Prevalence of Mangrove Progradation in the Gulf of Carpentaria Revealed by Satellite Time Series

Rogers, K.; Asbridge, E.A.; Goncalves, R.V.S.; Hamylton, S.M.; Kelleway, J.K.; Lovelock, C.E.; Lucas, R.M.; Mollick, P.; Owers, C.J.; Phillips, C.; Brooke; B.P.; Steven, A.D.L.; Lymburner, L., and Woodroffe, C.D., 2024. Prevalence of mangrove progradation in the Gulf of Carpentaria revealed by satellite time series. In: Phillips, M.R.; Al-Naemi, S., and Duarte, C.M. (eds.), Coastlines under Global Change: Proceedings from the International Coastal Symposium (ICS) 2024 (Doha, Qatar). Journal of Coastal Research, Special Issue No. 113, pp. 864-869. Charlotte (North Carolina), ISSN 0749-0208. The distribution of mangroves has been declining, predominantly driven by anthropogenic impacts, and increasingly climate change is proposed to be impacting their extent and condition. Projections of sea-level rise imply that the seaward extent of mangroves may retract with extension on the landward margin. The archive of Landsat imagery enables broad-scale regional assessment of changes through Digital Earth Australia, from 1988 to the present. Changes to the extent and condition of open coast mangroves can be interpreted using indices of vegetation and surface water conditions calculated from Landsat data. These dense timeseries data can be extracted and visualised along transects. In the Gulf of Carpentaria in northern Australia, where there are limited anthropogenic influences, we investigated shore-normal transects that were systematically spaced along a coastline of approximately 1500 km. This coastline includes a range of geomorphologically complex features such as chenier ridges, tidal creeks, spits and tide-dominated beaches. We found widespread occurrence of progradation of the seaward margin. Changes to the landward mangrove extent were highly variable. Despite the extensive dieback of mangroves in recent years, including events in the early 1990s and 2015-2016, recovery of vegetation cover was evident in subsequent years. This analytical approach offers potential to explore changes to both seaward and landward mangrove margins in open-coast settings. In contrast to highly populated coastlines where infrastructure and assets cause coastal squeeze, landward expansion of mangroves in northern Australia is anticipated across the widespread low-lying coastal plains, which offer accommodation space under a higher sea level. Observations of progradation of the mangrove seaward margin over the past three decades are contrary to expectations under sea-level rise.