One of Magnetic Island’s (Yunbenun) popular tourist drawcards is at risk without major reparation work, according to James Cook University research.
Volunteer researcher Chris Hopper, JCU’s Adjunct Professor Eric Wolanski and independent researcher Natalie K. Dowling, examined the impact of saltwater and erosion on Horseshoe Bay Lagoon, a freshwater lagoon and one of the few swamps with bulkuru sedge – also known as water chestnut – on the island.
“This lagoon is a hidden gem on the island and it’s in danger because of increased tidal inundation due to historical mismanagement of existing problems,” Mr Hopper said.
“There are two main areas of damage – the lagoon which now has saltwater flowing into what was a freshwater lagoon, and the AFL oval, which is affected by severe erosion and rocks used in a failed attempt to build leaky weirs.”
Professor Wolanski said their year-long study found the increased salinity had killed many of the weeping paperbark trees that surrounded the lagoon.
“The bulkuru sedge, which is food for the magpie geese chicks in the area, is also dying off because of the saltwater inundation volume is too high,” he said.
“This also affects agile wallabies, that are forced out to compete for food with the rock wallaby.
“We believe three human interventions are to blame for the damage – the diversion of Eastern Creek into the lagoon, unmitigated catchment erosion and the progressive dredging of the Outflow Channel, which removed a natural shoal or weir several years ago.”
The team used aerial photography and maps, satellite imagery and salinity logging during the 2025 dry season and 2026 wet season to gather data, backed by hydrodynamic modelling.
“Massive restoration measures are necessary, including annual dredging from the Inflow Channel, restoring the outflow weir to trap freshwater in the lagoon, stopping saltwater intrusion using one-way flap gates and catchment erosion control,” Professor Wolanski said.
“We’ve provided modelling as to why we think adaptive management is the best way forward for the health of the lagoon and the upper catchment and how fish passage can be facilitated through one of the flap gates at all times,” Mr Hopper said.
“We need to rebuild the natural sand weir and ensure there is enough freshwater for successful breeding of the magpie geese after the wet season.
“They need a certain number of days to lay and hatch their eggs and allow the chicks to grow.
“We want to avoid a decrease in the breeding of the species.”
“These measures would need the consent of the Great Barrier Reef World Heritage authorities, Queensland fisheries and Traditional Owners to reverse the slide into increasing environmental degradation of what was a very nice area,” Professor Wolanski said.
“It will be difficult to fix, but we think it can be done if we take a holistic approach.”
The peer-reviewed research was published in the journal Applied Science.
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