Dredging compromises harbour 'healthiness'

Continued dredging of Otago Harbour will make an impact on its health but as it is a heavily modified harbour, it is a question of what the community can live with, University of Otago marine science Associate Professor Abby Smith says.

In a report soon to be published in the Estuarine, Coastal and Shelf Science journal, Prof Smith and four students studied the carbonate in sediments in Otago Harbour to determine human impact on the urban port.

They modelled a carbonate budget for the harbour, looking at what deposits, such as cockle shells, went in to the harbour and what went out, to determine what carbonate was left.

The most important factor in the removal of carbonate from the system was dredging, with an average 283,000cu m a year removed since 1899.

The model showed the carbonate budget was half what it used to be and that by 2100, no carbonate would be preserved in sediments in the harbour.

"It's a measure of naturalness or healthiness.

"It tells us we're making changes, but they are changes we should be sure we can live with."

This had implications for future research of the harbour as shells in rocks and sands were a record of what the past environment was like, an indicator that was used by many scientific fields, she said.

"We'd lose all of that."

As the shells held carbon derived out of the atmosphere, they were also an important sink for carbon sequestration.

"If all the shells are dissolved, they're not going to sequester carbon, but put it back into the sea."

Changes in sedimentation could also impact on the organisms living in sediment.

Overall, Otago Harbour, as a heavily modified urban harbour, was not "too bad" but it was still losing some of its overall naturalness, Prof Smith said.

Port Otago's Next Generation Project, which included further dredging of the harbour, would certainly make an impact, but it was not unreasonable in an already modified place, she said.

An enormous number of studies on the Otago Harbour had never been published, so this paper showed scientific study on the harbour was very relevant, Prof Smith said.

"It has something to tell us that we might want to listen to."