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