Improvements to the health of Southland’s "resilient" Waituna Lagoon have been tentatively welcomed by DOC’s top science advisor.

Results from an annual survey have exceeded the department’s expectations, revealing a decrease in algae and higher levels of aquatic plant Ruppia.

The treasured wetland is home to a plethora of native birds, fish and plants, but has been plagued by algae blooms and deteriorating water quality in recent years.

Waituna Lagoon forms part of the much larger Awarua-Waituna Wetlands, a 20,000 hectare area. Credit: Living Water

.

DOC principal science advisor Hugh Robertson told Local Democracy Reporting he was feeling “positive, but always cautious”.

“This is a system where we really have to keep an eye on the water quality as well as water levels … for a number of years ahead of us,” Robertson said.

“The system is vulnerable each summer to whatever conditions are present.”

Located about 40 minutes southeast of Invercargill, Waituna Lagoon's management has proven contentious over the years.

Surrounded by farmland, it is prone to high nutrient levels which have led to toxic algal blooms and manual openings to adjacent Toetoes Bay.

A 20-year consent was granted in October 2025 to DOC, Te Rūnanga o Awarua and Environment Southland for opening the lagoon based on a variety of triggers; it has been appealed by Federated Farmers who feel the required level will compromise farms.

A Ruppia sample from the Waituna Lagoon. Credit: DOC/Supplied

.

Robertson explained the balance at play with the lagoon — if it stayed open the whole time, the system would change with water levels dropping and salt content increasing, making it harder for plants to grow.

But opening it also had the benefit of flushing out nutrients.

“So in an ideal world, if nutrient levels coming into this lagoon were much lower, [we] probably wouldn’t need to open it that often. Apart from when there’s concerns around flooding and other things like that.”

Robertson said the current lagoon level was 1.4 metres, which meant more water and habitat for plants to thrive.

Although many of the country’s coastal lagoons were in quite poor condition, Waituna had remained “fairly resilient”, he said.

The lagoon was last opened to the sea in September 2024 and closed in April 2025.

DOC’s annual survey was undertaken across 47 sites at the lagoon and showed low levels of algae.

Ruppia was present at 80 percent of monitoring sites compared to 36 percent last year, and stonewort had jumped from 7 percent to 54 percent.

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