By Erica Mather 27/07/2015

The importance of mangroves in estuaries and river deltas has been highlighted in a new study. Vulnerable parts of the coast are protected from erosion by the mesh-like roots of mangroves that trap soil, buffering the impact of waves and tidal currents.

New Zealand and UK scientists utilised data on the New Zealand mangrove (mānawa) in a new modelling system to predict the effects of sea level rise on different types of estuaries and river deltas.  The research, published in the journal Proceedings of the Royal Society A, found that areas where mangroves grow are less likely to erode in the same way as other vulnerable areas where mangroves are not found.

“Surprisingly, our coastal landscape can actually keep pace with sea-level rise to a certain degree, by accumulating sediment at faster rate,” says Dr Bryan from the University of Waikato.

Dr Bryan describes how coastal estuaries and bays fill in over time, due to the run-off from erosion.  A network of sandbanks and channels gradually form in shallow areas from the movement of tidal currents.  Utilising the model to simulate the interaction between mangroves, tidal currents and sediment transport, the researchers identified the evolution of the channels.

Mangroves have been found to play a critical role in this process.  Leaf and root structures incorporate into the accumulating sediment and contribute greatly to the trapping of new sediment, further increasing the elevation of the sandbank.  The sandbank rises, increasing a few millimetres per year and was found to be able to maintain an elevation in the upper intertidal zone, even if sea levels were to rise 0.5 millimetres per year.

“Mangroves appear to be resilient to sea level rise and are likely to be able to sustain such climatic change. The implications for the New Zealand coastline are considerable and will require new thinking in terms of sediment budgets and response to climatic changes,” says Associate Professor Coco from the University of Auckland.

The species of mangrove in New Zealand (Avicennia marina) occurs in every major mangrove habitat in the world, meaning that this research is globally significant.  In addition to its role in maintaining the integrity of the coastline, overseas studies have shown the mangroves remove carbon from the atmosphere and store it in sediment.  These carbon sinks become carbon sources when mangroves are cleared.

Dr Bryan says mangroves are being seen in a new light.  Traditionally mangroves have been unpopular in New Zealand, as they tend to transform sandy beaches into mud flats.  Our coastal landscape is changing as mangroves are more appreciated.

The research teams hope that the new findings will promote the protection of global fringing wetlands from clearance for development or aquaculture.

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