Stradbroke is the planet’s second largest sand island, after Fraser Island. Over hundreds of thousands of years, sand has been blown by winds and deposited in dunes and valleys to create the island we see today.
Beneath the surface lies a large freshwater aquifer that consists of saturated sand. In places like Blue Lake, Eighteen Mile Swamp and the island’s many streams and creeks, the aquifer comes up to the surface. The aquifer is replenished only by rainwater seeping into the sand. Blue Lake is called a ‘watertable window lake’ because it opens a window into the deep aquifer.
Some fresh water is held in more superficial (perched) lakes and wetlands, like Brown Lake, high up in the dunes, where a layer of cemented sand holds the water and feeds the lake. This layer is called coffee rock.
Everything living on Stradbroke Island, the forests and wildlife, the frogs, fish and insects living in streams and wetlands like 18-Mile Swamp, depends on freshwater from the deep aquifer, or from the superficial aquifers that form into perched lakes and wetlands.
Scientists who have studied the freshwater biology of Stradbroke’s freshwater lakes and streams have found new species never before known to science. One special insect is the dragonfly Orthetrum boumiera – a deep blue dragonfly first found at Brown Lake. It lives only around brown-water dune lakes on Stradbroke, Moreton and Fraser islands, at Cooloola and along the coast south to Lake Hiawatha in northern NSW. Another new insect species belongs is the caddisfly – Westriplectes angelae – first found in swamp near Blue Lagoon on Moreton Island, and in 18-Mile Swamp on Stradbroke.
The best way to look after the waterbodies where rare creatures live is to keep them as natural as possible – with clean water (brown and blue waters are both healthy), diverse surrounding vegetation and habitats for many aquatic and wildlife species. We must make sure that humans do not cause changes to waterways – no rubbish, no damage to vegetation, no driving of vehicles into the water, no washing dishes or people’s hair. Mangroves too are sensitive to disturbance; their exposed roots must be left free to exchange gasses with the air, their leaves left to fall and rot into food for crabs and prawns.