Hydrogeological controls on endangered frog breeding habitat in an urban coastal wetland: Insights for conservation strategies

Coastal ecosystems are shaped by the dynamic interaction of freshwater and saltwater, governed by both oceanic and terrestrial hydrological processes. However, anthropogenic development and climate change are disrupting these processes, necessitating targeted conservation strategies to sustain ecosystem functions. This study examines the hydrogeological processes influencing water levels and salinities in a coastal wetland near Avoca Lagoon (NSW, Australia), an intermittently open and closed system that is manually breached when water levels exceed a threshold to prevent urban flooding. The wetland was specifically designed to support the breeding of the endangered Green and Golden Bell Frog (GGBF), whose eggs and tadpoles require a narrow range of low-salinity conditions for survival. We established two surface water and three groundwater piezometers at depths of 3.5 to 5.5 m to monitor water levels and salinity. Additionally, multiple electrical resistivity tomography transects were acquired near the wetland, and the lagoon's depth and salinity profile were surveyed using a kayak. The results reveal that lagoon levels rise rapidly after rainfall and decrease gradually through evapotranspiration and water loss to the ocean during dry periods. The wetland’s water levels closely follow those of the lagoon, indicating hydraulic connectivity through the subsurface. Manual breaching of the lagoon’s berm prevents flooding of low-lying areas but leaves the lagoon level too low to sustain wetland water, causing it to dry out. Salinity within the lagoon is stratified, with brackish water overlaying seawater. While these saline conditions are unsuitable for frog breeding, the wetland is surrounded by fresh groundwater, which can discharge into the wetland under lower lagoon levels to create favourable breeding conditions. High lagoon levels, however, breach the barrier between the lagoon and wetland, causing salinisation and compromising habitat suitability. Our investigation reveals the delicate balance of water level and salinity conditions required for GGBF breeding, requiring a critical "goldilocks zone". Effective habitat conservation strategies must address a complex interplay of hydrogeological processes to enable breeding conditions, including challenges posed by climate change-induced shifts in rainfall patterns and future sea level rise. These findings underscore the broader challenges coastal areas face under increasing anthropogenic and climatic pressures, highlighting the critical need for improved management approaches that integrate surface and groundwater processes to protect frog habitats and maintain broader ecosystem functionality.