Around the Rocks at Island Point – Port Douglas

Sometimes you can't get to the Great Barrier Reef because there is not enough time, the sea is too rough or you can't afford it. It is always good to know a few areas that are easy to get to and that can provide a little adventure.  Today, I walked around the sea coast of Island Point which is at the end of the famous Four Mile Beach at Port Douglas.

View from near near Port Dickson (click photos to enlarge)

View from near Four Mile Beach

This time instead of clambering over the cliffs at the Four Mile Beach side of the headland, I walked in from the Dickson Inlet side, which proved far easier.  On rounding the first corner, all of the bustle of the busy tourist port was left behind and it was like being on a deserted tropical island.  It is the kind of place Bear Grylls uses to challenge modern humans.  It was 37 degrees Celsius, and despite a sea breeze in the Port, the rocky coast was almost airless and 100% humid, right in the danger zone for the unacclimatised.  Visiting this area requires the crossing of almost a kilometre of large round boulders.  These boulders have a layer of ancient blue green bacteria (Nostoc) which is natures first attempt at a non-stick coating.  Finally, with the mangroves being close there is a low but credible risk of crocodiles, there are nasty oyster reefs and the rocky coast is where the toxic jellyfish breed so swimming around the headland is not recommended.  If you are are fit and take care then you will be rewarded with a rugged coastline featuring dozens of rock pools full of darting fish.

A rock pool with a black sea sausage

Juvenile diamond-scale mullet and two sergeant majors

Most of the fish present are specialised for living in rock pools and include a variety of gobies, pipefish, dart fish and juvenile reef fish which use rocky coasts as a nursery.  If it is high tide or if our seasonal 15-20 knot trade wind is pumping large waves onto the coast, very little of this marine life will be visible.

Higher up are smaller pools in the black rock.  These have grazing snails and sometimes even have live coral! It seems that under the right conditions, coral is even tougher than algae.  The rocks were so hot that the limpets were almost being cooked alive.

Most of the bottom of this tiny pool is covered by colony of hard coral.  It has a touch of bleaching on one edge.

Nerita costata snails feed on the thin layer of algae

Heat-stressed limpets were holding their shells as high as they could to cool down

The sub-tidal zone is dominated by brown macro-algae and black sea urchins (Diadema).  I do not know if this community is a recent community that has developed due to climate change and loss of coral.  Until recently it was incredibly hard to make an environmental record, there were no GPS, no cheap underwater cameras and each photo cost a dollar which is probably 1000 times the cost of a digital photo.  I only have fading memories of what places used to be like twenty years ago and these often do not include vital information such as which season it was.

The coast that I observed a few days ago is not that different to temperate coasts where algae dominate the swash zone.  Under the water, algae cover the rocks and a few metres further out the rocks give way to sand flats.  In this underwater rock garden, sea urchins are the gardeners.

Boulders quickly give way to a sandy seabed 

Diadema sea urchin

Close to then end of the headland, rainforest gives way to patches of natural grassland with pandanus trees due to the constant blasting of wind and salt spray. Up the top, parts of the hill belong to very rich people so avoid the temptation of bush bashing up the slope.  To get to the top, find the path at the end of the headland.

As with any posts on this blog, original photos and data are available to anyone doing biodiversity studies.

Miniature Insects of Tidal Sand Flats

Sometimes if you just look hard enough you may see something no one else has seen.  Unable to find the time to go somewhere new, I decided to sit on the edge of a sandbar and watch at the seawater gradually draining from the sand and I kept up my concentration until I saw something that I had never seen before.  Tiny flecks of life appeared to be moving around on, under and above the water surface above black patches of mangrove debris.  Moving above the surface implies insects and insects the size of dust particles can fly through water as well as air! However it was hard to believe that such infinitesimally small creatures could survive in an environment that lasts only a few hours before the tide comes in violently reworks the very ground.  I can remember ten men trying to open a metre wide channel from a nearby blocked creek to the sea.  Such is the rate at which the sand is moved around that digging like mad they could not open the channel faster than the sea could fill it back in. Yet it is in this ocean of highly mobile sand that the flecks of life are present.

The sand flats can look like this one day (click to view)

I described these insects to the entomologists at the local university to a muted response.  It was uncomfortable to call what I had seen a community or ecology as how can a community exist when the very terrain is destroyed and recreated every few hours.  Worse still, finding the flecks of life a second time eluded me.  Even scraping up the mangrove debris and going through it under a microscope failed to yield anything other than a few larval crabs.  For nearly two years I have been looking. If I put a bright LED torch on the sand at night, flecks could be seen jumping through the beam.  However the nights are seldom still and it was difficult to be sure that the flecks were not flakes of matter propelled by the breeze.

Hours later the sand flats can be a very different place, in this case turned over by stingrays

A few days ago, I tried the old trick of just looking until I saw something, this time in a different place and I rediscovered this lost community.  Perhaps this community is only present in the winter (June-July) or perhaps it is only present in certain areas.  I am yet to work this out.  It was late in the day and the sun was so low that the shallows had become dark yet anything on the water surface was still illuminated and suddenly the true number of these little creatures was revealed. To see the creatures, you must approach the waters edge with great care as the weight of a foot fall even a half a metre away gently shifts the sand, causing the sand to slump just slightly and warning the little creatures to depart the area.  Video confirms that they are moving around the edges of the water, as I cannot easily see them with the naked eye.  I can only see them on the surface of the water and here they are caught by the wind and streak across the surface so quickly that in a few video frames they are gone.  Falling into the water does not seem to bother them and may in fact be part of their normal habitat.

A rill between sand ripples at the mouth of the Barron River

Springtails floating on the water seen in silhouette

I filled a small bottle with surface water and took it home for a look under the microscope.  Even under a microscope at 40 x the creatures are small.  They proved to be springtails, small wingless insects that can fly by flicking themselves into the air with a spring-like rod that folds under their body.  To the naked eye they look white or brown (could be a few species) but the brown ones are actually bright yellow.  The captive creatures were placed on a drop of water on a black surface and I photographed then with my compact camera held over the eye piece of the microscope.  The first photo shows the springtail beside a fine blonde human hair.  The other photos show the springtails rolling around on the surface of a drop of water.  Under the abdomen in the last photo is the spring that gives springtails there name.

Springtail beside a human hair

Are the springtails just a curiosity or are they important?  Their sheer numbers probably make them important and potentially they are food to the many juvenile fish that live in the pools.  Aside from that, I wonder if their yellow colour is a sunscreen.  I also wonder if they are part of a detrital community based on bacteria that live on buried organic matter as they seem to be present mainly in shallow areas where detritis is present in the sand.

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.