The Fitzroy River estuary

– a rich larder

The muddy reaches of the estuary and marine delta, where the river meets the ocean, are a rich, productive larder for Keppel Bay.

It starts when decaying plant matter from the catchment, picked up by the flooding river, eventually reaches the estuary and delta. Here, when dust, sediment and decayed matter meet the salt water from the bay, clumps can form (‘flocculation’), which, if not eaten, drift slowly to the sea floor in the phenomenon known as ‘marine snow’ [24].

Lobophora variagata (marine macroalgae) covered in flocculated silt and nutrient, known as ‘marine snow’. © Alison Jones.

Nutrients suspended in the water and in the sediments on the sea floor are a rich source of food for:  

  • all the tiny organisms that live on the mudflats at the mouth of the estuary
  • the plankton that drift with the currents
  • seaweeds and other macro-algae attached to the floor of Keppel Bay.

Clam Bay reef flat 3 years after the 2011 flood. © Alison Jones.

These plants and animals in turn enter the complex food web of crustaceans, molluscs, fish, turtles, sea snakes, dolphins and dugongs. 

Corals can consume the plankton or feed on the nutrients directly, or make their own food through photosynthesis [25]. But too much nutrient can encourage the growth of algae, bacteria, sponges and bioeroders that can be detrimental to corals [26].

A bioeroder can erode the skeleton of the coral, reducing calcification and introducing diseases [27].

Macroalgae such as Sargassum and Lobophora variagata (encrusting brown algae) can grow faster than corals, blocking out light and taking up space, which means less coral until the macroalgae dies back (usually in winter) or is dislodged after a storm. 

Favites russelli with bioeroders. © R. Berkelmans.

The seaweeds and coral are antithetical to each other, the weeds making the ground foul for coral by leaving to the corals no clean gritty surface to perch upon, so that the coral will be unable to regain this site while the weeds are in possession of it. But the cycle will run its course; when the algal complex now in possession dies down the corals will resume their heritage, as one crop follows another” [28].

Sargassum spp., Haven Point. © D. Brighton.