A lot more known about mangroves
the Whangamata Harbour, and to date that process has consumed over a decade in time and in excess of $1.5M in costs,” Mrs Goudie said.
The bill aims to empower Thames–coromandel District Council and Hauraki District Council to get the job of mangrove removal done more efficiently.
A council statement said prior to the 1960s harbours were clear of mangroves. However the mangrove is a native species that scientists say has been around for thousands of years.
The rate of expansion has, however, grown dramatically since that time.
Mangroves cover more than 26,000ha of New Zealand’s northern coast, equivalent in size to Wellington. The species is expanding its cover by an average of four per cent a year and episodic waves of expansion occur in the Firth of Thames about every 10 years.
NIWA scientist Andrew Swales has examined the pattern of mangrove colonisation in the Firth by taking sediment cores to 2m depth and dating the age of the sediment before and after mangroves have colonised.
It’s work that will be of interest to debates at political level about whether mangroves actually speed up the silting of harbours that were once sandy. The scientists’ work confirms that native forest clearances and changes in land-use are to blame – infilling estuaries with sediment, with mangroves merely taking hold afterward.
“Ultimately mangroves are just making use of this increase in sediment to our estuaries,” says Richard Bulmer, another NIWA scientist working in the field.
In fact, between 1940 and 2014, sediment has rapidly infilled our estuaries, coinciding with increases in mangroves expanding from 2300ha to 10,500ha in the Auckland region.
Similar sediment infilling and associated mangrove expansion has occurred on the Coromandel, in harbours such as Whangamata, Tairua and Whangapoua.
One thing the scientists want to know is how this may be impacting the tiny animal communities that dwell there, and whether removal of mangroves makes a difference to them.
“When mangroves were cleared in Tauranga, we didn’t see any evidence of change to the neighbouring shellfish beds,” says NIWA scientist Dr Carolyn Lundquist.
“But at the same time, we also didn’t see any new shellfish beds recolonising where mangroves had been removed. Rather we often see decreases in the shellfish because of the sediment inputs, so these beds may be already gone by the time mangroves come in.”
One impact that’s hard to ignore in several clearance sites is the stink. That is the smell of anoxic sediment, and the reality is that it generally sticks around rather than returning to sandflats. Dr Lundquist says a lot has been learned and newer methods of mangrove removal are better at avoiding longlasting odour. There’s also less damage to the animals in the sediment.
“In Whangamata, four or five years after clearances, worms, cockles and other bivalves were still more comparable to an intact mangrove area rather than a sandflat, though these animal communities are changing gradually.
“But the reality is you don’t get the pristine sandflat back in most places where mangroves have been removed. We’ve learnt a lot and we know there are places where mangrove removal is likely to be successful, whereas for other places you wouldn’t because it would be a poor choice as it is unlikely to return to a sandier state.”
While the rate of expansion is alarming, mangroves and the mud they inhabit may be nature’s way of fixing things. New Zealand scientists want to know what role our own coastal habitats — mangrove, coastal marsh, seagrass or mudflats — might play in storing greenhouse gases. Even the mud, it seems, is of value.
Dr Lundquist and her team have recently completed a pilot study to test the plant’s potential for reducing the impact of climate change. Overseas studies indicate tropical mangrove forests can take up carbon dioxide 100 times faster than land-based forests. Mangroves store carbon in the branches and other woody vegetation, but up to half the carbon in mangrove forests is stored in the sediments.
“We went to 40 places where mangroves had been removed,” Richard Bulmer says. “Even 16 years after removal, we’re still getting high rates of carbon dioxide being released from the sediment, while you have at the same time lost all of the carbon that is stored and accumulated within the mangrove trees, plus you’re not getting carbon stored in the trees either.”
A study funded by NASA in Florida says the spread of mangrove trees could hold the key to protecting shorelines as climate change impacts are felt.
“Mangrove habitats provide a staggering 800 per cent more coastal protection than salt marshes (and millions of dollars more value than manmade barriers),” says the study published in Hydrobiologia, the International Journal of Aquatic Science. In the US, wetlands provide an estimated $23.2 billion per year of protection against economic losses as well as deaths in major storm events.
Ecologist Meg Graeme thinks mangroves are underrated as a climate change tool and deserve to be seen as a quintessential part of New Zealand’s natural character .
“A favourite pastime of mine is kayaking around mangrovelined creeks with yellow-eyed mullet, parore and flounder swimming ahead. Apart from important carbon absorption and food web roles they play, mangroves accumulate and stabilise sediment to protect against rising sea levels.
“Mangroves provide vital habitat for shore birds, fish follow the tides to feed among the mangroves, and banded rail are dependent on them. We have a lot of lessons that can inform that TCDC bill,” Dr Lundquist says.
‘We’ve learnt a lot and we know there are places where mangrove removal is likely to be successful, whereas for other places you wouldn’t because it would be a poor choice as it is unlikely to return to a sandier state. DR CAROLYN LUNDQUIST
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