‘Don’t blame the mangroves!’

Admin
2 days ago
3 min read

For many locals the pros and cons of mangroves in the Ōhiwa Harbour have been a topic of discussion since the native shrub began to spread in ways not previously known. Ōhiwa Harbour used to form the southern limit of Manawa, the New Zealand mangrove’s natural occurrence. Tolaga Bay has been home to a population of mangroves for some years.

Established mangroves along Wainui Road. Image: Tanja Rother

It is now widely acknowledged that the increasing spread of this highly adaptive estuarine plant results from more sediment entering the harbour in recent times. It has worried some people in the Ōhiwa community for many years. Among their concerns were the impact on shellfish beds and fish breeding, the obstruction to natural drainage and the reduced access to the harbour for boating and swimming. As a result, the Ōhiwa Harbour Strategy partners developed an approach to management that included both dealing with the sediment flowing into the harbour from the land and controlling the mangroves themselves. It was well understood that mangroves, as a native plant, do have their place, providing habitat and dense mangrove stands acting as coastal buffer zones. (Read about the local history of mangrove management and outlying seedling control here.)

Now research into mangroves and coastal environments by Professor Karin Bryan at the University of Waikato has confirmed that comprehensive management approaches are needed. Professor Bryan points out that solutions need to be found within the larger scale catchments, for example the reduction of sedimentation loss from up-stream land-use practice. In fact, the key take-away of research on coastal restoration undertaken by Bryan and an international team of experts is: ‘Don’t blame the mangroves.’ The study, published in the journal Nature shows that ‘coastal mangrove removal initiatives cannot stop or mitigate mud-infilling of estuaries to restore previous sandy ecosystems’. By contrast, according to the researchers, ‘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’.

Through modelling of what would happen if mangroves were removed from certain areas of the New Zealand coast the researchers were surprised to find that with the plants gone, the mud did not start to wash away as expected. Instead, it stuck around — and continued to build up. The ‘mangroves on the front line — the fringe facing the sea — tend to thrive, building up an elevated barrier of mud that stops sediment washing in behind it. So instead of a thick mudflat, you’re left with something like a seawall, with less mud closer to the beach’. ‘Feeding the computer model various scenarios, the scientists found that no amount of mangrove removal would stop the mud — and it certainly wouldn’t bring back the sand. Only stemming the flow of sediment to the sea could do that. And that means changing the way we use land. Mangroves, Bryan says, are not the bad guys. “They’re just a symptom of what’s wrong with our coast.”’ (Read the full New Zealand Geographic article ‘Calling a truce on mangroves’.

Mangroves near the Kutarere stop bank. Image: Tanja Rother

In light of this knowledge a change of attitude towards mangroves may be on the cards for some. Locally, the value of mangroves for biodiversity (including providing fish rearing, bird nest and feeding habitat), in reducing coastal erosion and seaward sediment build up is increasingly recognised. Mangroves also protect seagrass habitat from acidification by making seawater more alkaline.

In Ōhiwa Harbour the removal of mangrove seedlings remains a permitted activity where they have been controlled previously, where there are no mature mangroves in the area and provided Rules DD 19 – 25 of the Regional Coastal Environment Plan are considered. Mangroves cannot be removed from any of the three exclusion areas: