Marlborough Sounds 'fingerprinted' to find mud culprit
GEOFF VAUSE11:57, Dec 17 2017
Crew aboard Pelorus at Mahau Sound use a high-frequency vibrating core sampler to take mud for forensic testing.
The waters of Marlborough are being fingerprinted to find the precise source and land use behind the ocean of mud smothering the seabed.
A team of National Institute of Water and Atmospheric Research (Niwa) scientists led by Dr Andrew Swales will soon know what land use is causing the mud, and exactly where it came from.
Swales and his team took 18 core samples in Mahau Sound on last week, and isotopic fingerprinting will tell them all they need to know.
Diving Services crew Daimon Holmes, left, and Bruce Lines prepare the high-frequency vibracore sampler aboard Pelorus at Mahau Sound.
The evidence will help the Marlborough District Council to give the sounds desperately needed protection and take remedial action to turn back the tide on the ongoing degradation of the seabed.
Pelorus crewman Bruce Lines with the vibrating end of the care sampler.
The scientists used a core sampler which vibrates at a high frequency to push a plastic pipe two metres into the thick mud on the seabed and retrieve the most accurate, undisturbed sample possible. Cores of up to two metres were extracted from the seabed in Mahau Sound in three locations.
The cores are then X-rayed, and slices of each core are dated and fingerprinted to identify where the mud came from, and the type of land use in place where it was dislodged.
Sediment deposited over the last century or so will be dated using the radioisotopes lead-210 and caesium-137, the latter being a product of past atmospheric nuclear-weapons tests. Radiocarbon dating will also be used to date shell material to estimate sedimentation rates prior to catchment deforestation.
Swales said the source of sediments at different times can be "fingerprinted" through particular fatty acids secreted by roots and bound to the soils.
"This means scientists can distinguish between native forest, pasture grass and pine forests with some confidence," he said.
"We'll learn how the composition of the sediments has changed over the course of the last few hundred years, and when broadly those changes occurred. The rates at which sediments were deposited into the marine environment at different times can also be calculated."
Pelorus, the 16.5-metre vessel owned by Nelson Based Diving Services New Zealand Ltd, provides transport for the Niwa scientists to sample Mahau Sound sediment.
Marlborough District Council environmental scientist Dr Steve Urlich said the council would be able to see where current sediment coming down the Pelorus and Kaituna rivers came from.
"This will help the council and the community to better manage land-use activities [and] reduce sedimentation," Urlich said.
Similar sampling had already revealed the reason mussel beds had effectively vanished from Kenepuru and Pelorus sounds.
"We know from the previous coring work in Kenepuru Sound and Beatrix Bay by Niwa, and also from Niwa's research on historical newspaper articles and old research reports, that the seabed environment in Pelorus Sound has changed dramatically since European settlement in the district," Urlich said.
"The ecological change has been from a place of great abundance to one of relative scarcity. We also know from recent estuary monitoring that the Havelock, Kaiuma and Mahakipawa estuaries are getting muddier and less productive ecologically."
In terms of recovery, marine reserve monitoring around the country showed the environment was capable of healing itself. In the case of Pelorus Sound, recovery would depend on significantly reducing existing sediment and halting the flow of mud pouring into the waters.
"If we can do this as a community, it may take several decades for positive changes to biodiversity to fully manifest. In ecological terms that is not a long time, and also not in the course of our multi-generational history," Urlich said.
"This type of research is important for guiding sustainable management in our region," said the council's environment committee chairman, David Oddie.
"Our recent science investments have helped us to appreciate the profound changes to the sounds over the last 150 years. This information is hugely valuable in informing decisions about how we can use our land and marine resources wisely well into the future."
Niwa's report will be available by March 2019.
The Marlborough Express