Wetlands in the sand – the rare and unexpected

North Stradbroke Island is one of south-east Queensland’s remarkable sand islands. Well known for its beaches and marine life, its myriad freshwater lakes and wetlands are also what make it special.

North Stradbroke Island or “Straddie” is one of the largest sand islands in the world. While you may think sand is not the best material to hold water, there are in fact over 70 recognised wetlands on the island*. These include streams, lakes and swamps that occur when aquifers (groundwater) intersect with the land surface.

As many a disappointed camper can attest, Straddie experiences regionally high average annual rainfall. This water feeds numerous small perched aquifers contained by layers of low permeability sand. These sit above a large regional aquifer that provides drinking water for islanders as well as mainlanders.

Sand-dune ecosystems on the island are known as ‘wallum’ after the local Indigenous name for the small tree Banksia aemula. Elsewhere in the region much of this vegetation has made way for coastal development. The aquatic ecosystems found in the wallum are not the most inviting for aquatic life. They are generally quite acidic and low in nutrients. Some also are “blackwaters”, naturally stained by organic material.

18 mile swamp - one of North Stradbroke's larger wetlands

18 mile swamp – one of North Stradbroke’s larger wetlands

As might be expected, not as many aquatic species are found on Straddie as in nearby non-wallum wetlands. However, what they lack in diversity they make up for in oddity. Some of the aquatic life found is rare and found in very few other locations. The presence of some species is also somewhat unexpected.

Unusual fish species include a one-gilled swamp eel that has only been recorded on eight previous occasions. Its so rare it has yet to be described. The endangered Oxleyan pygmy perch  (Nannoperca oxleyana) is also found in the wallum wetlands.

As someone who spent many years collecting and counting bugs I was amazed to discover that stonefly nymphs, which I associate with cool mountain streams, are also found on Straddie. It is thought that the constant flow of cool groundwater into streams allows for high dissolved oxygen saturation and provides habitat for stoneflies akin to that normally only found at high altitudes.

A stonefly nymph

A stonefly nymph

Another unusual feature of North Stradbroke Island’s freshwater life is the difference between what is found on the west versus the east sides of the island. Throughout the majority of the island’s 500,000 year existence, sea levels were much lower than today, and the island was joined to the mainland. This means the streams that are currently isolated on the west side of the island were tributaries of mainland river networks and animals intermingled.  Eastern streams on the other hand were isolated.

Wetlands on Straddie today reflect an intriguing combination of fairly harsh but stable conditions, strongly influenced by the water that lies underneath the sand and the sands of time – connection to then isolation from the mainland.

* The full article on which this piece is based is: Distributions of the freshwater fish and aquatic macroinvertebrates of North Stradbroke Island are differentially influenced by landscape history, marine connectivity and habitat preference. By: J.C. Marshall, P.M. Negus, A.L. Steward & G.B. McGregor. Proceedings of the Royal Society of Queensland, 2011, 117, 239-60.

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