Dr. Danghan Xie Publishes on Nature Communications: Removing Mangroves Would Not Aid Coastal Restoration
It’s not possible to reduce mud by removing estuarine mangroves
Mangroves are known to slow down currents and trap mud. However, mangrove removal, rather than reducing estuarine mud levels, may unexpectedly exacerbate the process of estuarine muddification. A recent study published in Nature Communications sheds light on why more mud is accumulated in the estuary after mangrove removal and what alternative restoration strategy could be taken to mitigate estuarine muddification. In New Zealand, upstream deforestation and agricultural activities due to historic European settlements have increased the supply of mud to the coasts. In turn transforming the coastal landscape in expanding mangrove forests. Local authorities try to revert this trend and reduce coastal mud contents through local mangrove removal. “We need to be considered multiple feedback loops, since mangrove removal might lead to more mud accumulation on the entire system,” says Danghan Xie, Postdoctoral researcher at Boston University and lead author of the study.
Mangroves play a crucial ecological role by providing essential services such as coastal protection and carbon sequestration and serving as critical habitats for various species. The evolution of coastal mangrove landscape is highly impacted by the amount of riverine sediment supply and the presence of mangrove forests. Sediment transported to the coast might settle on the ground such that soil level is raised, while the sediment might also be flushed out of the coast by tides such that less sediment is accumulated in the coast. Mangroves can interact with tides and influence sediment movement. Coastal restoration strategy involving mangrove management should be careful, as the way mangroves impact their environment depends on the interaction between local, estuary and source-to-sink scale feedbacks.
Local scale
New computer simulations show how the presence of mangroves reduce tidal currents locally and therefore facilitates mud deposition, creating levees and channels. In areas with mangroves, the soil level is higher as more mud is trapped by mangroves. “Based on such local-scale processes, i.e. mangroves enhance sediment trapping, mangrove removal is expected to reduce overall mud trapping, but the situation is more complex than it may seem” says Xie.
Estuary scale
If looking at the estuary-scale picture, removing mangroves surprisingly intensifies the problem, rather than mitigating mud accumulation in the estuary. This study shows that mangrove removal triggers a chain reaction at the estuary scale, altering water flow patterns and sediment dynamics, which, in turn, exacerbates mud buildup. “This happens because a greater amount of mud is deposited within the channels, and simultaneously, more mud is transported to the tidal flats located further away from the channels”, Xie illustrates. “Actually, keeping mangroves not only limit the ‘escape’ of mud from channels but also concentrate flow current so that more mud can be pumped out of the system”.
Source-to-sink scale
Xie adds: “We need to look into a bigger picture. Rather than focusing on mangrove removal in coastal areas, reducing upstream sediment supply through sustainable land-use practices can significantly mitigate the estuarine muddification. It is also essential to know the timing of mud reduction is important, as an ‘early’ mud reduction can have major implications on slowing down the mud increase”.
The publication entitled “Mangrove removal exacerbates estuarine infilling through landscape-scale bio-morphodynamic feedbacks” was published in Nature Communications. Co-authors are from Boston University, USA; Utrecht University, the Netherlands; KU Leuven, Belgium; University of Waikato, University of Auckland and Waikato Regional Council, New Zealand and University of Exeter, UK.
DOI: https://doi.org/10.1038/s41467-023-42733-1
(Photos are credit by Department of Conservation, New Zealand)