Tomahawk Lagoon open day

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Waterway in `reasonable state’

Tomahawk Lagoon is in a “reasonable state” of environmental health, according to the results of a year-long scientific study.
Young scientists from three Dunedin schools shared the story of Tomahawk Lagoon in 2016 during a public open day on Saturday.
Pupils from Bayfield High School, John McGlashan College and Tahuna Intermediate have made fortnightly visits to the lagoon this year to carry out testing at four sites.
Working under the watchful eye of Ecotago scientists Andrew Innes and Dr Jonathan Kim, the pupils tested the water’s chemical composition, observed the weather and counted invertebrates and small fish to gauge the biological status of the lagoon.
The project was funded through the Participatory Science Platform (PSP), administered through the Otago Museum, which aims to encourage participation in science projects.
It was also supported by the University of Otago chemistry and geography departments.
On Saturday, families and members of the public listened as the pupils presented their findings and discussed the potential reasons for them.
Among the data discussed were the fluctuating levels of phosphates and nitrates in the water, the presence of algae and E. coli, and the numbers of fish and invertebrates.
The pupils showed visitors samples of fish and invertebrates collected from the lagoon.
Summarising the results for The Star, Mr Innes said the phosphate concentrations in Tomahawk Lagoon were high in late summer and autumn but declined significantly in winter. The sources of soluble phosphate were likely to be swan activity and bacterial action.
A minor algal bloom occurred in late autumn, preceded by the peak in phosphates, and ecological communities were “moderately impacted” by this.
In both the stream and lagoon, nitrate concentrations showed a marked increase in winter and dropped in spring. The levels in the stream were about 50% higher, suggesting that the source of the nitrate was run-off in the stream catchment.
Levels of E. coli fluctuated in the lagoon, exceeding the guideline for “action for contact recreation” once in late autumn, while levels declined in the cooler months.
The stream exceeded the guideline five times, suggesting continuous sources of E. coli.
The biodiversity of the macroinvertebrates supported an MCI (macroinvertebrates community index) of 82-84, meaning the water quality could be described as “fair”, Mr Innes said.
“The lagoon is in a reasonable state of environmental health.”