When it comes to mangroves, the less salty the environment,
the taller and more diverse are the mangrove forests. The most magnificent of all mangroves are
found in the tidal reaches of large freshwater rivers. Unfortunately the same cannot be said for
fauna as diversity drops off quite dramatically as soon as the salinity
drops. In fact with fish, only about 2%
of fish species can survive in brackish zone and I am sure that even fewer
invertebrates would thrive in this environment. However to my surprise this effect can even
affect biodiversity levels in the sea near the mouths of large rivers.
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Aerial view of sand flats in 2013, with river mouth at top of photo |
For many years I had been itching to investigate the wide
sand flats on the southern side of the Johnstone River mouth in Far North
Queensland (Australia). The Johnstone
River flows from the wettest part of Australia, a place which has 4 m per year
of rain on the coastal plain and up to 8 m per year in the mountainous
headwaters of the river. Furthermore,
the river does not have a large mangrove estuary and pretty much flows straight
into the sea. At the mouth is a sand
flat approximately 1.5 km long and 0.5 km wide.
Inspecting the sand flats with GoogleEarth shows several parallel
sandbars with darker troughs between. At
Yule Point, the darker areas were fossil coral reef and in the
mouth of theBarron River, the darker areas were shallow pools that often contained
something whether seasnakes or sponges.
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Large, soft sand ripples and shallow pools |
Getting to the sand flats is tidally restricted as one has
to cross a small mangrove creek to get there. A few things stand out
immediately. There is grass on the lower
creek banks between the mangroves and the channel, an arrangement that I have
never seen before and something that is no doubt only possible were the tides
pump mainly freshwater. On the face of
the beach is a broad swath of black mineral sand. Heading out onto the sand flats, the surface
had oversize sand ripples which were strangely soft underfoot. The much hoped for intertidal biodiversity
was absent and the sand flats were almost sterile save for saltwater yabby
burrows.
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Black mineral sand on the beach |
Eventually some hundreds of metres from the river mouth, I
began to find a few common sand flat creatures but even then, only in low
numbers and small patches. Even on the
extreme low tide it seemed that the sand flats were struggling to shed water
and emerge into the air. There was
nothing exciting. Only small fish seemed
to inhabit the deeper pools as on account of their mobility they could avoid
the factors that were diminishing the intertidal fauna.
The simplest explanation for the lack of intertidal organisms is that floods and high river flow
regularly turn the sea fresh in this location. The poor drainage of the sand flats would also
mean that during a low tide deluge, any sea creatures present would be subject
to a torrent of freshwater. Such
exposure to freshwater would kill most sea life.
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Persistent sheet of water on flats at low tide well away from the beach |
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Freshwater seepage pours out of the base of the beach making the beach very flat |
The dark troughs between the sand bars turned out to be
deposits of dead rainforest tree leaves.
Many of the leaves were still recognisable to species level. Despite the gift of abundant organic
material, there was nothing that I could find that fed on this material, although hermit crabs did appreciate these deposits of flood detritus. Teredos or ship worms are a marine animal
that depends on logs washed into the sea to survive so the concept of marine
animals feeding on terrestrial material is possible. In places compacted deposits of leaf and stem
material formed low ‘reefs’ across the sand flats. I also suspect that these bars and layers of
silt were buried under much of the sand flat and accounted for the strange
softness underfoot. It is often said
that a particular fossil was formed when a river covered some animal or plant
with sediment and here it is possible to see the process in action.
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'Reef' of compacted flood debris |
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Close-up of reef of flood debris |
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In some places blankets of fresh rainforest leaf litter cover the ground |
Moving to the rocks at the southern end of the beach
revealed a rocky landscape with a community of algal turf, oysters and a low diversity of marine
gastropods. Some of the marine algae
seemed to depend on terrestrial seepage.
Bedrock formed a large part of the coast and every few metres it changed
texture and colour as it was probably a contact zone with hot intruding
volcanic material. Between the rock
outcrops was a pavement of uniformly sized boulders, all tessellated into
position by fierce storms. In the corner
of the bay was more recent evidence for such storms in the way of trees fallen
into the sea and root systems exposed by erosion.
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Rocky shore with tessellated boulders and outcrops of bedrock |
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Mangroves in this area have dayglo green trunks when wet |
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Strange moss-like algal colonies occurred in seepage areas |
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The protected corner of the beach was covered with piled up remains of trees due to beach retreat during major cyclones. |
In summing up, this location has lush vegetation overhanging the sea and a modicum of scenery but it is not a delightful place to visit.
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