Mangroves buffer the effects of rising seas

22 July 2015

Associate Professor Karin Bryan
Associate Professor Karin Bryan

Mangroves could play a critical role in preventing rising sea levels from eating away at vulnerable parts of the coast, according to a study by Kiwi and UK scientists being published in the journal Proceedings of the Royal Society A in August.

Researchers Associate Professor Karin Bryan (University of Waikato), Associate Professor Giovanni Coco (University of Auckland) and Dr Barend van Maanen (University of Southampton) used New Zealand mangrove (mānawa) data as the basis of a new modelling system predicting what will happen to different types of estuaries and river deltas when sea levels rise.

They found areas without mangroves are likely to widen from erosion and more water will encroach inwards, whereas mangrove regions prevent this effect - which is likely due to soil building up around their mesh-like roots and acting to reduce energy from waves and tidal currents.

Dr Bryan says the spread of mangroves is changing the New Zealand coastal landscape.

"In New Zealand, mangroves have been traditionally viewed as undesirable as they take over areas where there were once sandy beaches. In other countries, this is not the case as they are seen as a buffer for climate change in low level areas."

She says New Zealand's low-lying coasts are especially vulnerable to the inundation, which is predicted to accompany sea level rise.

"Surprisingly, our coastal landscape can actually keep pace with sea-level rise to a certain degree, by accumulating sediment at faster rate. Coastal estuaries and recesses in coastlines that form bays receive the run-off from erosion on our steep catchments, which give them the tendency to fill in over time."

As they infill, the movement of the tidal currents over the shallow areas create networks of sandbanks and channels. The sand banks grow upward to keep pace with water-level changes, while the channels get deeper to efficiently drain the excess water out to sea.

The researchers' latest work shows the expansion of mangroves can facilitate this process, by adding leaf and root structures into the accumulating sediment, which increase the elevation while enhancing the trapping of new sediment arriving from the catchment.

Overseas studies have shown mangroves have the ability to remove carbon from the atmosphere and protect people from hazards such as tsunami.

"Now we know that they also could play a critical role in buffering our coastal land from the effects of sea-level rise," says Dr Bryan. "Although the study is on Avicennia marina (the only species of mangrove that occurs in New Zealand), Avicennia occurs in every major mangrove habitat in the world."

The research team hopes that this work will enhance the case for protecting global fringing wetlands from the threats of drainage and clearance caused by development and aquaculture pressures.

"Being able to work on a globally significant project has been amazing. All of a sudden people are interested in all this research from New Zealand. It's great to be able to contribute something of such importance to so many people."

This work was published in a series of four papers, the first three of which was the doctoral thesis of Barend van Maanen at the University of Waikato and the National Institute of Water and Atmospheric Research.

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