that Keppel Bay corals rarely reproduce with corals from elsewhere.

As a result, Keppel Bay corals have remained genetically distinct [33].

Google Earth image showing how isolated the Keppel Islands are from other island groups.

Fast-growing corals

The conditions in Keppel Bay also allow corals to grow faster [35]. In the absence of floods, fast-growing Acropora can increase their weight in water by 3–4% per week [42].

The fast growth rates have resulted in Keppel Bay having some of the highest ‘coral cover’ (density of corals covering the seabed) on the entire Great Barrier Reef.

Acropora nobilis and A. formosa cover 100% of the seabed at Barren Island, 2009. © R. Berkelmans.