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:

1. Reducing external nutrient load/catchment restoration

2. Reducing internal nutrient load/within lake restoration

Anthropogenic activities in a catchment such as pastural, urban or industrial land developments can all result in nutrient, stormwater or wastewater run-off entering freshwater environments. These sources of pollution can be classified as point sources or diffuse sources.

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. An example of a diffuse source of pollution is a farmer applying fertiliser to their farm. Eventually these nutrients will enter lakes and streams through ground or surface water inputs. A diffuse source of pollution has usually been diluted and often takes a long period of time to enter the freshwater environment. A point source of pollution is discharged directly into the environment.

External Nutrient Loading

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

These point and diffuse sources of nutrients may not alter the freshwater ecosystem if they are only present in small amounts. However, if these sources continue to enter lakes and streams over a long period of time, even if they only enter in very small amounts, the nutrients and pollutants build up in the lake ecosystem, as they become trapped in the lake sediments.

Internal Nutrient Loading

Consequently, if a lake has only 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 e.g. under anoxic conditions or due to sediment resuspension, they become available for phytoplankton and plant growth and can lead to the lake becoming eutrophic. These pools of 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 in some lakes may be vital to improve the trophic state.

Nutrient imbalance

Generally lakes that have suffered from only minimal modification in New Zealand are nitrogen limited, not phosphorus limited. New Zealand soils tend to be high in phosphorus and have 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.

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