Dynamics of the open coast mangrove fringe in Chambers Bay, northern Australia
摘要
Coastal mangroves respond to a range of environmental drivers, including sea-level rise. This study investigates changes in extent of the mangrove fringe on the open coast of Chambers Bay on the southern shore of van Diemen Gulf in the Northern Territory, a location that has experienced the most rapid inland expansion of mangroves and the fastest rate of coastal erosion in Australia. Spatiotemporal changes in mangrove distribution were assessed using the Digital Earth Australia (DEA) archive of satellite imagery from 1987 to 2022. Digital Shoreline Analysis System, incorporating analyses of 1192 transects for 36 km of coastline, and Object-Based Image Analysis were applied to examine the influence of geomorphological features. Our analyses revealed non-uniform responses of the seaward mangrove extent over the past 35 years, categorised into four modes: contraction, progradation, stability, and establishment. Approximately 6 km2 of contraction and 2 km2 of progradation and/or establishment were observed along the open coast. The broad coastal plain south of Chambers Bay prograded during the mid- and late Holocene, and historical aerial photography indicates a near-continuous mangrove fringe several hundred metres wide. The mangrove fringe has been completely eroded away west of Point Stuart with a rapid mangrove contraction rate of about 18 m/year and shoreline retreat at up to 15 m/year. Much of the remaining mangrove fringe has been contracting at slower rates; in some places, the seaward margin appears stable; elsewhere progradation continues at rates of up to 8 m/year. Prominent along sections of the coast are sandy ridges, which overlie coastal plain muds and are termed cheniers. Similar chenier ridges, preserved on the plains surface, indicate previous phases during which progradation has been interrupted, reinforcing the significance of these landforms. Mangrove fringe retreat has been slower where intersecting minor tidal creeks and mangroves persist, acting as mangrove groynes. Elsewhere, where the fringe is completely breached, sandy sediments are overwashed, and a pattern of retreat is apparent that may foreshadow processes that will dominate when retreat becomes more widespread as anticipated with accelerating sea-level rise. The observational findings highlight feedback mechanisms between mangroves and coastal landforms, indicating the critical role of coastal geomorphology in mediating mangrove responses to environmental conditions, which may be further exacerbated in response to sea-level rise.