Dredging lagoons seen as option

Dredging may be the most practical way of reducing toxic algae blooms in the Tomahawk lagoons, an Otago Regional Council report says.

The lagoons have been hit by outbreaks of cyanobacteria, or blue green algae, this summer.

Council water resource scientist Dr Dean Olsen said in the report to the technical committee this week the lagoons were nutrient rich due to historical land-use practices such as aerial top-dressing with super phosphate.

''Such blooms have been recorded as early as the 1960s.''

While the most practical means of improving water quality was to reduce the nutrients entering the waterway, in the lagoon the phosphorus was locked in its bed, so that would not be effective.

Methods of management were limited, largely due to the internal load of phosphorus in the sediments of the lagoon, he said.

Technically feasible methods included dredging and the use of a capping agent such as Phoslock which would bind the phosphorus, but exposure to strong winds meant it might not be a long-term solution.

Dredging might be the most practical way of reducing internal nutrient sources and would have the added benefit of increasing the water depth in the lagoon.

However, it was likely to be costly and the sediments removed would have to be disposed of appropriately, Dr Olsen said.

Cr Gretchen Robertson said the council needed to understand a lot more about its coastal environments before it considered any work on the lagoons.

''We need to weigh up the outcomes, costs and benefits. We're not signing up to anything at this stage.''

Council director of engineering, hazards and science Gavin Palmer said the report was put together in response to questions from the Tomahawk community about the algal blooms but was not suggesting options to fix the problem.

The report would be taken to the community.

Cr Doug Brown said the issue needed to stay on the council's radar but he imagined any cost-benefit analysis would show dredging was too expensive.

- rebecca.fox@odt.co.nz


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