A guide for managing mangroves

The Far North is home to huge areas of mangroves, including those bordering parts of the Twin Coast Cycle Trail.

Niwa has released a guide for managing mangroves, prompted by public interest in learning about mangrove ecosystems and what happens when they are removed.

Marine ecologist Dr Carolyn Lundquist said the aim was to provide guidance for managing mangrove expansion, while maintaining the ecological functioning of estuaries and harbours.

Mangroves grew on coastal mud flats from Kawhia north, and as they had expanded rapidly in recent decades were often targeted for mass removal in attempts to return harbour seascapes to what they were in past, Dr Lundquist said.

Reasons for removal included improving recreation values and access to the ocean, improving views, returning habitats to firm sand flats and improving drainage or flood protection.

Until recently little had been known about the effect of mass removal on the environment, or even if it worked, a NIWA survey of more than 40 areas where mangroves had been removed establishing that often there was not a return of sand flats, and that many removals had had detrimental effects on the local ecosystem.

"In the past, a decision to remove mangroves was not always informed by the best science. In this guide, we have compiled what we hope will be useful information for community groups, councils and individuals," she said.

Opinions on mangroves were often divided, ranging from those wanting them protected to those seeking to eradicate them. Dr Lundquist said she was not taking sides in what is often a fraught debate, but would prefer that people were properly informed, whatever action they decided to take.

The guide explains that mangroves act as a buffer zone from waves and storms, and protection from coastal erosion. They also store carbon and nutrients, and support a diversity of animal life that feed off the plants' organic matter.

However, decisions made on land, such as increased conversion of land for agriculture, forestry or urban use, had huge downstream implications for estuaries with mangroves.
"What we do on land has a huge impact on sediments that enter estuaries and raise the height of tidal flats, increasing the area that mangroves can colonise," Dr Lundquist said.

The best way to manage the expansion of mangroves over the long term was to limit the amount of sediment reaching the coastline from the surrounding land. Areas where mangroves had been removed also needed regular maintenance to keep mangrove seeds from re-establishing, an annual cost that could range from $1000 to $5000 per hectare, a cost that was often overlooked.

The guide discusses a range of management strategies that address mangrove expansion, and advises where removal is unlikely to achieve the desired outcome or could be very costly to maintain.