Queensland Coast: Where the Sea is not Salty Enough

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.

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.

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.

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.

Persistent sheet of water on flats at low tide well away from the beach

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.

'Reef' of compacted flood debris

Close-up of reef of flood debris

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.