Keppel Bay today

Today, Keppel reefs are stable and and well adapted to their current conditions.

Since they first began to form thousands of years ago, Keppel Bay’s fringing reefs have taken advantage of stable sea levels. Corals chased the light, cementing themselves on to older reefs and even to sand, rock, clay and rubble on the seabed [29].

The Keppel Bay reefs developed during a time of immense geological changes when the bay was turbid (muddy) and regularly injected with fresh water from the Fitzroy River [30, 31].

Then, about 1500 years ago, rapid growth slowed and the reefs stabilised.

A typical modern reef growing over old Pleistocene reef, rock and sand in Keppel Bay. © Alison Jones.

Flood-adapted corals

The reefs’ development at a time of adversity is one of the reasons that the Keppel region has such a diverse range of corals, such as  acroporids, faviids, gorgonians and other soft corals which have evolved to:

  • tolerate sediments [32]
  • regrow rapidly after floods
  • survive, and even use, pulses of nutrients
  • compete with turf algae, seaweeds and other macroalgae.

Turf algae covers dead coral skeletons following a flood, Miall Island. © Alison Jones.

Winds and currents in Keppel Bay

Another reason for the high coral diversity may be the winds and tides.

During the cooler months of the year, winds in Keppel Bay are mostly south-easterly but in summer northerly winds spring up in the afternoon.

When windy conditions continue for more than a few days and coincide with strong currents driven by the spring tide, mud and marine snow from the seafloor are stirred up and re-suspended in the water, causing the bay to look muddy.

On those reefs closest to the river, this re-suspension may prevent mud from smothering the corals.

View from Haven Point over white-capped waters towards Great Keppel Island. © Adrian Ambrus.

Algal slicks

In summer, when warm oceanic surface waters are driven into the bay by the ‘northerlies’, long brown, pink and yellow chains of cyanobacteria (Trichodesmium) appear as streaks on the water surface. 

These blue-green algal slicks are often mistaken for coral spawn. 

The algae convert nitrogen from the air into proteins that are essential to the entire marine food web.

To the annoyance of coastal communities, the slicks can deposit on mainland beaches where they lie rotting as the algae die.

Trichodesmium washes up on a Capricorn Coast beach. © Adrian Ambrus.

Other cyanobacteria ‘mats’ contribute organic carbon and nitrogen to the food web, encouraging the growth of fleshy macroalgae which, if prolific, can out-compete corals.

Cyanobacteria mat smothering corals at Pelican Island, 2009. © Alison Jones.