Grassy Salt Pans of Cairns

Almost exactly 100 miles/160 km down the coast from the succulent covered salt pan near Cooktown is one of the few remaining salt pans near Cairns. Despite being so close that a lazy crocodile could drift from one to another in less than a week, the botany of these two salt pans is very different. At Cooktown, it was all succulents, near Cairns it is usually nearly all grass. In Google Earth the photo pattern of each vegetation type is remarkably similar. Surprises like this remind me that neither I nor anyone else I know of can fully understand why things grow where the do. Until we can understand why what grows where, environmental management is just educated guess work.  I think students and rangers could do mini science projects to find out why some places that should be similar are entirely different.  Professional scientists don't answer these questions as they need to maintain their fame and are not interested in easy research.

Saltwater couch grassland/salt marsh at Yorkeys Knob

Most of the original salt pans and samphire flats around Cairns have been lots to development so I have to be careful about deciding that Cooktowns salt pans are different to those found near Cairns. Many salt pans within Cairns City were reclaimed. Cairns Airport is on what once was a salt pan and the major agricultural folly of East Trinity also included saltpans. Fortunatelly, we have a couple of good examples left and I think the remaining examples are similar to the ones lost. It is also unlikely that any halophytic (salt tolerant) species would be lost from the entire region. Salt pan plants are great colonists and show up even in salty ruts around the edges of sugarcane fields. For these reasons, I suspect that the succulent flats like those near Cooktown never occurred on salt pans near Cairns.

The natural park lands are quite pleasant provided you wear long clothes and don't dally around after sunset

Cooktown is in the Cape York Peninsula Bioregion and Cairns is in the Wet Tropics Bioregion. Bioregions are like countries for flora and fauna so this suggests that the salt pan vegetation could be different. Queensland has 13 bioregions. The bioregion boundaries were originally drawn by smart people with marker pens on paper maps. They were not something that came from a sophisticated analysis. Yet it is strange that even intertidal communities seem to have a sudden transition at the bioregion boundary. It is strange because most mangrove plants and animal species (fish, crabs, molluscs) occur from here to Africa and on almost every coastline in between. Given that most species are so widespread, why to comparable salt pans that are so close by natures standards have such different vegetation? I can only guess. Climate and geology and tidal inundation cycles are all quite similar. The founder effect may be at play. The founder effect is natures version of losers/keepers. The founder effect is when the first plant species to find a new island or in this case salt pan multiplies and occupies all the space making it very hard for new species to establish.  In very flat places, I have noticed that even a few centimetres in altitude makes a big difference to how long an area is flooded in inland areas or how many tides per month are experienced in coastal areas and I would start my investigations here. Succulents (Halosarcia sp.) do grow on slightly lower ground around the margins of the grassy swards in Cairns but are low and straggly.

Saltwater couch (Sporobolus virginicus) is the grass that covers the salt pan. We also have a second species of saltwater couch (Paspalum vaginatum) which often grows with the first species and looks very similar. The leaves of the first species are spirally arranged on the stems and the on the second only grow on opposite sides (called distichous).

Sporobolus virginicus has little tufts of leaves from underground runners.

Crabs and mud lobsters thrive in the saltwater couch and the ground is so full of hummock and hollows that I stumble through the grass rather than walking through it.  I wonder how the productivity of this marine grassland compares with mangrove ecosystems.  

Mud lobster mound peppered with crab holes

Shed exoskeletons were hanging in the grass

Ant plants also like the margins of salt pans and I suspect this is related to mists but I don't really know.  

Myrmecodia beccarii - the spiny ant plant

Miniature Forests of the Salt

The salt pans and samphire flats of South East Queensland receive a lot of attention.  Yet the salt pans of Cape York Peninsula have a strange flora that is barely described on the internet or even in scientific reference sites such as the Atlas of Living Australia.  On a saltpan that is within  bike riding distance of Cooktown I found a veritable rainforest of succulents.  Within this miniature forest, the canopy was almost closed, the understorey was dark and humid and a thriving crab population.

Batis argillicola mini tree

Being on a salt pan is to experience open space.  If you value open space and enjoy the visual intensity of colour and detail, being on a salt pan is like freedom to breath.  In this particular salt pan, it is also about being a giant that needs to tread carefully through a forest that reaches as high as our knees.

Should I even call this a salt pan?

Batis (Batis argillicola) is the main plant although approximately six succulents were present in total. Grass which is ususally present was not to be seen. Batis is quite large for a succulent and stands about half a metre tall.  The leaves are bright green and of similar shape and size to grains of rice and the seed pods are like green peas.  I am not sure how old the Batis are and I don't know if any one does but their stout trunk suggest they are old.  As there are no good photos on the net, here are a few more (yes it is true, stacks of stuff is still not on the net).

Batis argillicola is big enough to compete with young mangroves

Leaves and flowers of Batis argillicola - the flower are tiny and only stamens are clearly visible

In the salty margins are some red and pink Halosarcia succulents, which come from the salt bush family.  The red is a protective pigment (a carotene) that allows plants to avoid damage caused by intense sun.  It is a sunscreen and a way of dissipating excess energy.  In such a dry landscape, plants cannot open their stomates to get CO2, so the energy captured by the photosynthetic pigments cannot be used and would generate free radicals that would corrode the plants cells if it the energy was not passed to pigments which reradiates the energy.  The pigments also have a sunscreen effect by blocking some of the incoming energy.  Back in the days of the ozone hole, it was predicted that many plants would go orange or red as they increase pigment levels to protect themselves as UV light also creates free radicals.  Breeding programs were established to put protective pigment genes into crops as many crops are badly damaged by intense UV.  The succulents on the salt pans also experience high UV from the sky and reflected UV from the salty surface.

Red succulents dominate very salty areas

Thinking of fauna, the crabs were in hiding and all I could find were mud whelks.  It seems that mud whelks thrive in a narrow band of ground around the margins of the saltpan.  Here shallow pools exposed to the sun provide better grazing than the surrounding mangrove forest.  However on this flat terrain, a mud whelk can easily move in the wrong direction and by after as little a five metres, it can find itself lost in the miniature forest with fatal consequences.

Hundreds of mud whelks clustered seeking shade under an isolated mangrove

There is a sad fate for those that crawl a few metres in the wrong direction

A Clean New Land

It is a surreal experience to go to a place that you know well and to find that it is entirely different.  Ellie Point has never been attractive.  It is a stark, shadeless place with an incredible sandfly population and appears to be the final resting place of any rubbish that the good folks of nearby Cairns throw into the sea.  It it a place that I wished could be better and maybe I have gotten my wish.  I found my self standing on clean new land where only three years ago, I was motoring my boat.

A new beach facing the City of Cairns

Ellie Point with a new beach - it would be more than 400 m long and would extend 100 m into the sea

In my life time, the coast line has only ever seemed to retreat, but now I am beginning to see this trend reversed.  The long period of beach erosion was caused in part by human appetite for sand.  For many years, sand was dredged from the nearby Barron River as it was transported toward the sea.  More than 20 000 tonnes per year were taken.  As a result, the Cairns northern beaches were starved of sand supply and began to erode.  Seawalls are still being built to protect houses from disappearing into the sea where the beaches have been all but lost.  Even as I write, I can hear machinery crunching on massive rocks to build a new seawall for Machans Beach on the northern side of the Barron River.  Sand was also mined from the sand flats at Ellie Point.  This was unfortunate as it delayed the development of sand flats around the recently moved Barron River mouth.  The new mouth emptied into deeper water and sand was carried by the flow of the river into deeper water where waves could not bring it back to the beach.  For these reasons sand extraction from rivers was banned (~1990) and sand mining at Ellie Point ceased.  Sand mining is now only allowed in so called in-active sand deposits which are old beach ridges or alluvial deposits which are located mainly under sugarcane fields.

View from the new beach back to the previous shoreline across a shallow sandy basin

Geology maps tell me that the oldest beach ridges are about 5 km inland and that they are about 5000 years old.  This suggests that the coastline should be prograding at an average rate of about 1 m per year.  In recent years, the beach at Yorkeys Knob has been prograding even faster as it receives sand from both the Barron River and Richters Creek.  Only this year have there been clear suggestions that beaches are again growing within the delta.  Shallow waters over sandflats now extend out hundreds of metres from the beach and waves can been seen refracting around the higher sandbars, resulting in sand being moved toward the beach.  However within the delta, prevailing winds and waves can also strip sand from one area and deposit it in another so prograding beaches are often matched with nearby areas of coastal erosion.

Just to the north the coastline is regressing and trees are falling into the sea.

There is another reason why sand is not mined at Ellie Point.  It contains acid sulfate materials.  Behind the new beach is a backwater that has filled with mangrove detritus.  When this material is buried, the organic matter feeds bacteria which combine sulfates from seawater with iron from seawater or from the mangrove detritus to create iron sulfide.  On exposure to air, the sulfides oxidise and become sulfuric acid which can eat concrete and steel and burn living organisms.  Whilst the process of acid sulfate soil formation is well known, as far as I can tell, it has never been visually documented.  Hopefully, I can fill this gap.  Another post describes the acid and hydrogen sulfide being produced by organic matter that was buried in the old Barron River mouth 40 years ago.

In the basin behind the new beach is a vast deposit of mangrove detritus

Where it has been buried by sand, black rivulets issue from the ground

Queensland Mangroves Die from Volcanic Eruption

To be precise I have found a creek choked full of pumice that comes from an undersea eruption. I am investigating the range of environmental insults that mangrove trees are subject to and being buried in pumice is one of the strangest.

An undersea volcano to the north of New Zealand and about 2900 km from Cooktown is the source of the pumice. A small fraction of the reported 20 000 square kilometres of floating pumice arrived in North Queensland after a journey lasting more than 9 months. Large 'rafts' of pumice were reported near Cooktown in August-Sept 2013 and bands of pumice were present on Bramston Beach in November. The Cairn's Northern Beaches which lie in the middle did not get any pumice until several weeks later. Out at sea, there were widely scattered blocks of pumice and I did not see any of the rafts personally. Most of the larger blocks had goose barnacles attached and and the beach was on the nose for a few days after the pumice washed up as the goose barnacles putrefied.

40 m wide drift of pumice at the mouth of the inlet

Some scientist postulated that all sorts of marine organisms are rafted across the ocean on floating blocks of pumice. All I could find was a thin coat of filamentous algae and goose barnacles so I suspect that the sea mount was too remote from reefs or life on a floating block of pumice is too harsh for most organisms.

The pumice on Cairn's beaches has long since disappeared. Most is buried within the beach I suppose. It gets blown inland by cyclones and covered. Pumice is also supposed to eventually become waterlogged and sink but this must be a slow process as a creek on the southern side of Archer Point is totally clogged with the stuff.

Archer Point is one of the windiest places in Australia and it has a south-facing beach with a small inlet that is a natural pumice trap. At the mouth of the inlet is a trapped raft of pumice some 40 m wide. Flowing into the inlet is a tiny creek with a channel approximately 100 m long and 5 m wide. The headwaters of the creek lie in a very small catchment which appears to be feed mainly with seepage rather than overland flow. Seepage fed systems do not flood violently like normal creeks so the pumice in the creek is not likely to be washed out. Small creeks like this are common where hilly land meets the sea.

Pumice within the mangroves, creek in in the middle

Now that the surface of the creek has been choked with pumice for almost a full year, has there been any effect. Several of the mangroves on the bank of the creek have expired including a myrtle mangrove and some stilt mangroves. The dead mangroves appear to be in the minority and live healthy mangroves dominate in back swamps away from the channel. It seems the impacts are limited to the channel and channel margins, which is not surprising as backswamps often act like normal forests rather than tidal forests. The pumice raft did not cover the ground in these areas.

Several dead mangroves on the margin of the creek

The channel however appears to have been devastated. The thick raft of pumice would have cut off the air supply to the creek and the organic matter with the raft of pumice or previously deposited within the creek would be decomposing and releasing hydrogen sulfide. I poked my underwater camera into the small opening in the pumice created by the strong wind and attempted to record the amazing bacterial films that were coating every surface. There was a definite structure to the strands, quite like the root system of a plant. When I lifted my hand from the water, there was a powerful stench of hydrogen sulfide. It is likely that all of the crabs and fish in this little system perished when the pumice raft blocked the sun and air (see previous posts). I prefer crabs and fish to bacterial slimes however bacterial slime are also a topic of great scientific interest as the bacterial colony which consists of several species is self-organising to organs that resemble multi-cellular life. The size and structure of these slimes is the best I have ever seen.

feathery lace of bacterial slime

long filaments of bacterial slime with white colonies

Small gap in pumice through which photos were taken

This is not the first pumice raft I have seen. They are semi regular occurring every decade or so. If the volcano is closer then raft can be very thick. In some of the Pacific Islands, they have even used pumice from similar creeks to make light-weight floating concrete that can be cut with a saw!

Investigating the Naturally Toxic Watercourse

I am on a mission to determine what mangrove and mangrove creatures can actually put up with.  At what point does environmental stress start to cause degradation.  It is an incredibly important question from an environmental management point of view.  With that in mind, I revisited the naturally toxic watercourse to test the acidity of the water.   Acidity is highly detrimental to plants and two significant environmental problems in Australia are acid mine drainage and acid sulfate soils.  Plants just will not grow on acid soils – a few specially adapted species can, however they are typically present in unusual habitats such as peat bogs which are not relevant to the general environment.  Acid affected areas are normally devoid of vegetation or have shallow rooted grasses which can grow in the shallow layer of sand on the surface where the acid is washed away by rainwater.

To test the waterway, I bought an aquarium pH test kit with a wide testing range for $12 and headed to the watercourse.  When I arrived, there was a moderate odour of hydrogen sulfide.  A sample draining from the toe of the beach via a blackened rivulet was taken an attempt to get a really acid reading.  The result was green (pH 7) which is neutral.  Seawater is normally alkaline (~pH 8.5) the water is slightly acid in comparison to the nearby sea.  Such a slight effect should not impact on many creatures and indeed, there were gobies in blackest, mankiest rivulets.  Some mangrove creeks receive acid waters from freshwater swamps which generate tannic acid (as low as pH 5.5) and the mangroves appear to be fine, even after prolonged exposure.  In coming to a proper scientific conclusion, all of the confounding factors have to be accounted for or controlled and in the case of tannic acids, these waters are also fresh, which may confound the effect of the acid.  No firm conclusion can be draw about the slight acidity of this waterway, which lacks freshwater inputs.

Weird orange (iron-stained?) sand suggests that acid-sulfate chemistry is happening under the beach

Even with a stand of dead mangroves in the seepage zone, the water was neutral

Nearby creeks have a pH in the normal range

Then I found a dead patch of mangroves and had to investigate.   Below the patch of mangroves, there was a nasty black seepage zone, with sticks that looked like bones.

Recently killed mangroves

A blackened seepage area

A blackened rivulet, with sticks covered by a white bacterial slime

Behind the patch of dead mangroves was a blocked creek.  In recent times, sand washed up by a cyclone had blocked the mouth of the creek and waters now drain through the sand to emerge in the nasty seep.  Putting all this together we get a possible scenario.  As the tide retreats, it carries leaves to be point where the creek is blocked and they become trapped.  In freshwater creeks, thick rafts of leaves build up where a creek flows through a sandbank and the same may be occurring here.  When the leaves were buried with sand, they formed a food source for bacteria.  Once the oxygen from the incoming creek water has been consumed the water would continue flowing through the dead leaf litter where anaerobic bacteria decompose the vegetation and release H2S.  The flowing water carries the H2S through the sand and into the mangrove root zone.  Normally, mangroves can cope with H2S but the sand has buried their breathing roots and the crabs which pipe air into the mud have also moved on.  This particular combination of a limitless supply of H2S and loss of soil aeration seems to be fatal.  A few metres away other mangroves also have buried root systems but are thriving, so sand alone does not hurt mangrove trees.  Blockage of the creek and the prolonged flooding also seem to have no effect.

View up the blocked creek

In this post, I have listed several potential environmental stresses to mangrove forests and have found that mangroves can easily cope with prolonged fresh and saltwater flooding (a few months) and can cope with acidic freshwater.   Slightly acidified saltwater probably has little effect, however burial of breathing roots in an environment that generates H2S seems to be rapidly fatal.  What little detailed information there is on the net supports the idea presented here that H2S is one of the key or perhaps the key factor affecting the survival of mangroves.  There is even a possible case of two lovers perishing at the hand of H2S as they enjoyed each-others company beside a mangrove waterway.

Tannin stained water in Wyvuri creek when mouth was blocked by sand

More reading on acid sulfate soils:

• Commonwealth Government information page; and
• NSW Government information page.