CuriousCity: What lurks beneath Wellington Harbour

Wellington Harbour is the capital's crown emerald jewel, but it's also a murky enigma. In the latest instalment of our series exploring the capital's curiosities, Stuff asks what's lurking beneath swimmers' dangling feet.

Niwa marine biologist Serena Wilkens reckons people would be shocked if they realised what was under the sea.

"It looks so pristine and green from the surface and sparkly and lovely. But in actual fact it's a real tip under the water at times."

Wilkens specialises in monitoring marine pests that travel in the "bio-fouling" on ships' hulls and in ballast water - the growth of seaweed, barnacles, sea squirts, mussels, and sponges that accumulate over seafarers' long journeys.

Her team keeps an eye on invasive marine species arriving into the country's waters, and on locals they don't want to spread further, performing a twice-annual survey nationally.

They may look pretty but one particular species of starfish, the Northern Pacific Sea Star, is one such unwelcome visitor in Wellington.

READ MORE:
* On the hunt for marine pests in Wellington harbour with Niwa
* Dangerous jellyfish appear in Wellington harbour
* Trapped shark seen in Wellington harbour near Te Papa
* Archaeologists lift old anchor and not-so-old purse from Wellington Harbour

Scientists use traps to catch the starfish, as well as crabs on the sea floor, to determine whether they are guests to the harbour's natural habitats that could prove a threat to the local ecosystem.

"Wellington, fortunately, hasn't had too many," Wilkens says.

They are trying to keep newcomers at bay, like the mediterranean fanworm and the "feisty" paddle crab, both of which have taken up residence in Auckland.

But sometimes it's the harbour's human visitors that foul up the seabed.

On Saturday, volunteers mucking in for the Wellington Harbour Spring Clean went diving to fish out the rubbish that ends up in the harbour.

Some of Niwa's marine experts are involved with the clean-up, along with volunteers from the Department of Conservation, the Ministry for Primary Industries, Victoria University, Wellington City Council, Sustainable Coastlines, and members of the public in kayaks and dive gear.

The clean-up, which has been running since 2009, has brought kilograms of rubbish ashore. It retrieves everything from shopping trolleys and road cones to tin cans, sunglasses, tyres, bikes, scooters, cellphones, and tablets.

Divers have even found a pairs of shoes, Wilkens says.

"Sometimes you wonder how people have ended up losing both pairs of shoes at the same time. You name it and people throw it over the side, unfortunately."

The accidental junk - ships and car wrecks - can prove a boon for marine life. But Wilkens would not like to see any more added to the harbour's collection, which includes one vehicle submerged in Mahanga Bay.

"That's actually become an amazing artificial reef and it's full of baby crayfish, and it's teeming with fish life."

Niwa Marine geologist Dr Scott Nodder is interested in a different sort of seabed lurker.

He was part of a team that discovered and mapped the Aotea faultline, which was discovered in 2014.

They got funding to map its extent and fault characteristics and found it traced a path of about two kilometres from popular swimming spot Oriental Bay, out past Aotea Wharf.

"[It] looks like the seabed has been moved by about a metre on this fault," Nodder says.

"Over the last 10,000 years we think this fault has moved maybe three or four times, from what we can tell from our data."

The scientists estimated the fault could trigger a magnitude 6 to 7 earthquake, although a tsunami was "unlikely":

"So a little bit less than the recent Kaikoura earthquake, but a significant feature nonetheless."

Their data suggested the fault was about 10 to 30km long, reaching inland, "which would mean that it would basically go from Oriental Bay through Te Aro and possibly out on the South Coast."

Historic seismic activity is responsible for much of the uplift that created flat land for Wellington to be built, Nodder says.

So the shakes are frightening, but they are also responsible for giving our waterfront its charm.

"You've got masses of hills and a beautiful harbour and I think all of that has been generated by millions of years of tectonic activity."

Ask away: Have we piqued your CuriousCity? Have you ever wondered how something in the Wellington region works? Email your question to news@dompost.co.nz with 'CuriousCity' in the subject line.