Estuaries and coastal wetlands of the southern hemisphere – An overview

Our understanding of the coastlines of the Southern Hemisphere has generally lagged behind that of the Northern Hemisphere. Research in each of the three continents has usually progressed independently, often stimulated and informed by paradigms that were developed in Europe or North America. In September 2018, the Estuarine & Coastal Sciences Association held its conference, ECSA57, in Perth, Australia, with the theme ‘Changing estuaries, coasts and shelf systems – diverse threats and opportunities’. This special issue arose from a themed session at ECSA57, entitled Similarities, stressors and sustainability of southern hemisphere estuaries on wave-dominated coasts. The session, sponsored by the Commission on Coastal Systems (International Geographical Union), brought together researchers with a focus on estuaries and coastal wetlands from across the southern hemisphere. This opportunity stimulated consideration of the sustainability of estuaries in the southern hemisphere, including those that occur in wave-dominated east coast settings, as well as more tropical tide-dominated settings. The 21 papers in this virtual special issue are based on presentations at ECSA57, with several additional complementary studies of southern hemisphere coastlines. The papers draw out similarities between estuaries and associated wetlands in Australia, Brazil, South Africa and New Zealand. Several of the studies describe the evolutionary history of estuarine systems, identifying broad geophysical factors, such as relative sea-level history, which characterise each of these coastlines, and distinguish them from more tectonically active or glaciated coastlines in the northern hemisphere. Many of the studies emphasise the links between biotic processes and abiotic drivers; others report on methods to measure sediment accretion or quantify carbon sequestration. The trends that are emerging from this research have been enhanced by the adoption of newly available, sophisticated remote sensing technologies, such as airborne Lidar and photogrammetry from drones to investigate substrate topography and vegetation structure, enabling detection of changes in sensitive coastal habitats such as mangroves and saltmarsh. Connectivity is a key theme cutting across disciplines, and in this case across the three continents and neighbouring islands. After examining ways in which coastlines of the southern hemisphere might differ from those of the northern hemisphere, the principal findings are summarised under the headings: estuary evolution, physical processes, environmental constraints on estuarine ecosystems, perturbation in wetland systems, and biomass and carbon storage.