Whakamana te Waituna

Salinity dynamics around the lagoon

Opening the lagoon to the sea can be a useful tool for flushing out sediment and nutrients, and reducing effects of eutrophication. But a rapid change to saltwater conditions may threaten aquatic communities and other aspects of the lagoon's ecology. During the July 2012 opening at Walkers Bay, we investigated how far and how quickly saltwater penetrated around the lagoon.

‘West’, ‘centre’ and ‘east’ are our water quality monitoring sites. ‘T9’ is in the far west of the lagoon, ‘T7’ is in the central basin and ‘T1’ is in the far east. Salinity is measured by what’s called the Practical Salinity Scale, which has no units. Freshwater is usually less than 1, and seawater has a salinity of 34-36.

The results showed the central basin became salty almost immediately, whereas it took longer for salt to penetrate into Shand Bay and the extremities of the lagoon. The Waituna Creek mouth also became salty quickly, but returned to freshwater conditions after rain events, so the salinity at this site was quite variable.

The lagoon closed very quickly on the 25th of July 2012, which probably prevented the extremities of the lagoon from becoming as salty as those levels reached in the central basin. After the lagoon closed, there was less variability around the lagoon and most sites trended towards a salinity of 20. This reflectedthe lack of tidal processes creating variation in salinity, and wind mixing up the lagoon. The optimal salinity for ruppia germination is considered 4–8. As the lagoon fills up with freshwater, salinity decreases.

Salinity dynamics around Waituna Lagoon in 2012.