Sediment success – our expanding mangrove forests

By Lynley Hargreaves 28/01/2015

With World Wetland Day next Monday, celebrate the most successful of New Zealand wetland types – the mangrove. While we’ve lost 90 percent of our swamps and bogs, the area covered by our only salt water tree has been expanding, driving many bach owners and other recreational beach users crazy. The University of Waikato’s Associate Professor Karin Bryan and co-principal investigator Dr Julia Mullarney have a new Marsden grant investigating how the trees’ unique peg roots help sediment settle in. She reports from the waist-deep mud of the Firth of Thames.

How long have mangroves been in in the Firth of Thames?

Associate Professor Karin Bryan, (left) and Dr Julia Mullarney.

Mangroves weren’t in the Firth of Thames much at all in the 1950s. There is now a forest about a 1km wide that makes one of New Zealand’s biggest mangrove swamps. We will start with sampling a simpler sandy site, and move on to mud when we have tested our theories and have our systems working properly. Mud is much more difficult because it flocculates (clumps) and binds together in a way that people do not fully understand yet.

Why are New Zealand’s mangrove forests expanding?

Mangroves like a certain range of sediment infilling, and because we’re a relatively young country in terms of how long it has been since deforestation, there are high levels of sediment still washing off the land and infilling our estuaries. This will change over the next 100-200 years as the coast gradually becomes sediment starved.

Many other areas of the world have declining mangrove forests. We’re also studying mangroves in the Mekong Delta of Vietnam, where the rate of decline is a cause for concern to the point where people are trying to restore swamps.

What problems can declining mangrove forests cause in Vietnam?

Climate change scientists are beginning to realise that a huge amount of carbon is locked up in global mangrove swamps. In places like Vietnam, there is intense pressure to convert mangrove swamps to economically-productive ventures like shrimp farms, with frighteningly-little thought on the potential effect on our climate.

What techniques do you use to understand how sediment affects mangroves?

We use a high resolution profiler which takes multiple current measurements – it essentially looks at the eddies that kick up the sediments and move them into and out of the swamp. We’re trying to see what role the root structures play in causing these eddies. We’re also trialling a technique which involves taking 1000 pictures from different angles so we can build a three dimensional construction of the roots.

Can you predict where New Zealand mangroves will expand to next?

They only exist between about mid and high tide level, and they need fairly low wave conditions for the seedlings to establish, so if sediment infilling causes expansive high intertidal areas (and the temperature is warm enough), mangroves are likely to be very happy.

These interviews are supported by the Royal Society of New Zealand, which promotes, invests in and celebrates excellence in people and ideas, for the benefit of all New Zealanders.