How did Mangroves Evolve?

When I Google mangrove evolution all I find are a few pithy generic statements and no real information.  I think that if all the mangroves we currently have were to disappear, other plants would race to fill the ecological vacuum.  Next time you are in the mangroves, look for non-mangrove trees that are dipping their toes in saltwater.  It is surprising how many there are and how many species can actually survive in the edges of mangroves swamps, where they are surrounded on all sides by mangroves and are growing in mud.  The question is why are these species not completing the transition?  That is a very big question so lets investigate a small part of it, how mangrove stilt roots may have developed.

The one scientific paper on mangrove evolution that I could get for free (most are locked up behind science journal paywalls) was about the biogeography of mangrove evolution and it said that almost all the mangroves present today evolved in the Tethys Sea which was sort of where Arabia and India are today.  The earliest species were Acrostichum, the mangrove fern and Nypa, the mangrove palm.  These species like lower salinities so that is a clue.  The main mangrove species (Avicennia and Rhizophora) were around 50-70 million years ago so forests just like those lining the worlds tropical oceans today would have seen dinosaurs.  Perhaps the Tethys Sea was like the sea between Australia and the island of New Guinea is today.  The information from the paper tells me what evolved and where but does not tell me how the plants evolved.  If I knew how mangroves evolved, then perhaps I would be able to understand the barrier that is preventing more mangroves from evolving.

Recently I have been seeing stilt roots on some species of mangrove that normally do not have them.  These mangroves usually have pneumatophores which are breathing roots that protrude from the mud.  If the trees have a choice of stilt roots or pneumatophores, why do they choose pneumatophores?  Fortunately it is possible to collect evidence on conditions that favour stilt root development.

In a swale, which is a shallow valley between sand ridges, I found several white mangroves (Avicennia marina var. eucalyptifolia) with stilt roots instead of the usually pencil roots.  This is very strange as Avicennia is the champion producer of pneumatophores. The stilt roots are not as well developed as those of stilt mangroves (Rhizophora) but are clearly stilt roots none-the-less.  

Avicennia tree with stilt roots instead of pencil roots (pneumatophores)

The swale traps a pool of freshwater in the wet season and saltwater in the dry season.  I happened across the swale on one of the few days when it was completely dry.  During the wet season, the swale appears to be a window lake, which means that the freshwater table is above ground level.  Several large paperbarks live or lived among the mangroves.  However as sea level rises by a few millimetres each year and the drainage line to the sea expands due to tidal and storm water flows, the ingress of seawater is increasing and the paperbarks are declining. 

Mangroves growing around a paperbark tree (Melaleuca leucadendra) 

Two days later a full moon brought in one of the highest tides of the year and flooded the swale and covered the entire stilt root system of the Avicennia trees.  It seems that in the centre of the swale the water is just too deep for pneumatophores.  Avicennia on the margins of the swale grow thickets of pneumatophores.  

Mangrove swale with standing water at full depth

Stilt roots and a few pneumatophores when pool is full

Avicennia can also grow adventitious roots (roots growing from the trunk above ground level) in more exposed situations, such as river mouths, where regular tides are experienced. In these areas, however the adventitious roots rarely become stilt roots.

Adventitious roots on tree a creek mouth

Another mangrove species that can also form stilts instead of pneumatophores is the black mangrove (Lumnitizera racemosa).  Normally, black mangroves have very few pneumatophores unless they are in areas with freshwater seepage, then they grow strange root loops.   In one special place, near Pormpuraaw in the Gulf of Carpentaria, I found a forest where black mangroves have stilt roots instead.  The Gulf has only one high tide per day, whereas the east coast of Australia has two high tides each day.  There is much less tidal exchange of water and the headwaters of rivers probably remain brackish longer than their east coast equivalents.  Upstream of the tidal creek is a vast flat swampy plain that slowly drains into the mangroves for several months a year, creating freshwater tides for part of the year. It is these special conditions which create this strange forest.

Lumnitzera mangroves in headwaters of a creek which has strong seasonal freshwater flows

Lumnitzera on stilts with super long Avicennia pneumatophores

Brackish water swamps with still pools of water do not foster stilt roots, in these places pneumatophore development goes crazy.  In a back swamp close to the black mangroves is a brackish water swamp with pools of water surrounded by mangroves and reeds.  The mangrove pneumatophores are exceptionally long and dense.

Avicennia around a brackish pool with Baumea reeds

The Avicennia pneumatophores are as dense as the reeds

So my concluding conjecture is that stilts seem develop more in areas with seasonal freshwater flooding or tides.  In fact this is just the type of place that the forest mangrove grows.  The forest mangrove (Carallia brachiata) is the only non-mangrove member of the stilt mangrove family in Australia. Carallia has a propensity to grow masses of adventitious roots even when it is in rainforest on wet foot hills, so perhaps the stilt mangrove families propensity for this type of root structure combined with an opportunity is what lead to stilt mangroves.  

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.

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.