Flooding in Keppel Bay

Keppel Bay reefs are occasionally inundated by floodwaters from the mighty Fitzroy river.

Floods are the most damaging natural event in the Keppels, killing virtually all living organisms on the reef. The lower salt content of flooded seawater causes the coral polyps to swell and die.

Acropora millepora affected by the 2011 flood in the Keppels. © Alison Jones.

Acropora millepora can tolerate salinity as low as 22 parts per thousand (seawater is 35 ppt) for up to 3 days, or 28 ppt for up to 16 days [52].

Mud in the floodwaters smothers those corals that cannot remove the silt fast enough with their tentacles; it also feeds parasites and blocks the sunlight that is so essential for growth.

A south-facing reef flat at Miall Island lies directly in the path of the 2011 flood plume. © Alison Jones.

Sloping reef (6 metres) after the 2011 flood. The flood plume moved south to north and this reef is on the northern side of Sloping Island. © Alison Jones.

Staghorn corals are the most susceptible to floods. The loss of entire beds of staghorn corals on the shallow reef flats can appear devastating [53].

The more weather-exposed sides of islands and rocky headlands have deeper, more structurally complex coral communities that are adapted to strong swell, waves and currents. These reefs are much less susceptible to floods and bleaching.

Coral rubble at Middle Island after Tropical Cyclone Marcia in 2015. © S. Dunlop.

A history of floods

Since 1863, a tropical cyclone has hit the Capricornia region on average about once every four years during summer, and more frequently during La Niña conditions than El Niño [54]. 

Every 10–20 years the monsoonal rains have resulted in major flooding of the Fitzroy River which spews out into Keppel Bay, sometimes reaching the islands.

In 1918, 1954, 1991 and 2011, floods killed many corals in Keppel Bay, but the affected reefs recovered within about 10–15 years [23, 34, 55, 56, 57, 58].

Flood tide, Keppel Bay, 2011. ©  P. Williams.

In the 1950s, Gordon La Praik noted that Maisie Bay (North Keppel Island) had an area of staghorn corals that “filled the curve of the bay”.

Then, suddenly, in 1956 or 1957, the same area was dead and covered with moss. 

After 20 years, Gordon reported that “it was all back again, just as luxuriant as it had been before” [59]. 

Flood-damaged reef at Miall Island, 2011. © Alison Jones

In 2008 and 2013, less severe floods caused only localised coral death and these reefs were quicker to bounce back.

Soft corals are often the first to regrow after a flood and, if found on a reef in large numbers, can be a sign of previous flood damage [60].

Between 2010 and 2016, four consecutive floods damaged the reefs in Keppel Bay. They are slowly recovering as conditions become more favourable.

I have seen the sediment plume from the Fitzroy River after the big 1990/91 flood; that was the one that killed all the reef out here at Keppel Island. It was enormous. You wouldn’t believe that it could go so far out, you know. And yet, why not?” 

— Lionel Bevis, Yeppoon

A mangrove takes advantage of the 2011 flood, taking root on a shallow reef flat at Maisie Bay, North Keppel Island. © Alison Jones.

Mangroves thrive on the sediment and nutrients that floods deliver to estuaries and mudflats, and can even travel on the flood plume to colonise new areas.

Oysters can benefit when the fresh water kills their natural enemies, such as starfish, sea urchins and borers.

Black lip pearl oyster. © Alison Jones.

Fish populations can expand when seasonal flows carry nutrients and chlorophyll to breeding areas, providing food for the larvae to grow and survive in the years after the flood.

‘Happy moments’ (rabbitfishes) feed on particulate matter above a flood-damaged reef. © Alison Jones.

After the flood – coral cloning

Floods kill virtually all living organisms on the reef. So we might expect corals to regenerate only when baby corals from neighbouring surviving reefs are carried to the damaged reef by ocean currents.

This is not the case for the Keppels. The Keppel reefs are geographically isolated from the rest of the Great Barrier Reef and most regeneration is from corals cloning themselves from surviving fragments [33].

Acropora millepora recolonises a reef following the 1991 flood. © Alison Jones.