Peat lakes

Peat forms from the build up of partially rotted plant material in wet environments. Peat lakes form in peatland areas such as bogs. Peat soils have a marked effect on the physical, chemical and biological nature of peat lakes. For example, the water is usually stained a brownish colour due to the high levels of dissolved organic matter leaching from adjoining peat soils. Water in the bottom of the lake is usually mildly acidic and often low in dissolved oxygen.

Peat lakes have unique plants and animals that have evolved to cope with their acidic conditions. Changes in water quality due to the surrounding land use could threaten these specialised plants and animals.

Peatlands take many thousands of years to develop. They develop where the rate of plant growth is greater than the rate of decay, and contain plants that are specially adapted to the waterlogged conditions and low nutrient levels.

Peatlands are fragile ecosystems that are susceptible to fire, increases in nutrients and the effect of drainage. 

Waikato peat lakes are valued for their unique genetic diversity, scientific interest and recreational opportunities. They are also valued for their cultural and spiritual values.

Peat lakes are a valuable habitat for many unique animals and plants.

Resident and visiting birds

Rare native species include:

• Australasian bittern
• banded rail
• grey duck
• grey teal
• marsh and spotless crake
• New Zealand dabchick
• North Island fernbird
• white heron.

Common introduced species include:

• mallard
• Canada geese
• black swan.

Aquatic animals

Rare endemic fish (only found in New Zealand) include the black mudfish and giant kokopu. Interesting invertebrates include:

• whirligig beetles
• a water mite (recorded for the first time in Lake Maratoto)
• freshwater jellyfish
• leeches (medicinal leeches in Lake Rotongata).

Plants

Some of the plants found in our peat lakes are rare in the Waikato region. These include:

• Wolffia australiana - a type of floating duckweed that is the world’s smallest vascular plant (plants that have vein-like structures for transporting sap).
• Native characean algae (underwater plants) – Chara coralina and Nitella cristata present in Lake Rotopiko.

Before Europeans settled in the Waikato, most of the lakes had no inlets or outlet drains. Changes in lake levels were due to seasonal rainfall variation. Water entered the lakes from nearby peat bogs and directly from rainfall, and was lost from the lakes mostly through evaporation.

Since 1840, many of the peatlands in the region have been drained for farming. Outlet drains from the peat lakes were dug to drain the land, often significantly lowering water levels. For example, the Komakorau, Rukuhia and Moanatuatua peat bogs once covered over 40,000 hectares. However today, only the 110 hectare Moanatuatua Scientific Reserve remains.

Vegetation clearance in the past, and more recently, land management practices (such as drainage for agricultural development), have affected all peat lakes in our region by:

• reducing water quality
• lowering lake levels
• degrading native habitat
• reducing biodiversity – fewer plants and animals present
• changing the natural character of the lake from a native vegetated environment to a grassed farmed environment.

Drainage and cultivation

Drainage and cultivation of peat soils for pasture and cropping results in oxidation and shrinkage of peat soil that can’t be reversed. Landowners often need to deepen drainage systems to maintain production on shrinking peat soils.

Deepening drains lowers the water table in surrounding farmland. However, the water level in nearby peat lakes becomes higher than the surrounding water table, causing water to ‘spill out’ onto neighbouring land. Lake levels in the peat lakes are then lowered to prevent flooding. This cycle of land drainage and lake lowering can continue until all the peatland and associated lakes are drained.

Today many of our peat lakes are much smaller and shallower than they were in the past. Shallower lakes tend to have less stable environments than deeper lakes because they have:

• increased turbidity from wind stirring up bottom sediments
• greater temperature extremes.

Continued lowering of peat lake water levels threatens the survival of the lakes and their unique wildlife.

Aquatic pests

Aquatic pests also threaten our peat lakes. Pest fish can play a major role in the disappearance of native aquatic plants from peat lakes. For example, introduced koi carp and brown bullhead catfish affect water quality by stirring up the lake sediments and uprooting aquatic vegetation. Rudd also affect native plants by grazing them.

The main plant pest surrounding peat lakes is grey willow (Salix cinerea). It replaces native woody vegetation like manuka and shades out native grasses and sedges.