Year
2024
Degree Name
Master of Philosophy
Department
School of Earth, Atmospheric and Life Sciences
Abstract
The changes in mangrove extent define the lateral and horizontal changes of mangrove fringes on the open coast. These changes in mangroves will impact the topography of the coastal areas and the entire coastal ecosystem. As the sea level rises, the impacts on mangrove fringes, their adaptation capacity, and their changes on different geomorphological landforms are still not fully understood through previous studies. Therefore, to minimise the gap in the mangrove research and infill the understanding of seaward mangrove fringes and their changes, the primary goal of this study is to analyse the changes in mangrove extent on the mangrove-dominated landforms in northern Australia, especially the case study of Mary River and Point Stuart regions of the Northern Territory. The Mary River estuarine plains have an intricate network of tidal creeks surrounding the mangrove expansions, which makes it challenging to quantify the lateral variations in the extent of mangroves in this estuarine environment. Additionally, the narrower extent of mangroves located in the current open coast settings in Point Stuart after the most significant shoreline erosion occurred on the coastal plains, that shoreline erosion highlights the choice of the Mary River open coast as the final study site. This mangrove extent changes analysis of the Mary River open coastal plains was conducted over a significant time covering from 1987 to 2022. This study utilised sophisticated remote sensing techniques integrated with extensive national-level datasets. The change rate in the mangrove seaward extent and change lengths in specific transect positions of mangrove seaward extent were quantified using the Digital Shoreline Analysis System (DSAS). This study classified the variations in mangrove extent into four distinct modes of change: contraction, progradation, establishment, stability, and their associated change areas and mangrove extent behaviours on these three distinct geomorphological landforms, i.e., cheniers, mangrove groynes and overwash flats. These different change modes occupied mangrove extent areas: 2.42 km2 was occupied by contraction, 1.9 km2 of areas experienced progradation and establishment, and 2.26 km2 experienced stability from more than three decades. Simultaneously, this study determined the patterns of changes in which the change rate in the mangrove seaward extent was slower than the change rate in the shoreline on the open coast. During contraction, the highest rate of -13.18 m/year was quantified on the overwash flats for mangrove seaward extent, where the highest shoreline contraction rate was -16.04 m/year. The highest mangrove seaward extent progradation rate was recorded at +7.65 m/year on the cheniers, where the highest shoreline progradation rate was +6.32 m/year. Concurrently, the mean change rate of mangrove seaward extent and shoreline were recorded as -2.12 m/year and -3.06 m/year, respectively. Conversely, there was a significant positive relationship between the extent of mangroves along the coast and the shoreline, with a correlation coefficient of +0.81. Consequently, the mangrove seaward extent exhibited variability based on their specific coastal landforms. When the tidal creek or mangrove groyne was situated on the cheniers, the evident progradation rate recorded for mangrove seaward extent was +4.32 m/year. However, when these cheniers were absent from the tidal creek mouth on other mangrove groynes, no progradation was observed for the seaward mangrove extent in these specific mangrove groynes-occupied areas. This part of the results supported that the mangroves were prograded toward the seaward extent on the cheniers-occupied areas while the surrounding tidal creeks or mangrove groynes experienced retreat or stability in their extent. Hence, the conversion of tidal creeks into mangrove groynes significantly prolonged the stability of mangroves in these tidal creeks-dominated areas. Five different mangrove groynes are classified and recorded on the open coast to understand the influence of mangrove groynes. Concurrently, there are two distinct chenier types on the open coast settings in the Mary River: beach-dominated cheniers and mangrove-dominated cheniers. The results revealed that mangrove seaward extent was less susceptible, more consistent and prograded on the seaward extent when located on the mangrove-dominated cheniers in contrast to the beach-dominated cheniers. On the other hand, most of the coastal plains occupied by overwash flats experienced mangrove extent contraction; among those areas, two major types of overwash flats were recorded on the Mary River open coast: mature-stage and early-stage overwash flats. By examining the two distinct overwash flats’ occupied areas, the mangrove seaward extent on the mature-stage overwash flats is more susceptible and has a higher trend of contraction than the early-stage overwash flats. Overall, this study reinforces the importance and influence of specific geomorphological landforms in determining the resilience of mangrove ecosystems to the current sea-level rise scenarios. These findings will also contribute to the management of coastal areas, conservation and restoration of mangrove ecosystems, and their associated policies, and strategies in open coast settings of northern Australia.
Recommended Citation
Mollick, Porni, Mangrove Extent Dynamics in Northern Australia, Master of Philosophy thesis, School of Earth, Atmospheric and Life Sciences, University of Wollongong, 2024. https://ro.uow.edu.au/theses1/1882
FoR codes (2020)
3709 Physical geography and environmental geoscience, 370901 Geomorphology and earth surface processes, 4101 Climate change impacts and adaptation
Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.