Tuesday, 6 December 2016

Giant Tides and Intertidal Fauna

Central Queensland has some highest tides in Australia, with a tidal range of as much as 7 m, and I was wondering what effect such a tidal range has on the intertidal fauna. 

The rocky headlands on the Mackay Coast are spectacularly jagged with angular boulders seemingly arranged to present as many sharp angles as possible.  The way the boulders are stacked is reminiscent of the concrete the landing craft traps that were used in World War 2.  Could tsunamis or super cyclones have contributed to this artistry?  I do not know, all I know is that the boulders are shaped and arranged differently to what I have seen elsewhere.

The intertidal boulder field at Perpetua Point, Sarina is over 100 m wide. (click to zoom in)
So many of the rocks are sharply pointed
More than 100 km north at Ball Beach, fields of angular rocks surround the headland.
These complex and stable rocky landscapes should provide the perfect habitat for a wide range of intertidal fauna, but they are virtually barren.  In both the Cairns region and in the oyster growing areas of NSW, oysters are clustered in the mid to upper intertidal area but on the Mackay Coast, they are only to be found in the lower intertidal zone.  Barnacles present a similar story.  I would even suggest that this pattern extends to the small snails such as Nodilittorina which graze on the algal crust on the surface of boulders.  Usually, these small snails are present is such numbers that it is hard not to step on them, yet in Central Queensland, one almost has to search for them.  It will not be a small undertaking to figure out why this apparent intertidal paradise is all but empty. 

Oysters and barnacles cling to the low tide line, normally they are much higher.
It is not that oysters and barnacles are not trying to colonise.  Low growing mangrove trees have barnacles and occasional oysters growing on the twigs in their canopies.  Tropical periwinkles (Planaxis sultcatus) often had small oyster shells on their shells, something I have not seen before.  Even the gravel that forms shoals in the sand flats is almost carpeted with small mostly dead oysters.  On reflection, I think that salt is what makes the rocks so hostile.  Exactly how, I do not know, but the vegetation of the headlands is cropped by the wind until it is less than ankle deep.  I think the factor that cuts the vegetation on the hills is also the factor that makes the rocks so barren below high tide level. 

At least a quarter of the periwinkes at Slade Point had dead oysters on their shells
The low mangroves near the headlands are coated with barnacles (Slade Point)
Trees that are tall enough to walk under have barnacles in their canopy!
With 7 m tides, the sandy beaches are vast, being more than 150 m from the high tide mark to low tide level.   For comparison, Cairns has a 3.6 m tidal range and the beaches are about 30 m wide from vegetation line to the toe of the beach and Bribie Island near Brisbane which has a tidal range of about 2.2 m also has beaches of 30 m width.  The huge tidal range seems to fill the beach with water on a high tide and this water seeps out of the beach when the tide falls.  For a beach to be steep and narrow, swash running up the beach has to soak in, something that does not happen on beaches where water is seeping from the face of the beach, and the beach slopes in Central Queensland are very low.  A lower slope makes the beaches very wide.

You almost need wheels to get across the beach, Grasstree Beach near Sarina.
One of my interests are the filter feeders of the swash zone.  While I have only briefly looked at Central Queensland beaches, it seems that the filter feeders are almost absent.  A walk along the high tide line reveals very few wedge shells, which are the most common filter feeder in northern Australia.  My guess is that the rough choppy seas and huge tides combine to create an environment where the tidal migration that swash zone creatures need to undertake become impossible.  Filter feeding animals are present in the surf beaches of Southern Queensland, but surf is gentle on beaches in comparison to large chop.

Almost no bivalve shells are present in the tide line but pebbles are abundant (East Point)
At low tide, most of Grasstree Beach at Sarina is carpeted with the round balls made by sand bubbler crabs.  However, the upper part of the intertidal range seems to belong to insects not crabs.  The insects are almost invisible as they have excellent vision and tend to flee from people before we get close enough to see them.  Sometimes I see the shadow of flying insects more easily than the insect.  It is strange to share a place where our vision is excellent and visibility is great, yet there are creatures that can see you perfectly well but you can’t see them.  Shorebirds including seagulls somehow feed on these creatures which is frustrating when you cannot see anything.

Billions of tiny balls carpet the beach at low tide thanks to sand bubbler crabs
Sand bubbler crabs look like sand balls but seagulls still find them and eat them
On some beaches, tiger beetles run around at high speed to battle with other tiger beetles or prey on smaller creatures.  Tiger beetles flee from us and it is quite hard to get any closer than about 2 m.  I suspect the tiger beetles are preying on midges or sandflies.  On looking for these tiger beetles on the net, I discovered that the beach tiger beetle of the North American Atlantic coast looks very similar.  The North American tiger beetle in now an endangered species and there is an interesting story about how a once common species is now on the brink.  For me, it is vindication that the time I invest in understanding creatures that no one else notices is worth something.  
Tiger beetles mating on Grasstree Beach
The smooth wet zone belongs to the tiger beetles and the rest to the sand bubbler crabs
To see how different Central Queensland is from Far North Queensland, see links below: