What started as small, white specks quickly turned into a potentially toxic blue-green algae.
Cyanobacteria was found in the lower part of the Tomahawk Lagoon last month, prompting the Otago Regional Council to warn people and their pets to stay away.
The bacteria, which can be toxic, was discovered by Tomahawk Citizen Science Group science and communication adviser Lena Schallenberg while undertaking a routine water quality check.
Miss Schallenberg was walking into the lagoon in her gumboots and noticed small, white specks in the water, which she suspected was a cyanobacterial bloom.
“They’re so tiny you can’t see them normally, but they form these colonies which look sort of like little flecks,” she said.
She emailed the ORC, which later confirmed it was Anabaena lemmermannii, a type of cyanobacteria.
Cyanobacteria can produce toxins harmful to humans and animals if they are swallowed or come into contact with skin.
ORC environmental resource scientist Rachel Ozanne said cyanobacterial blooms occurred when there was sufficient sunlight, warm water and enough nutrients, such as phosphorus and nitrogen.
The upper and lower lagoon, including the stream outlet to Tomahawk Beach, has been tested twice since the bloom was first identified.
The latest results, released yesterday, found cyanobacteria was still present in the lower lagoon and outlet stream, so the warning signs would remain in place, Ms Ozanne said.
The results on October 24 showed “very similar concentrations of Anabaena lemmermannii” to the test results on October 19, she said.
“Due to the surprisingly early appearance of an algal bloom, the lagoon will now be monitored on a weekly basis until the weather gets colder in March/April next year.”
She expected the bloom would naturally die due to insufficient sunlight for photosynthesis or the plant cell line becoming too old to reproduce.
Miss Schallenberg said since the bloom was discovered, the lagoon turned an opaque “pea soup” colour, went bright blue with “white scum” and was now almost back to its usual colour but had a slightly brown tinge.
She said it was common for the lagoon to experience these blooms and information from the ORC showed they had been occurring regularly since 2013, including once in February this year.
Anecdotes from the community suggested there had been blooms since the 1970s, she said.
She would continue to monitor the lagoon and hoped to compile a record of when the blooms were happening.
Otago Peninsula Community Board chairman Paul Pope was concerned about the health of the lagoon.
Earlier in the year he lodged a submission on behalf of the community board to the ORC about lagoon’s condition. The ORC had committed to creating a management plan, but nothing had happened yet, he said.
A council spokesman said the council was aware of Mr Pope’s concerns, the issue of a management plan remained in the 2017-18 Annual Plan, and the matter was being progressed.
A sign at the lagoon advised anglers to clean their fish well and discard guts.