Observed and predicted impacts of climate change on the estuaries of south-western Australia, a Mediterranean climate region

Regions with a Mediterranean climate are generally predicted to become warmer and drier with climate change. Estuaries in these regions are influenced by a broad range of climate drivers and are particularly vulnerable to the effects of climate change. We examine observed and predicted effects of climate change on the estuaries of south-western Australia (SWA), where sustained warming and drying trends have caused dramatic declines in freshwater flows of up to 70% since the 1970s, as a case study of the impacts that might be expected in other Mediterranean regions. Current and projected impacts of climate change in SWA include progressive warming and 'marinisation' of estuaries; extended closure of periodically open systems; an increased frequency and severity of hypersaline conditions; enhanced water column stratification and hypoxia; and reduced flushing and greater retention of nutrients. We document the effects of these environmental changes on the habitats, biota and ecology of SWA estuaries, including phytoplankton, macrophytes, invertebrates and fish. For example, decreasing river flows will cause periodically open estuaries across SWA to remain closed for longer periods, inhibiting the extent to which marine taxa can access these systems, thus reducing species diversity, whereas marinisation of permanently open systems will increase species diversity. We discuss the broader relevance of our findings, placing them in a global context and highlighting implications for ecosystem services and human populations. Finally, we consider the adaptation options that could be implemented to reduce the impacts of climate change in Mediterranean climate regions.