Media release

From: University of Waikato

Don’t blame the mangroves: Study shows surprising results around coastal  restoration, mangroves and sediment

‘Don’t blame the mangroves’, is a key take-away of an international collaboration looking  into vegetation removal, sedimentation, and coastal restoration.

The study, published in Nature, shows coastal restoration on a smaller local scale isn’t going  to cut through the mud without larger scale catchment change.

Contributing author, oceanographer Professor Karin Bryan from the University of Waikato  says: “Going into the work we thought we knew what was going to happen, but the  modelling showed surprisingly different outcomes – mangroves do not trap and cause
sedimentation build up as we’d anticipated”.

The researchers used computational models based on New Zealand estuaries to assess  sedimentation inputs, and the frictional effects of mangroves on the sedimentation.

Professor Bryan worked within the NIWA Future Coasts Aotearoa programme to build  evidence of impacts and restoration efforts on local low land coastal ecosystems.

“We know changes in upstream land-use like deforestation have significantly changed  downstream coastal and estuarine ecosystems. We wanted to look at how levels of up-stream sedimentation interact with down-stream vegetation like mangroves to assess the  efficacy of certain remedial efforts at a local scale,” says Professor Bryan.

The results show that coastal mangrove removal initiatives, guided by knowledge on local- scale effects, cannot stop or mitigate mud-infilling of estuaries to restore previous sandy ecosystems.

“The removal of mangroves enhances estuary-scale sediment. This means human  interventions like vegetation removal in estuaries can lead to counterintuitive results that  actually impede restoration efforts, highlighting that more holistic management approaches
are needed,” says Professor Bryan.

Again, research points to solutions needing to be found within the larger scale catchments –  for example the reduction of sedimentation loss from up-stream land-use practise.

Lead author on the paper Dr Danghan Xie (Boston University), undertook research as part of  his PhD at Utrecht University.