Legacy of Cyclone Yasi

Cyclone Yasi was a category 5 storm that crossed the Queensland coast just north of Cardwell in February 2011, which is more than 3 years before the photos in this post were taken.  Cyclones happen nearly every year in Queensland and most have little effect on mangroves, a few large, old mangroves fall but smaller trees survive and the ecology continues uninterrupted.  The difference with Cyclone Yasi is that it had a storm surge.  At Cardwell town which is only 1.5 km south of Meunga Creek, the storm surge was about 2-3 m above highest astronomical tide level and in places was accompanied by waves that were a few metres high.

The effect of the cyclone on the mangroves surrounding Cardwell was catastrophic.  A very high proportion of mangrove died along a 20 km stretch of low lying coastline was destroyed.  Many square kilometres of mangrove forest look like the photos below which were taken on 3 June 2014.

This was previously very tall and productive mangrove forest with a canopy at about 20 m

Rafts of timber which are held of the ground by stilt roots make walking challenging.

Wear a hard hat if you go in here and don't grab the trees as they are ready to fall.

Flakes of bark cover the ground - crabs eat leaves not bark and only a few sesarmid crabs are present.

Debris forms a timber floor in parts of the forest closer to the high tide line

Thirty days before the cyclone, a satellite image of the mouth of Meunga Creek was captured.  The site was also captured in late 2013, six months before the photos above were taken.  In the post-cyclone aerial photo, the living forests are mostly non-mangrove vegetation, mainly littoral rainforest or strand vegetation. The grey areas are central areas of mangroves swamp which were hit the hardest.  Patches of mangroves survived around the fringes.

Meunga Creek Mouth days before the cyclone - Jan 26 2011

Image taken 3 years later showing large grey dead areas.  Blue numbers show photo locations*.

*Photos relating to point 1 are in a previous post, those from point 2 are shown above and the regenerating mangroves are at point 3.

In Edmund Kennedy National Park which begins on the opposite side of Meunga Creek, there was a board walk for tourists through a similar mangrove swamp (no closed to the public).

Edmund Kennedy NP Mangrove Board Walk (now closed)

There are many varieties of mangrove swamp.  Spurred mangroves occur on higher ground, which is not flooded by every tide.  These mangroves are adapted to tolerating higher levels of salinity and do not  have the stilt roots or breathing roots that most other mangroves have.  Death in these forests was almost total.

Destroyed spurred mangrove (Ceriops) swamp - spurs are another name for buttress roots

View of ground - every hole has a crab and the come to the surface and look at you when you walk

Recovery is slowly taking place.  In some areas, new stands of mangroves are developing.  After three years these dense stands have reached eye level.  I do not know whether these stands are developing from seedlings that were present before the cyclone or seedlings that established afterwards.  The stands are 20-30 m wide and beyond the stands, establishment of new mangroves is scattered to widely scattered.

Regenerating mangroves

Mass death of mangroves also occurred as a result of Cyclone Tracey and was documented but not investigated.  Searching Google did not reveal any clues as to why the mangroves died.  The canopies of the mangrove trees were intact.  Cyclone Larry shattered many trees but did not kill them.  Smaller mangrove that would have been relatively protected within the forest also died.  One clue is that strand vegetation which grows on the beach between the mangroves and the sea survived.  Mangroves on the leeward edge of the foredune also survived.  To me this implicates the soil as these mangroves were exposed to a least as much wind and wave action as the central areas of mangrove swamp.

Living mangroves on lee of foredune then dead mangroves further inland

Foredune vegetation is battered but recovering - many trees survived.

 I think it is time we found out what kills the mangroves.  We should also plan to replant mangroves should a cyclone every hit Cairns, as leaving the vast swamps that surround Cairns looking like a wasteland is not an option.

Egrets and Sandflies

Great white egrets are pretty friendly.  On one occasion one even fed from my hand.  When I got that close, I was astonished to see how many sandflies attack the area around their eyes.  In the photo below, I was drifting downstream in my row boat and can within about 10 m of a great white egret, standing in the bright afternoon sun in front of a dark mangrove forest.  With a good superzoom camera, I could take a photo of the egrets face, remembering to underexpose by about two stops to compensate for the extremely bright white feathers.  Even the wings and legs of the sandflies are visible on the original seven MB image.  The sad part is that the egret must remain motionless and just endure the sandflies if it is to catch fish.  Sometimes you can see that the egret is suffering.  They live a life of intense concentration and I am sure that sandflies make things really hard.

Face of a great white egret about one minute after it landed

Something eats Fiddler Crabs

Just when I was beginning to believe that fiddler crabs were so good at avoiding predators that predators didn't bother with them, I find evidence of predation.  In a 30 m section of creek bank, I found 3 broken off fiddler crab claws and a partially eaten body.

As I had observed that same creek bank at 9 am, I knew that the predation had occurred during the day as it was 3 pm when I observed the claws.  At night the forests are stalked by the Australian water rat (Hydromys chrysogaster), which is a voracious predator of crustaceans and is like a tiny otter rather than a rat.  Their footprints can often be seen in the mud and on the beach.  So what preys on crabs in the middle of the day.  It was not people – no fresh footprints and people did crabs up which makes a mess.  In fact their were no footprints which made me thing that the predator was a bird.  Birds can carry their victims away from the site of capture before processing their prey and swallowing it.  In this case the large claw was removed.

Without footprints the only way to check out suspects was to photograph birds and look for mud on their beaks.  In the patch of mangrove forest where the crabs were taken were kookaburras, black butcher birds and scrub fowl.  On the sand flats next to the mangroves were several more candidates including a beach stone-curlew and various herons.

Beach stone-curlew, white-faced heron and great heron

My bet is that it was the kookaburra that took the crabs.  Scrub fowl scratch for their prey and generally are not present in the mangrove except when they are moving between islands of beach scrub within the mangroves.  Butcher birds generally attack prey by gliding in with a dead straight trajectory so that their approach creates very little apparent movement in the eyes of the prey.  However crabs are fast and they run when other crabs run and the other crabs with different angles of view would seen the butcher bird move.  Herons seem to be interested in fish not crabs.  The beach stone curlew had mud on its beak, but this species is very shy and is a creature of open spaces.  I doubt that it would go into the mangrove to catch crabs.  That leaves the kookaburra.  Kookaburras often sit on a lawn waiting for something to move so I suppose they could do this in the mangroves.  The photo seems to show mud on the kookaburras beak, however they wipe their beaks on branches to clean them so there is only what looks like a film of dry mud on their upper beak.

Beach stone-curlew - Esacus magnirostris

Kookaburra with mud on its beak a few metres from the crab remains

Seeing whats Under the Mud

Mangroves are famous for their aerial root systems but until recently, I did not know how much of the root system was hidden below the surface.  Cyclone Yasi, which hit the Queensland Coast at Cardwell in 2011, completely stripped away some of the smaller, more exposed swamps and revealed the substrate.  To my surprise, the ground was largely composed of mangrove roots rather than mud.

Remains of a stand of mangroves, Cardwell

Shredded remains of roots cover much of the surface

What the stand would have looked like before the cyclone

Mangrove mud is actually toxic to mangrove trees as many toxic compounds occur in the anoxic mud. Mangrove trees pipe air down into the mud to create an oxyzone around each root that destroys the toxic compounds and that is why mangroves have stilt roots.  The roots contain aerenchyma, which is a tissue with air spaces in it to allow the flow of air.  Mangrove roots also follow crab holes which also introduce air into the mud and create non-toxic zones.  Knowing that mud is hostile to roots, I assumed that the roots would taper off quickly.  Many wetland plants have small root systems due to the lack of oxygen in the soil.  When Cyclone Ita came along in 2014, it exposed some mangrove roots on the sandy mangrove coast at Cooya Beach allowing their structure could be seen.

New roots growing out from the stilt root

Where the root strikes the ground, there is a star burst of lateral roots.  The roots give rise to a finer set of roots four times which is to say that the roots have forth order branching.  The effect is almost fractal like a snowflake.  Each of the rootlets resembles a small pine tree.

This root structure is entirely different to that of trees that live on the same sand just above the tidal zone.  Most species of trees have roots that are like cables and these roots run for distances of many metres and branching is generally simple dichotomous branching where the root divides into two similar sized branches.  Strand trees have shallow root systems which run through the top soil which is where the fertility is.  Their roots also seem to stay above the water table which can be close to the surface near a beach.  Calophyllum trees, beach she-oaks and paperbarks have this pattern.

A giant Calophyllum tree toppled by Cyclone Yasi reveals cable-like roots

Another root system design is to have strong scaffold roots which radiate from the trunk.  These are elliptical in section and composed of stiff timber to resist flexing.  Secondary tap roots descend from the radiating scaffold roots.  Coral trees, Beach Almonds and Bloodwood trees have this pattern.

Scaffold roots and secondary tap roots of a Beach Almond (Terminalia catappa)

So what does all this mean?  Roots keep trees alive so it is important to understand roots.  Mangrove roots are special as they have an additional function that roots of trees on land do not have, the piping of air into the soil. The shape of trees and tree leaves is largely controlled by their ability to pipe moisture to their leaves - the water potential theory.  Mangrove tree root systems may be optimised for piping air rather than seeking moisture and fertility.  The old theory that stilt roots are needed to prevent trees from falling over in the soft mud is clearly rubbish.  In no case have I seen a mangrove tree topple over and pull up a root ball as rainforest trees often do.  In every case, the mangrove trees roots have snapped at surface level and the tree has been torn away so the ground is stronger than the tree.

Mangroves are also putting a huge amount of carbon below the ground surface where it may be retained indefinitely. Growing mangroves may be a way of stripping some carbon from our atmosphere.  Sewage plants usually discharge into the mangroves and in many coastal towns, the surrounding mangroves have grown taller and denser as a result of the lower salinity as treated sewage dilutes the seawater and irrigates the mangroves in the dry season.  Some carbon is already being captured due to this accidental process. Perhaps we can use mangroves to sequester carbon?  Conversely, clearing mangroves releases carbon into the atmosphere.  When cane farmers cleared mangroves to grow cane, the soils firstly became acid and poisonous but as the carbon in the soil oxidised and disappeared into the atmosphere, the ground surface fell sometimes by a few metres and some areas ended up useless and permanently underwater.  Clearing of mangroves is no banned in Australia for this and other reasons.

More info on carbon sequestration in mangroves

Crocs, People and Creek Mouths

Creek mouths are magnets for people on warm sunny day. The crab habitat gets trampled and I wonder how the crabs manage to survive.

On a sunny day, everyone is chasing fish at the creek mouth

If you are not interested in fishing, you can swim in the mud with your brothers and sisters

Sentinel crabs that share habitat with the mud-swimmers

In the search for more pristine habitat I went to another creek mouth about 50 km from Cairns to look for the  locally elusive Uca polita fiddler crab but had to rethink my plans.  There was a crocodile that was larger than my boat and faster too so I had to put some distance between it and myself.  Around the bend and about 100 m away were the weekend crab tramplers.  It is not uncommon for crocodiles to be within visual distance of people and the people to be totally unaware.  I sometimes took binoculars to a creek mouth, lay down on the sand so that I could see under the overhanging branches and looked for crocs and I would see them and nobody else was aware of their presence.  When boats approached, they would quietly slide into the water unseen and come back out when the boat moved on.  Crocs are not put off by noise and human activity.  One of the best places to see crocodiles was on the Barron River where the Cairns Airport landing lights span the river.  Screaming jets tensing up before landing would pass only tens of metres above the crocodiles.  They even nest in unlikely habitats like than.  

3.5 m croc and girl friend at creek mouth

Crocs run and jump off banks and can end up in your speed boat if you cruise at speed close to the bank - they have hit the side of my boat on a few occasions.  Years ago, when I had been padding around in the sea on a surf ski for hours, I paddled into the creek mouth shown above and up the edge of the channel to stay out of the wind.  A croc came running out of the mangroves and stopped with one foot hanging over the lip of the bank just in front of me.  On a 17 foot long surf ski, turning fast is not possible so I had to maintain my composure paddle within 2 m of a 2.5 m croc perched on the bank.  Sometimes you can even run into crocs in coastal waters.  With me it has usually been near misses caused by me surprising crocodiles.  Crocs generally avoid us but always stay on guard.  I recommend that you make an effort to see the crocodiles that share our waterways as it helps to know your potential enemy.

Catching some Winter Sun

Fiddler crabs wave their nippers for various reasons. Some crabs seem to get stuck halfway through a wave and hold their nipper out for minutes on end. When I saw that the little feeding hand was also held out, it became clear that the crabs were sun baking. The crabs are also oriented to catch the most sun. Being Cairns on a warm day, it was not cold by any stretch of the imagination and I can only guess how warm these crabs were getting. Down in the boundary layer next to the ground, where the wind is almost non-existent, the crabs may get quite hot. Years ago, on a mangrove planting project, many of the mangrove droppers/seeds (Rhizophora hypcotyls) got so hot at the point the dropper entered the ground, it killed the droppers. There would be a ring of dead material at ground level, with the below ground part and the above ground part being fine. Based on that experience I would think that a temperature of forty degrees or more would be possible. These sorts of temperatures are often dangerous for invertebrates and it would be interesting to know why the crabs are warming them selves up. A thermal imager would have been handy to see how hot the crabs were getting – they are now only $350 or the price of a good compact camera.  

Uca vomeris in the sunbaking pose

All dried out

Who Eats Fiddler Crabs?

You would think that fiddler crab colonies would be a magnet for predators as the density of fiddler crabs is often really high.  However in all my years in the mangrove environment, I have rarely seen anything eating or attempting to eat fiddler crabs.  I suppose the lesson is that you often have to work hard to see predators in action.  It is also possible that fiddler crabs are not very good to eat.

Animals that are known/said to eat fiddler crabs include ibis, terns, possibly the giant mudskipper and the mangrove kingfisher.  At high tide, fish and rays will also eat any fiddler crab they can find. Mangrove kingfishers certainly eat crabs and recently I spotted one that appeared to be hunting fiddler crabs.  Being short of time, I could not sit and watch, however you can see several sets of fiddler crab eyes watching the kingfisher – look along the ridge line behind the kingfisher.  Mangrove kingfishers (Todiramphus chloris) are are much larger than other kingfishers and make a loud kak kak sound.

Mangrove kingfisher hunting crabs on the mudflats of Tingalpa Creek

Below is the would be prey of the kingfisher, Uca longidigitata, a fiddler crab which is common on the mudflats of Moreton Bay, near Brisbane.  They are mostly not very colourful, however some are a turquoise with sky blue nippers.

Of all the birds, the Great -billed Heron (Ardea sumartrana) would seem to be the ultimate fiddler crab catcher.  It is a massive and powerful bird which stands around 1.15 m tall and is only slightly smaller than a Jabiru.  They usually hunt the mangrove waterways near river mouths.

Great-billed heron hunting along waters edge

I watched the individual above as it slowly walked along a river bank and through 50 m of particularly rich fiddler crab habitat.  The bird showed no interest in the crabs and was only hunting something in the water, probably mullet but possibly other fish and prawns.  This does not prove that great-billed herons don't eat fiddler crabs, only that this individual was not interested at the time of observation.  It would require observations of many great-billed heron individuals for tens of hours and in many places and seasons before a properly substantiated statement could be made.  That said, all of my observations of great-billed herons suggest they hunt fish not fiddler crabs.

Fiddler crabs behind a great-billed heron.

The one animal I have seen attacking a fiddler crab was a small juvenile mud crab.  It had was wresting with a mature male two-toned fiddler crab of the same size that had wandered past the mouth of its burrow.  Unfortunately I could not stop and see what happened.

Update June 2017
Since this post was prepared, I have been actively looking for Fiddler Crab predators.  Here are some other posts on the topic.

• Something eats Fiddler Crabs;
• Fiddler Crab Predators;
• Predation by Fish; and
• River Monsters in Mackay - I have sent into to wide range of experts and still have no solid ID on what the creature is.

A Rare Hairtail

Sometimes I see mangrove creatures that are really interesting and never see them again.  Due to the toxic creek discussed in the previous post, I have rediscovered one of those creatures, Lepturacanthus savala which is hairtail or cutlass fish.  The fish died due to poor water quality which provided me with a chance for a look.  The fish I found is a juvenile, as adults are up to a metre long.

Hairtail or Cutlass Fish

According to fishbase, this fish is rarely caught and little is known of its biology.  Other fish with this shape often hunt in the dark and have special senses, so I assume that this is the case.

Transparent dorsal fin

This is the shiniest fish I have ever seen, note also the highly developed lateral line

This fish also has barbed fangs

I have only seen the fish once before.  A similar sized specimen was swimming at the surface not in the serpentine manner of an eel but like a plastic rod being bent into a curve then bent the opposite way. I suspect that this fish is like a deep sea predator that avoids flexing its body as it swims and attacks from below.  From above, it was very thin with a pair of very large eyes protruding from the sides.   It did not look agile at all and I thought to catch it by hand but it never came close enough to the bank to grab and it was sucked out to sea but the falling tide that raced out the creek mouth.

A Naturally Toxic Watercourse

Hydrogen sulfide (H2S) or rotten egg gas is often the smell of mangroves.  This poisonous gas is produced  by decomposing organic matter in the below the surface of the mangrove muds.  Normally the gas is only present in trace amounts and is wafted away in the wind or currents.   Very occasionally in prolonged still conditions, it builds up to levels that become toxic to life resulting in fish kills.  At very high concentrations, I suspect it kills patches of mangrove trees and it could be the main cause of the mass mangrove tree death caused by Cyclones.  Obviously understanding what hydrogen sulfide can do to ecosystems is important.  Yesterday, I found a strange example of an ecosystem stressed by this gas - the Blind Barron.  

In about 1942, the Barron River suddenly broke through the beach and created a new mouth about 1 km to the north.  The channel to the river old mouth filled with sediment and a flat beach ridge formed in what was the mouth.  A 1952 view of the Barron River mouth is presented below.  A pair of orange points have been placed on the image to mark two features that can be identified in both photos.

However mixed in with the river sand and mud that filled the old channel was a lot of mangrove leaf detritus, which creates hydrogen sulfide as it decomposes.  And this is where it gets strange.  The sand flats at the mouth of the Blind Barron (name of the old mouth) are higher than the bed of the Blind Barron so the water flows from the sand flats into the Blind Barron when the tide goes out – it looks like the watercourse is flowing backwards.   When the tide has gone out, the flow keeps going!  It is sustained by groundwater coming out of the beach ridge that has built up in the mouth the Blind Barron.  As the groundwater has been in a low oxygen environment that is rich in organic matter, it contains hydrogen sulfide.  When the tide comes in, it refills the beach ridge ensuring that on each low tide there will be plenty of groundwater.  The net result is a watercourse that fills with clean seawater at high tide and runs with contaminated water at low tide.  

Freshly deposited mangrove detritus at mouth of Blind Barron which will generate H2S when buried.  The white material is shell grit.  Normally the presence of shell grit neutralises acid generated by H2S so I am assuming that the water is not acidic, only contaminated with H2S.

This 50 m long pond is filled with contaminated seepage from the adjacent beach and sand flats

Pond water flows back into the Blind Barron at a high rate- shown here still flowing strongly many hours after high tide

Strange water colour - clearer than estuary water with a hint of colloidal cloudiness and blue tint.

View up the Blind Barron - it has high banks and a sandy bed

The extent of the toxic backflow is shown in this 2008 imagery.

The impact of the hydrogen sulfide is only on animals that live in the waters of the Blind Barron.  Life on the banks is not affected as the hydrogen sulfide is a low tide phenomenon.  The water was clear and inviting but smells like a volcanic spring.  It was noticeable but not powerful.  Toad fish were swimming around randomly in the water column and a mud crab was sitting in a hollow with its tail hanging down (It didn't want to be photographed).  One fish, a rarely seen hairtail or cutlass fish was lying on the bottom and died as I photographed it with my underwater camera.