For a new study, researchers used New Zealand mangrove data to develop a modeling system to predict what will happen to different types of estuaries and river deltas when sea levels rise.
The models show that areas without mangroves are likely to widen from erosion and be affected as more water will encroach inwards. Mangrove regions prevent this effect—probably because soil that builds up around their mesh-like roots reduces energy from waves and tidal currents.
Coastal estuaries and recesses in coastlines that form bays receive the run-off from erosion on steep catchments, which give them the tendency to fill in over time. As they fill, the movement of the tidal currents over the shallow areas create networks of sandbanks and channels. Those 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 new study shows that 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.
“As a mangrove forest begins to develop, the creation of a network of channels is relatively fast. Tidal currents, sediment transport and mangroves significantly modify the estuarine environment, creating a dense channel network,” says Barend van Maanen from the University of Southampton.
“Within the mangrove forest, these channels become shallower through organic matter from the trees, reduced sediment resuspensions (caused by the mangroves) and sediment trapping (also caused by the mangroves) and the sea bed begins to rise, with bed elevation increasing a few millimetres per year until the area is no longer inundated by the tide.”
The study suggests that the ability of mangrove forest to gradually create a buffer between sea and land occurs even when the area is subjected to potential sea level rises of up to 0.5mm per year. Even after sea level rise, the mangroves showed an enhanced ability to maintain an elevation in the upper intertidal zone.
The spread of mangroves is changing the New Zealand coastal landscape, says Karin Bryan, associate professor at the University of Waikato.
“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.
“Now we know that they also could play a critical role in buffering our coastal land from the effects of sea level rise. 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.”
Overseas studies have shown mangroves remove carbon from the atmosphere and protect people from hazards such as tsunami. The researchers hope the new work will enhance the case for protecting global fringing wetlands from the threats of drainage and clearance caused by development and aquaculture pressures.
“These findings show that mangrove forests play a central role in estuarine and salt marsh environments,” says Giovanni Coco, associate professor from the University of Auckland. “As we anticipate changes caused by climate change, it’s important to know the effect sea level rise might have, particularly around our coasts.
“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.”