Natural Factors

Natural factors can cause changes to the quality of freshwater, and can also exacerbate the damage caused by human-made pollution. Algae grows best in a warm climate, so water bodies in warmer regions are more likely to experience algal blooms than those in colder parts of the country. The amount of rainfall that a water body receives controls the amount of water that flows in rivers and aquifers, and affects water quality by carrying pollutants from land to the water. At times of heavy rainfall, particularly after a dry period, much higher levels of pollutants may enter water bodies. The geology of the catchment will have an impact on the level of sediment carried by the water: mountainous regions where the catchments are steep-sided, or where the sediment is soft, are more likely to experience natural erosion causing rivers draining from these catchments to carry high levels of sediment.

Although these factors are important to consider when evaluating water quality, the main concern for managers of water resources are the impacts generated by New Zealand’s human population. A wide range of human activities impact on freshwater bodies including: agriculture, forestry, horticulture, urban development, and mining. Climate change is likely to place further pressure on lakes, rivers and streams. For more information on activities impacting on freshwater see Activities.

Invasive fish and plant species are also of particular concern in freshwater ecosystems. Exotic species can displace native species upsetting the ecological balance.

A well known pest species is didymo, (Didymosphenia geminata) a freshwater diatom (alga) which was first reported in New Zealand in 2004. 2008  Didymo, also known as rock snot, is a microscopic pest that is spread in water. It can form massive blooms on the bottom of streams, rivers and lakes, attaching itself to the bed of the water body on stalks. Didymo has a negative effect on the aesthetic values of water bodies for recreational users. It can also cause significant problems for irrigation by clogging up pump screens, sprinkler nozzles and filters, and can have the potential to reduce the area available to native fish for spawning, foraging and refuge.  Didymo may cover the spawning areas of trout, reducing oxygen available to the larvae and fry, and also potentially preventing them from escaping to the surface.

Another common pest species, Koi carp, were brought to New Zealand in the 1970s. They are now very common in the North Island. For example, koi carp now make up 67 per cent of the total fish biomass in the lower Waikato River downstream of the Karapiro Dam. 2009  They have been illegally spread through parts of the North Island, mostly the Auckland and Waikato regions, by recreational fishers who prize them for their size and fighting ability. Koi are very damaging to the environment, as they churn up the beds of rivers, lakes and wetlands when they search for food, and they have proved very difficult to eradicate.


  2. Hicks B J, N Ling and B J Wilson, 2010, ‘Introduced fish’, in K D Collier K, D Hamilton, W N Vant and C Howard-Williams, Waters of the Waikato, University of Waikato, Hamilton, 209-228 at 226

Last updated at 1:51PM on February 25, 2015