Clearing mangroves makes ‘muddification’ worse
Mangroves have been cut down in some parts of New Zealand, with the aim of reducing mud build-up and exposing the mud to currents and tides that can wash it out to sea. Widespread deforestation and farming was begun by European settlers in the late 19th Century in New Zealand, increasing the amount of sediment in rivers. Over many years, this has caused mud to build up in estuaries – providing habitat for mangroves to expand. But clearing those mangroves to stop estuaries getting clogged with mud actually makes the problem worse, new research shows.
Mangrove removal causes estuaries to trap more mud, not less.
We were surprised to find that mangrove removal increases muddification of estuaries, says lead author Dr Danghan Xie. He carried out this research as part of his PhD at Utrecht University, and currently works at Boston University.
Mangroves create relatively deep, fast-flowing channels that allow sediment to flow out to sea. Mangroves also trap mud efficiently near river channels, leaving less to settle elsewhere. When you remove mangroves, sediment can reach parts of estuaries that previously received very little. Overall, mangrove removal causes estuaries to trap more mud, not less.
While mangroves are under threat in many parts of the world due to human activity, the expansion of muddy mangrove habitats in once-sandy New Zealand estuaries is often unwelcome. The study used a bio-morphodynamic model – a computer simulation that takes account of factors including tides, currents, sediment patterns and mangrove growth in New Zealand estuaries. The findings show that coastal management of muddification probably won’t work.
Mangroves are a symptom of a wider problem – not the cause.
Symptom of a wider problem
We need to look into a bigger picture, says Dr Barend van Maanen, from the University of Exeter.
Our findings show mangroves are a symptom of a wider problem – not the cause. Rather than focusing on mangrove removal at the coast, the solution is more sustainable land use upstream. By reducing the amount of sediment going into rivers, we can safeguard sandy ecosystems and eliminate the pressure for expensive – and possibly counter-productive – management downstream.
Xie, D., Schwarz, C., Kleinhans, M.G. et al., 'Mangrove removal exacerbates estuarine infilling through landscape-scale bio-morphodynamic feedbacks', Nature Communications 14, 7310 (2023). https://doi.org/10.1038/s41467-023-42733-1