Cairns' Disappearing Mudflats

The famous Cairns mudflats are almost no more. This muddiest of places and magnet for migrating wading birds is becoming a sand flat. Some people think that the Council’s efforts to make a sandy beach is responsible for the gradual loss of this internationally important shorebird feeding area.  Certainly the amount of sand being placed on the esplanade should be investigated, however other factors might be at play including cyclones and coastal processes. 

Cairns' fine man made beach

Cairns is one of a very few places where you can sit on a boardwalk and view a dozen or more species of usually shy wild birds feeding only metres away. The birds can’t afford to be shy as there are few good feeding grounds in either direction. I once commissioned professional ornithologists to study one of the major estuaries to the north of Cairns. Port Musgrave on Cape York Peninsula should be ideal wader habitat. However we found that when the waders are migrating, it was always high tide during the day so the waders could not feed there. It takes a special set of circumstances to create good wader habitat. 

A wading bird less than 10 m from my seat and feeding in the small remaining muddy area near the birdwatchers lookouts.

The rich wader habitat in Cairns may also be partly of human creation. The shape of the coastline has been changed due to developments like the Pier and this has created a poorly flushed area which is great for accumulating mud deposits. Long before the Pier arrived, Fogarty Park which juts out into the estuary had been reclaimed. In the Cairns hinterland, land clearing was at its peak and there was much less concern for soil erosion at the time. Sediment supply from the Barron River, Trinity Inlet and even Saltwater Creek near the airport would have been much greater. Saltwater Creek now only drains urban areas, but when I was young this area was cane fields and would have yielded more sediment. The local Port Authority also pulls out all the mangroves which are attempting to colonise the mudflats just off the esplanade. This creates an open muddy habitat in an area that would normally be a dense mangrove swamp. Harbour dredging in times past would have had less consideration for sediment plumes and many think that these are the source of the Cairns mudflats.  Historically, Cairns is said to have had a sandy beach. However in my imagination, the sandy beach would have risen above a mudflat as in Cardwell today. This was the case in aerial photos from 1952.

Another wader feeding on the lumpy mud-scape near the wading pool - where the mud builds up today

I have been trying to establish a time series photographic record to make the changes over time visible. Unfortunately, nobody expects a fact of life such as the Cairns mudflats to disappear, so there is not much in the way of old data. However it is obvious that at least in some places, the mudflats are becoming sandy as you can now walk out a long way without sinking up to your knees. The question is where is the sand coming from and/or where is the mud going.  Similar changes are also occurring at Ellie Point which extends from northern end of the Esplanade. If massive changes have been taking place even quite far away from the sandy beach where the council dumps sand, it suggests other factors are involved. 

Dredge working in the shipping channel - note lack of life on exposed tidal flats which occupy the bottom half of the photo .

An almost matching view from 2005.

Telephoto view of the flats - the black object is a beer bottle (2014)

The same area in 2005 was seething with life 

The full story is complex and I am only undertaking informal investigations. It is probable that there is less mud coming out of the rivers. Even the sea on the Cairns Northern Beaches is more blue than brown these days. It used to be the colour of milk coffee in rough weather. Sand supply has also increased due to banning of sand mining in the Barron River and at Ellie Point. This supply which is estimated to be 23 000 tonnes a year is beginning to pour around the tip of Ellie Point. The renewed supply of river sand could already be making a minor contribution to the sand supply on the esplanade. Cyclones may have had a bigger impact. At Cardwell, Yasi washed away vast volumes of surface mud and left only the heavier sand. Several big cyclones have passed close to Cairns in recent years.  It is the swells that they generate that do the damage and these swells can change our coastline even when the cyclone is hundreds of kilometres away by creating inshore currents and by lifting sediment into the water column. Cyclones have always been around but there has been less time between large events recently.

In 2005, seagrass beds were clearly visible from the Esplanade (the dark band).  They appear to have disappeared from most of the Cairns foreshore.

The gradual loss of large areas of mudflat from Cairns probably has multiple causes and it would take a lot of work to apportion blame. If you are interested in helping to figure out what is happening leave a comment.  All the photos here were taken from near Muddies Playground.  

Hiding where there is Nowhere to Hide - Life on the Sand Flats

Sand flats can be like deserts however if you look long enough, you will find some very strange creatures.  Many of the beaches in the north have vast sand flats at low tide but many of the creatures that live there are buried in the sand so take some finding.  The shifting sands and lack of hiding places make life very difficult for creatures that shelter in burrows or which need something hard to hide under and this is the primary reason why sand flats have fewer animals.  There are also fewer plants to feed on as macro-algae require something to anchor themselves to.  Seagrasses also avoid very sandy sand flats which are subject to high wave energy, preferring more sheltered areas with muddy sand.

Sand flats at mouth of Barron River, near Cairns

Sand flats at Ellie Point near Cairns with mountains in background

The most abundant creatures by far on the exposed sand flats near the low tide line are sand dollars.  Sand dollars feed by moving just under the sand surface and letting the plankton fall through the short felt-like spines to channels below which sweep the material from the top of the animal around the edge and into the mouth at the middle of the under surface.  In some other parts of the world, sand dollars hide in the sand at low tide and feed on the surface at high tide and I will have to check if our local species does this myself, but my recollection is that they are always covered with at least a fine layer of material.  The sand dollars may also be hoovering up diatoms that live in the sand and which come to the surface to photosynthesis at low tide.  

Sand dollar with a typical hat of sand

On my last trip to the sand flats I observed hermit crabs eating sand dollars, however the hermit crabs live in shallow pools close to the beach.  Perhaps some sand dollars are washed up from their normal habitat into the pools where the crabs live and are scavenged.

A sand dollar being eaten by two hermit crabs which are hidden by their shell houses

Very occasionally, I find file snakes buried in the sand.  File snakes are harmless and are one of the least energetic of all predators.  In fact they have trouble moving on land as they have so little muscle power that their own weight pins them down.  They can bury themselves in wet sand by slowly shuffling their rough skin against the bottom.  To permanently record the details of the last file snake I found, I recorded the sighting on the Atlas of Living Australia.  If you open the preceding link, you can see what the snake looked like before I picked it up and put it on top of the sand to take the photo below.

Little file snake at Cooya Beach, in the process of shuffling back into the sand

Close-up of head and rough skin, which is being rotated around the body to dig into the sand.

Some creatures you see only once and this unidentified eel is an example, so it pays to have camera on you.  The eel was able to retreat backwards into the sand.  From memory, there were no yabby holes or any other gaps, just loose sand, so going into the sand backwards is quite a trick.  The eel was however sucked out of the ground with a yabby pump which is how we got to see it at all.

Unknown species of eel

The final creature which I have seen in Brisbane, Townsville and the Barron River Delta is the leaf-carrying crab.  The crabs last four legs are mounted on its back and point upwards so that the crab can grip a leaf.  Usually these crabs are found in very shallow water such as the leading edge of the incoming tide where they look like mangrove leaves being carried along by the flow, that is until they decide to change direction and then it becomes obvious that something is happening.

A leaf-carrying or leaf porter crab

Further reading:
Sand Dollars