Impacts of Mangrove Dieback and Recovery on Coastal Wetland: A Case Study in the Gulf of Carpentaria, Australia.
Summary
Mangrove forests are valuable ecosystems, offering socioeconomic and ecological services and thereby supporting the livelihood of millions of people. However, man- grove areas are declining due to various threats. These include coastal development, increasing extreme weather events (e.g. droughts and cyclones) and sea level rise. One recent example of a mass mangrove area loss is the dieback event in the Gulf of Carpentaria, Queensland Australia. With the loss of the mangrove trees, their valuable ecosystem services such as coastal protection and carbon sequestration are lost. Therefore, a need to further understand these valuable ecosystems and their morpho- and hydrodynamic interactions becomes apparent. In order to mitigate such dieback events and identify recovery projections.
The aim of this study is to gain insights into the causes of the dieback event and project the possible recovery of the mangrove characteristics within an estuary, by conducting numerical simulations with a biomorphodynamic model. Thereby, the hydromorphodynamic model, Delft3D is coupled to a dynamic mangrove vegetation model. The model was used to simulate the development of mangrove trees within an estuary, de- pendent on inundation and competition. Thereby the effects of tides, river discharge and sediment availability are included. The model input parameter and boundary conditions were chosen to resemble the case study of the Leichhardt estuary in the Gulf of Carpentaria. In five phases, the estuary development, mangrove establishment (with and without fluvial mud supply), mangrove dieback due to sea level drop and river discharge decrease and mangrove recovery were modelled.
The results indicate that, during the mangrove colonization, the inclusion of fluvial mud supply lead to lower mangrove area coverage in the estuary. This was found to originate from sedimentation and mud accumulation on bars, elevating the bed to a level that no inundation can reach these areas. Therefore, not enough water is avail- able in those areas for mangrove growth. During the mangrove dieback simulation, the results further show that mangrove dieback can be provoked within the model by a sea level drop and river discharge decrease. Thus, the hypothesis of sea level drop and drought conditions causing the large-scale dieback was validated within the model. But at the same time some differences in dieback pattern suggest that further factors could have potentially played a role in the dieback in certain areas. The mangrove dieback has effects on the morphology and hydrodynamics of the estuary. The absence of mangrove vegetation leads to an increased flow velocity in the area of mangrove loss and decreased flow velocity in the channels and thereby, channel infilling, channel shifting, and decreased sedimentation in the previously vegetated areas. This was found to be based on the reduced flow resistance in the dieback areas. While this leads to a decrease in flow velocities in the channels causing the channel infilling. Channel shifting is on the one hand caused by reduced vegetation bank stabilization and vertical velocity increase due to the vegetation cover. During the simulation of the recovery it was found, that these hydromorphodynamic changes due to the decreased mangrove cover have effects on the mangrove recovery, leading to a hysteresis effect. The mangrove area dieback leads to morphological and hydrodynamic change within the estuary which in turn has a negative effect in the re- settlement of mangroves in the dieback area. For the natural system, it was therefore concluded that considering the likelihood of further disturbance events (e.g., sea level rise, droughts, cyclones) during the recovery process a full recovery seems unlikely.