Regional lakes
Lake Ototoa
On this page:
- About Lake Ototoa
- How did this lake form?
- How healthy is Lake Ototoa?
- Threats to Lake Ototoa
- What is the ARC doing?
- Technical Publications
About Lake Ototoa
South-westerly aerial view of Lake Ototoa
More than half of the lake is over 12m deep. This means that, in summer, the surface water gets much warmer than the bottom water. This creates distinct layers of water in the lake, each at a different temperature; an effect known as thermal stratification.
As a result of thermal stratification, the oxygen-rich surface water is not mixed with the deeper water. Therefore the amount of oxygen in the deeper water declines over summer because decaying plant and organic matter in the deeper water uses up the remaining oxygen as they decompose and release nutrients.
The lake is filled mostly by groundwater that seeps into the lake and in wetter periods by a small stream flows into the northern end of the lake.
Water is lost by evaporation from the lake surface and is also taken for domestic use and to provide a water supply for stock. Water also percolates out through the lake bottom if the groundwater level falls.
Numerous waterfowl live on and around the lake. Many of these birds are shy and enjoy the generally quiet environment provided at Lake Ototoa. They include the New Zealand dabchick, New Zealand scaup, grey duck, and shoveler. Bittern, pukeko, and fernbird live around the lake where extensive beds of raupo fringe the shoreline.
Facts
| Location guide | At the northern end of South Kaipara Head, along the west coast of the North Island, New Zealand. See a location map of Lake Ototoa |
| Surface area | About 110 hectares |
| Maximum depth | About 29m |
| Catchment area | About 510 hectares |
| Type of catchment | Pasture (39%) Exotic forest (27%) Native forest/scrub (34%) Urban (none) |
| Surface water temperature | Average 18.0°C |
| Number of sampling sites | One site, but water samples are collected at two different depths. |
How did this lake form?
The Kaipara South Head is a massive, ancient sand barrier that separates the South Kaipara Harbour from the Tasman Sea.
A long line of interdune hollows runs up the Kaipara South Head, parallel to the coastline. These interdune hollows are lower than the groundwater level so they fill up with groundwater to create lakes.
Lake Ototoa is the largest of these interdune lakes (Lake Kereta is another smaller example).
How healthy is Lake Ototoa?
Since 1998, the quality of the lake water has been monitored as part of our State of the Environment (SoE) Lake Water Quality monitoring programme.
The quality of any lake water is shown by its trophic state. Lake Ototoa has the highest water quality of all the lakes that are monitored in the Auckland region.
The lake water appears to cycle between oligotrophic (clear and blue, with low levels of nutrients and algae) and mesotrophic (moderate amounts of nutrients and algae). Currently it is mesotropic:
| Microptrophic | Oligotrophic | Mesotrophic | Eutrophic | Supertrophic | Hypertrophic | |
| 2006/07 | x | |||||
| 2005/06 | x |
Threats to Lake Ototoa
The most significant threats to Lake Ototoa are the:
- introduction of invasive pest plants, particularly hornwort,
- large numbers of exotic fish, particularly perch.
Introduction of invasive pest plants
In 2005, a survey of the submerged plants found large meadows of native charophytes growing in the lake, with native reeds fringing the shores. There was little difference from an earlier survey done in 1988, although the invasive swamp lily was noted in shallow water near some parts of the shoreline.
The biggest immediate threat is the potential introduction of hornwort, an invasive weed already found in other nearby lakes. Hornwort is New Zealand's worst invasive submerged weed.
What is the ARC doing?
- In 2005 the ARC contracted NIWA to assess the condition of the submerged aquatic plant community in Lake Ototoa.
- In 2006, the ARC commissioned NIWA to help develop a regional management plan for freshwater weeds and fish.
- The ARC and Rodney District Council purchased a 247 hectare property at South Head on the South Kaipara Peninsula in 2005. This property offers access to Lake Ototoa and to one of the best beaches on the Kaipara Harbour. It will be developed into a regional park.
- The water quality is sampled once every two months as part of our State of the Environment (SoE) Lake Water Quality monitoring programme. Find out more about the Lake Water Quality monitoring programme.
- The ARC has operated and promoted the Environmental Initiatives Fund (EIF) since 1999. This support individuals and groups to improve and care for the region's natural, cultural and physical environments. Around $500,000 is available annually. Find out more about EIF.
Technical Publications
The ARC has a number of technical publications relating to this subject. Follow this link to see the full list of ARC technical publications.
