Ecohydrology Factors in the Environmental Degradation and Restoration of an Urbanised Lagoon on Magnetic Island (Yunbenun), a World Heritage Area Fringing the Great Barrier Reef

The ecohydrology of Horseshoe Bay lagoon on Magnetic Island, a World Heritage Area off Townsville, was studied using aerial photography and maps (1892–2023); Landsat/Sentinel-2 imagery; LIDAR and RTK-GPS surveys; in situ water-level and salinity logging during the 2025 dry season and 2026 wet season; and hydrodynamic modelling. We suggest from that data that three cumulative human interventions, namely the diversion of Eastern Creek into the lagoon, the unmitigated catchment erosion delivering sand to the lagoon, and the progressive dredging of the Outflow Channel culminating in the removal of a natural shoal (“weir”) in November 2022, have altered the lagoon’s hydroperiod and allowed tidal seawater intrusion. This likely caused the salinisation of the lagoon, mass mortality of Weeping Paperbark (Melaleuca leucadendra L.), loss of Bulkuru Sedge (Eleocharis dulcis L.), and possible impacts on Magpie Geese (Anseranas semipalmata) and Agile Wallabies (Notamacropus agilis). Restoration measures are needed all over the watershed of the lagoon and its estuary. These include restoring the outflow weir, using one-way flap gates, and catchment erosion control, to be implemented under adaptive management principles. Long-term monitoring of the hydrodynamics and the ecology of the lagoon is needed to guide the implementation of ecohydrology-based solutions for the environmental implications of the dead trees in the upper lagoon, the new sand delta in the lower lagoon, and long-term sea level rise.