The Waituna Lagoon was mechanically opened to the sea this morning, with the aim of preventing imminent, severe ecological harm.
The opening work, commissioned by Environment Southland, involved clearing work in preparation for excavation of a channel from the lagoon to the sea using an excavator. Site preparation work was undertaken yesterday using a bulldozer to move material so that a cut can be made today by an excavator to allow the flow of water between Waituna Lagoon and the sea.
Following consultation with mana whenua and the Department of Conservation, and weighing of environmental and health and safety risks, Environment Southland used emergency works powers available to it under the Resource Management Act to mechanically open the Waituna Lagoon to the ocean.
Urgent management action was required to disrupt a toxic algal bloom (cyanobacteria) which emerged in the lagoon last month, and to reduce the amount of nutrients within the lagoon fuelling the algal bloom.
Technical advice from Environment Southland’s Science Division, the Waituna Lagoon Science Advisory Group and other independent experts, all concur that the lagoon is currently experiencing adverse environmental effects and the situation is likely to be further adversely affected if it remains closed. Monitoring has identified that there is a risk to ecosystem health if the lagoon remains closed, with the latest samples showing toxic algal bloom levels higher than any recorded previously.
“The most suitable method available to achieve this is to mechanically open the lagoon to the ocean, and to ensure it stays opened for long enough,” Environment Southland’s General Manager Integrated Catchment Management, Paul Hulse said.
“We will work to keep the lagoon open for several weeks, unless a shorter closure is followed by immediate filling of freshwater that is ideally low in nutrients, so we will be closely monitoring the lagoon opening, and locating heavy equipment close to the site in case further work is required to maintain the opening. We will also continue our close monitoring of the lagoon’s ecological health.
“While there is some uncertainty as to how effective this opening will be, our belief is that it is appropriate for us to take this action to seek to mitigate adverse effects on this internationally significant area,
“Our view is supported by interagency science opinion, which recognises mana whenua and local community concerns.”
The Waituna Lagoon has been closed to the sea since March 2021 but was periodically opened under a previous consent for a many years before that. While such a closure can have ecological benefits in terms of certain organisms that are important for Lagoon health, long periods of closure result in reduced flushing of nutrients and can be a barrier to fish passage.
Waituna Lagoon and wetlands were among the first sites in the world to be named “a wetland of international significance” under the Ramsar Convention on Wetlands, an intergovernmental treaty signed in in 1971.
The area is an important habitat for birds, fish and eels and is home to some unusual plant species. The lagoon is also a significant trout fishery and a popular walking area.