The state of a Southland lagoon is being described as “concerning” by the Department of Conservation as emergency works are being undertaken at the site.

This week, Southland Regional Council announced it would open the Waituna Lagoon — a wetland of international significance — to the ocean after the toxic algae Cyanobacteria emerged in the area last month.

The council hoped that opening the body of water would reduce and prevent the “severe ecological harm” from the amount of nutrients fueling the bloom.

DOC freshwater and ecosystems threats manager Nicki Atkinson said that was the right move.

“The current state of the Waituna Lagoon is concerning for all involved in the health and management of this internationally significant wetland.”

Atkinson said DOC had worked closely with a number of organisations — including the regional council — to improve the area, with positive changes observed over the years.

“However, this wetland is extremely vulnerable, and recent monitoring showing extremely high levels of algae make it clear immediate action is needed to prevent permanent harm to the ecological, cultural and recreational values of the lagoon.”

In a statement, the council said technical advice received from its science division, the Waituna Lagoon Science Advisory Group and other independent experts all showed the lagoon would be worse off if it remained closed.

Mana whenua and DOC had also been consulted.

Cyanobacteria blooms had previously occurred in the lagoon, but this was the highest level recorded since monitoring began in 2003, council general manager integrated catchment management Paul Hulse said.

Data showed the bloom worsened between December 7 and January 10 but had not changed markedly since that time.

The lagoon was expected to remain connected to the ocean for several weeks once heavy machinery had completed the task of opening it to the sea.

Although similar work had periodically been undertaken at the site over the years, the most recent consent allowing for that had expired in 2022.

With no current consent in place, the council was using a section of the Resource Management Act 1991 referencing a “sudden event” as justification for this week’s work.

“These powers are not unlimited and must be exercised judiciously,” Hulse said.

“If they are exercised there is also a requirement to obtain a retrospective consent.”

The excavation was expected to cost less than $20,000.

The lagoon and its wetlands were among one of the first in the world to be named “a wetland of international significance” in the 1970s following the establishment of intergovernmental treaty Ramsar Convention on Wetlands.

In April 2022, the area made headlines when a large fire ripped through during a drought.

Cyanobacteria can have severe health impacts on both humans and animals with the potential to result in death.

By Matthew Rosenberg, Local Democracy Reporter

LDR is local body journalism co-funded by RNZ and NZ On Air