Clearing the murk around Mangawhai mangroves 

 

BY JULIA WADE

Over many years Mangawhai residents have discussed, debated and been divided when it comes to determining the merits of the harbour’s expanding mangrove forest. With science and myth becoming as entwined as the plant’s muddy root system, an environmental group has decided to create a fact-finding forum for the community. 

Mangawhai Harbour Restoration Society (MHRS) is currently developing an educational initiative to provide accurate scientific data regarding the controversial marine plant, inspired after a recent article in the Mangawhai Focus (April 29) which posed the idea that the removal of mangroves from the harbour could be a possible link in the drop of endangered fairy tern numbers. MHRS spokesperson Ray Welson says the article was ‘very timely and raises a number of issues that need addressing’. 

“While well intended, we believe the story lacked credible supporting data to warrant the concerns they raised,” he says. “The need to have a community agreement on the merits or threats of our local mangroves has never been more important.”

Current beliefs disputed
According to some ecologists, biologists and ornithologists [1], mangroves have significant value, understood to be an essential eco-system filtering sediment and contaminants, reducing erosion as well as functioning as nursery and habitat for a range of marine life and feeding grounds for coastal birds. 

However Welson says there is another body of research to consider which indicates that, due to a lack of scientific data available on the native Avicennia marina, New Zealand’s mangroves have been incorrectly affiliated with their overseas cousins the complex tropical ‘mangals’, giving the home-grown variety an incorrect status.

“For the majority of us living in Mangawhai our knowledge of the role local mangroves played in the environment was in fact based on characteristics of mangroves in other countries, generally in tropical climates,” he says. “These beliefs are now being broadly disputed, with scientific research recognising that New Zealand’s single-specie, temperate mangrove forests have different characteristics… that there are no mangrove-dependent marine or avian species in New Zealand.” 

Mangrove use questioned
MHRS’s claim is based on a New Zealand National Geographic article ‘Mangroves – allies or invaders?’ authored by coastal consultants and ocean engineers, Andre and Robin LaBonte, a Waipu couple who designed the Mangawhai Harbour Restoration and Maintenance Plan in 1994 as well as the mangrove management plans that have been implemented in the estuary since 2014. 

Although written 15 years ago, Andre LaBonte states that the study has not to date been discounted or proven wrong. 
“In fact research that has been done since, with much of it performed in Mangawhai Harbour, has confirmed our contentions,” he says. “Our article suggested that more research should be undertaken, much of which has been performed since the article was published in 2004, including monitoring of actual mangrove removal projects [2]. Therefore, I would consider the article to be a reasonable introduction to the topic and paper to be referred to with subsequent findings.”

The article questions a number of commonly held beliefs on mangroves including being a protective habitat for marine animals, an essential feeding ground for birds which are found also in non-mangrove areas and sediment catchments. 

Mangroves positive and negative
Since 2004 MHRS say they have supported scientific research investigating the effects of mature mangrove removal on sediment characteristics, the biodiversity and density of lower-water life forms and effects on wading and coastal birds. Studies [3] have shown the long-term effects associated with mature mangrove removal to be positive at Mangawhai Harbour Welson says.

“We are supportive of a comprehensive mangrove management project being undertaken on the eastern side of the estuary causeway. If no further management action is allowed or undertaken, in some instances, the ultimate outcome could be the closure of the historic inlet, loss of intertidal habitats, reduced density and diversity of marine and avian species,” he says. “A significant number of coastal communities in northern New Zealand share the same concerns that the spread of mangroves present.”

New Zealand is the only country in the world where mangroves are expanding their area of occupation which LaBonte says is at the expense of other habitats including shellfish and sea-grass beds, flounder and wading-bird grounds, as well as impacting on sandy estuarine shorelines and recreational activities.

Surveys show concern
Public surveys taken at two local meetings in 2009/2010 revealed that out of 690 respondents, 688 expressed concern that Mangawhai Harbour’s natural character has been, and continues to be, altered by mangrove expansion. 

Forming after the 1991 ‘Big Dig’, MHRS members share a collective wealth of both scientific and hands-on practical experience including committee member Jerry Pilmer, referred to in the fairy tern article, who has been highly involved in the dune planting and approved mangrove removal programs Welson says, and his participation benefits the entire community. 

“Jerry has had a love affair with the estuary, sand dunes and surrounding beaches for his entire lifetime,” he says. “His experience as well as the knowledge of other locals, coupled with scientific research data, will enable the MHRS to provide the Mangawhai community with quality information, ensuring future decisions relating to the environmental impact of mangroves are always informed and correct.”

More info
 nzgeo.com/stories/mangroves-allies-or-invaders/Issue 067
 nzpcn.org.nz/publications/ARC-325%20Mangrove-review.pdf
 niwa.co.nz

References: [1] www.boffamiskell.co.nz; Misunderstandings and management; [2] Reference for mangrove removal project monitoring reports: LaBonte A and R, 2010 – 2017, Pahurehure Inlet, Lincoln Street, Insley Street and Sand Island Removal Sites; [3] Alfaro, A.C. (2006-2010]: Estuarine, Coastal and Shelf Science 66, 70 and ICES Journal of Marine Science. 

To keep or to clear: MHRS hope to bridge the division which complex and conflicting information creates over Mangawhai’s mangroves. PHOTO/Julia Wade

“For the majority of us living in Mangawhai our knowledge of the role local mangroves played in the environment was in fact based on characteristics of mangroves in other countries… .“