By BRIDGET CARTER
One of New Zealand's most treasured environmental specimens, the mangrove, is under fire for choking up harbours, estuaries and inlets.
Northland scientists say in a study, Report on Mangroves - Thoughts, Comments and Observations, that their spread results from poor land management.
Northland Regional Council chairman and geographer Mark
Farnsworth and biological scientists Andre and Robin La Bonte studied mangroves in the Mangawhai estuary for the report.
Mr Farnsworth said they were growing rapidly in Northland estuaries, blocking them with silt and sand. They were making the estuaries shallower and stopping water flowing in and out.
The report says uncontrolled spread is likely to have "major socio-economic impacts" in estuaries which are home to holiday settlements and used for water-based activities.
Concern about unchecked growth of mangroves has also emerged in other areas, including Raglan, Thames and Tauranga.
Mr Farnsworth said they started to spread in the 1900s in Northland when trees near Mangawhai were cut down to clear the way for farmland. That meant more runoff, and with fertiliser, sediment and farm waste, the mangroves had the opportunity to flourish.
He said mangroves had been considered crucial to the marine environment since the 1970s.
But a lot of what we knew was based on overseas research, which led academics to call for mangrove preservation.
Not much research was done on mangroves on New Zealand, said Mr Farnsworth.
"We just put the mangroves up on a pedestal, saying based on overseas work they must be good."
He said the Northland report was the first time someone had "stuck up their hands" and questioned why mangroves in New Zealand had been so highly regarded.
"We are not trying to demonise mangroves. They do some very good jobs. They armour some shores from wave actions and they hold sediment on steep channel banks. We are asking for balance."
Northland waters did not require the additional nutrient loading from mangrove leaf litter, said Mr Farnsworth.
"Northland's 2m to 3m tidal ranges mean the mangroves roots are "drained" for a good part of the tide cycle, which calls into question speculation that mangroves are nursery areas and important habitats for fish."
The research will be presented at tomorrow's Northland Regional Council meeting, where the possibility of management plans will bediscussed.
nzherald.co.nz/environment
