New Zealand has 3,820 lakes over one hectare in size and many more that are smaller. Less than 40 lakes are greater than 1,000 hectares in size. [Based on the database of lakes produced by NIWA for developing a lake classification system.] The largest lake in New Zealand is Lake Taupo with an area of about 62,000 hectares and maximum depth of 163 metres. The next largest North Island lake is Lake Wairarapa with an area of almost 8,000 hectares and a maximum depth of less than three metres. The deepest North Island lake is Waikaremoana, at 248 metres deep, formed when a landslide blocked a valley 200 years ago (Spigel and Viner 1991, MfE 1997).
The South Island's largest lakes are Lake Te Anau, with an area of around 35,000 hectares and a maximum depth of 417 metres, and Lake Wakatipu with an area of 29,000 hectares and a maximum depth of 380 metres. The third largest South Island lake is Ellesmere (or Waihora) with an area of 18,000 hectares and a maximum depth of less than three metres. The deepest lake in New Zealand is Lake Hauroko in Southland with a maximum depth of 462 metres (Spigel and Viner 1991, MfE 1997).
Several broad categories of lakes can be identified that reflect formation processes. Volcanic eruptions created many of the North Island's larger lakes. Glacial ice gouged out the basins for many of the South Island lakes. Dune lakes are common in Northland and on the west coast of the North Island. Peat lakes are a distinctive feature of the Waikato region; they typically have slightly acidic and humic-stained water. Riverine lakes are formed when rivers change their course. Landslides blocking valleys can form large lakes. Lagoons are created by the movement of sandbars - they often have brackish water and are occasionally open to the sea. Finally, we create artificial lakes or reservoirs for hydro-power stations (Spigel and Viner 1991, MfE 1997).
NIWA is currently developing a classification system for New Zealand lakes for the Department of Conservation. This is a multivariate classification system based on lake variables (eg, physical attributes of area, depth, fetch etc) and catchment attributes (eg, proportion of catchment in beach forest, glaciers, peat, pasture, geology with high phosphorus etc). It is designed to discriminate the variation in the natural and existing character of New Zealand's lakes.
Lakes are intimately linked to their catchments. Land-use activities in a catchment affect the amount of water, nutrients, sediment and other contaminants that enter a lake. These inputs affect the water quality and the functioning of the lake's ecosystem. This is called eutrophication. Typically, an increase in nutrients to a lake stimulates growth of phytoplankton which reduces water clarity. At high nutrient levels algae blooms may occur, often of potentially toxic cyanobacteria species, causing surface scums and a decline in dissolved oxygen as the bloom decomposes. In very bad situations fish may die from the low dissolved oxygen levels. The state of the water quality and degree of nutrient enrichment is described by trophic state, with oligotrophic, mesotrophic and eutrophic lakes having progressively more nutrients, more algae biomass and poorer water clarity.
Although we report water quality in simple terms such as trophic state, the factors affecting lake water quality can be very complex. Some of the key factors that interact to affect lake water quality are: sediment resuspension and release of nutrients, grazing of phytoplankton, phytoplankton community composition, macrophytes and fish.
Wind has a strong influence on lakes; it can mix the water and resuspend bottom sediments - particularly in shallow lakes. This can increase the nutrients available for algae growth, at the same time the more turbid water can inhibit algae and macrophyte growth by reducing the amount of light available.
Lake Coleridge is a deep glacial lake in the South Island. In 1993, an earthquake triggered a large release of suspended sediments into the lake, reducing the water clarity. Over the next two years a die-off of native aquatic macrophytes (characean algae) occurred in the deep water so that the depth of macrophyte growth reduced from 30 metres to 20 metres. The depth of macrophyte growth has recently extended back down to 30 metres as the water has cleared (Elliot and Sorrell 2002).
The growth of aquatic macrophytes also has strong interactions with lake water quality. A collapse in the coverage of aquatic macrophytes has been observed in many shallow lakes around New Zealand - always with a corresponding decline in water quality, eg, Lake Rotokauri in 1996/97, Lake Rotomanuka 1996/97, Lake Rotoroa 1989/90, Lake Whangape in 1987 (in the Waikato), and Lake Omapere since 2000 (in Northland).
A crash in the macrophyte population can occur very quickly and be triggered by a storm, grazing by swans or fish, an increase in nutrient inputs or lowering of the water level. Following a die-off, the macrophytes decompose, releasing nutrients into the water which stimulates the growth of aquatic algae resulting in a decline in water clarity. Without the macrophyte cover, the lake sediments are more prone to resuspension - further increasing the turbidity of the water and the availability of nutrients. This turbid, phytoplankton-dominated state can last a long time and the macrophytes recover only slowly. When they do recover, as recently found in Lake Rotoroa (Hamilton), there is generally a corresponding increase in water quality.
Grazing by swan, fish or koura (native freshwater crayfish) has a very strong impact on the growth of aquatic macrophytes. In the North Island, koura (Parenephrops planifrons) reduce the density of charophytes in deeper water where the light is also limited. Introduced fish such as Japanese koi have a much larger impact on macrophytes and can completely destroy populations and prevent regeneration. A recent improvement in water quality in Lake Wainamu, near Bethels Beach in Auckland, has been partially attributed to the trapping and removal of coarse fish (eg, perch, goldfish, rudd) by the Auckland Regional Council and the local community (more information can be found on the ARC website www.arc.govt.nz).
For further information on factors affecting lake water quality refer to the Lake Managers Handbook (Vant 1987) and the updated version on Land-Water Interactions (Elliot and Sorrell 2002).