Science
The lagoon system is highly complex. The quantity and method of the nutrients and sediment entering the lagoon is still being investigated but the cumulative effects over many years of land development and drainage, and recent land use changes and intensification, are known to be significant contributors.
Flipping would mean we would see the lagoon change from having clear water and an aquatic environment dominated by Ruppia to turbid, murky water dominated by algal slime. Such a shift would likely be irreversible and degrade the fishery, birdlife, wetland values, as well as cultural, and recreation values of the lagoon.
Indications that the lagoon is under stress and on the verge of flipping are:
- Stream nutrient inputs (N and P) have increased over the past 5-10 years.
- Lagoon nutrient and chlorophyll-a concentrations are currently at eutrophic levels.
- Symptoms of eutrophication within the lagoon include;
- Extensive sediment anoxia (lack of oxygen) which has become widespread throughout the lagoon since 2007.
- Extensive growths of nuisance slime algae (the brown alga Bachelotia antillarum) which have become widespread throughout the lagoon since 2007.
- A marked decline in Ruppia cover since 2009 (attributed to slime algae blocking the light, sediment anoxia, and the artificial opening of the lagoon).
- Release of sediment nutrients to the water column from anoxic sediments.
On top of increased nutrients and sediment in the lagoon, artificial lagoon opening also places Ruppia under stress, primarily by increasing the frequency of salinity changes and drying out periods.
Our monitoring programme
In the tributaries:
Environment Southland monitors five sites for water quality in the Waituna catchment monthly on the Carran, Moffat and Waituna Creeks. One of these sites was established in 1995 and others added in 2001. Water quality parameters measured include nitrogen, phosphorus, dissolved oxygen, suspended sediment, turbidity, pH, electrical conductivity, water temperature, and E. coli.
In the lagoon:
Environment Southland also monitors water quality monthly at four sites in the lagoon. Monitoring began between 2001 and 2003. Water quality parameters measured are the same as in the tributaries with the addition of chlorophyll-a to measure algae levels.
Environment Southland monitors water quality monthly at five sites in the Waituna catchment as part of its long term State of the Environment (SOE) monitoring programme. Click here to view the study.
Reports
Environment Southland is the brand name of Southland Regional Council.