New Zealand Lakes
Lakes are localised bodies of water that are surrounded by land. They may be fed by, or feed into rivers or may be maintained by rainfall and groundwater inputs. Lakes exist in all shapes and sizes and may be natural (for example formed by glaciation or landslides) or man-made (for example reservoirs created after a river is dammed).
Lakes are an important feature of New Zealand’s landscape. They provide a habitat for birds and fish, are highly valued culturally by Māori, and are used for a variety of recreational activities including rowing, fishing, swimming and boating. Lakes provide a key source of municipal water supply for many towns and cities throughout the country and they act as natural nutrient sinks for run-off from the land. New Zealand’s deepest lake is Lake Hauroko (462 m), and the largest is Lake Taupo (606 km2).
Shallow lakes are less than 10 metres deep. They are generally well mixed throughout the year and can be very productive environments.
Deep lakes (deeper than 10 metres) are often stratified in summer, meaning environmental conditions such as temperature and dissolved oxygen level change over the depth of the water column. When a lake is stratified, it usually forms three distinct layers: the epilimnion, the thermocline and the hypolimnion.
During the summer, warm water, which is less dense will sit on top (epilimnion) of colder, denser, deeper water (hypolimnion). The thermocline (or metalimnion) separates the top and the bottom layer and is a thin but distinct layer where temperature changes more rapidly with depth than it does in the layers above or below.
As summer proceeds, there can be less and less dissolved oxygen in the hypolimnion in stratified lakes. This is because the water below the thermocline depth does not circulate to the surface where re-aeration occurs and organisms and the decay of organic matter in the bottom waters deplete the available oxygen. In some instances lakes can become anoxic in their bottom waters as a result of these processes. As winter approaches, the temperature of the surface water will drop. A point is reached where the density of the cooling surface water becomes greater than the density of the deep water, then the different layers will start to mix (overturn) as the dense surface water moves down under the influence of gravity. This process is often aided by wind, inflow floods and/or other processes (e.g. currents) that agitate the water.
Lake stratification over the course of a year showing changes in dissolved oxygen and temperature with depth (https://lakeecosystems2014.wordpress.com/lake-data/physical-characteristics/).
What lake water quality indicators are shown?
Lake water quality on LAWA is presented showing Trophic Level Index (TLI), (LakeSPI) and a range of other water quality indicators, including total phosphorus, total nitrogen, chlorophyll a, secchi disc depth and E. coli.
Trophic states of lakes
Nutrients such as nitrogen and phosphorus tend to be limiting resources in standing water bodies, so increased concentrations tend to result in increased plant growth. The ‘trophic state’ of a lake may sometimes be used to make a rough estimate of its biological condition or water quality. There are three main classes of trophic state: , or . Oligotrophic lakes are typically low in nutrients and are unproductive, mesotrophic lakes have moderate levels of nutrients and productivity, while eutrophic lakes exhibit excessive nutrient levels and high biological productivity. The Trophic Level Index (TLI) is used on LAWA to give an overall picture of the health of New Zealand lakes. See our factsheet on how TLI is calculated.
Where do I find more information?
Burns, N., G. Bryers, and E. Bowman 2000. Protocol for monitoring the trophic status of New Zealand lakes and reservoirs. Prepared for NZ Ministry for the Environment. Lakes Consulting Ltd. 190 p.
Clayton J., Edwards, T. A method for monitoring ecological condition in New Zealand lakes. LakeSPI. User manual Version 2. National Institute of Water & Atmospheric Research Ltd. CRBV062
Jolly VH, Brown JMA 1975. New Zealand Lakes. Auckland University Press, New Zealand. 388p.