Lake Restoration

Restoration objectives

Restoration objectives depend on the target lake characteristics and lake users, but may include one or all of the following:

• Improving trophic state (water quality)
  
• Enhancing recreational activities (e.g. swimming, boating, hunting, fishing, birdwatching).
  
• Enhancing lake values (historic, conservation).
  
• Enhancing lake use (water source).

Restoration techniques

Restoration techniques fall into two main categories:

• Reducing external nutrient load/catchment restoration
  
• Reducing internal nutrient load/within lake restoration

Anthropogenic activities in a catchment such as pastural, urban or industrial land developments can result in nutrients, bacteria or heavy metals entering freshwater environments through surface run-off or leaching through soil into groundwater. These sources of pollution can be classified as point source or diffuse source pollution.

Point and diffuse sources of pollution

An example of a point source of pollution is direct discharge of sewage treatment ponds into a lake or stream. A farmer applying fertiliser to their farm or a cow urinating on grass are examples of diffuse sources of pollution. A diffuse source of pollution is usually diluted and often takes a long period of time to enter the freshwater environment whereas a point source of pollution is discharged directly into the environment.

External nutrient loading

When we try to reduce the external nutrient load to a lake both point and diffuse sources of nutrients and pollutants must be addressed.

Point and diffuse sources of nutrients may not alter the freshwater ecosystem if they are only small quantities. However, if these sources enter lakes and streams over a long period of time, the nutrients and pollutants build up as they become trapped in the lake sediments. For more information, go to the catchment page.

Internal nutrient loading

If a lake has been subjected to low levels of diffuse nutrient sources entering the lake over a long period of time (e.g. 100 years) it may have very high levels of nutrients trapped in the sediments. When these nutrients are released from the sediments under anoxic conditions or through sediment resuspension, they become available for phytoplankton and plant growth and can lead to the lake becoming eutrophic. Nutrients trapped in the sediments can prevent the trophic state from improving for decades after the external nutrient load has been reduced. Consequently, reducing the internal nutrient load may be essential to improvement of the trophic state in some lakes. For more information, go to the inlake page.

Nutrient imbalance

Generally, lakes that have suffered from minimal modification in New Zealand are nitrogen-limited, not phosphorus-limited. New Zealand soils tend to be high in phosphorus and there is a high natural erosion rate in many parts of the country. However, eutrophication of a lake can modify its original nutrient balance. Lakes that were once nitrogen-limited can switch to become phosphorus-limited.