A range of factors influence the types of species that live in different parts of New Zealand.
- At a broad level, evolutionary history and climate determine which species are found in certain regions.
- At a finer level, factors such as water quality, land use, channel shape, or lake type influence the range of species found in a river reach or lake.
- Small-scale factors (light and food availability, cover, predation, competition, water depth and speed) also control which species live at certain sites within a river or lake.
All these factors can interact to shape freshwater ecosystems.
Without light, plants cannot grow. There are big differences between the communities living in the well-lit surface and edges of lakes, and those in the depths, which receive little or no light. Turbid (muddy) water also restricts the amount of light that reaches the bed.
Lakes with large shallow areas tend to be more productive than lakes with little shallow water (because more light reaches the lake bed). A similar situation exists in river systems, where heavily shaded streams rely on leaves and other organic material from the surrounding catchment to fuel the ecosystem. In larger rivers the canopy opens up, allowing algal production to become more important.
Predators versus available food
There is debate about whether the number of invertebrates (insects and other animals without backbones) in a habitat is controlled by the number of predators that will eat them, or by the amount of food available for them.
Both factors are important, but their relative importance may vary. For example, the introduction of trout appears to have changed some streams from systems controlled by the available food to those controlled by trout as predators.
Disturbances: floods and droughts
Rivers and streams are particularly prone to disturbance, due to huge changes in flows, as in floods and droughts. Although these are often seen as harmful, it appears that a moderate level of disturbance can promote higher diversity in some systems. This makes sense: if there is some variation in an environment, more life forms may be able to live there.
Irrigation and hydroelectricity projects
Water is currently in high demand, and the value of freshwater systems is often weighed against the potential value of using the water. Hydroelectric dams and large irrigation projects can turn running waters into lakes, affect water quality, and restrict fish movement along river systems.
Many rivers and lakes now have high concentrations of nutrients, sediment and faecal bacteria, and problems with algal blooms (a heavy growth of algae). This is because more intense farming leads to excessive runoff of sediment and fertilisers into waterways. Taking water for irrigation has also harmed the water quality of lowland streams and some lakes. The Rotorua lakes are classic examples. Too many nutrients enter the lakes, causing algal blooms, which in turn starve the water of oxygen. At times, the lakes are closed for swimming, due to low water quality.
Various pests have become established in freshwater systems. Pest fish prey on native species and can stir up mud and alter the water quality. Invasive aquatic plants grow profusely, changing the habitat and causing problems for recreational activities. For example the introduced weed Lagarosiphon major can grow so profusely that it clogs the shoreline, making it unattractive for swimming. Even tiny algae can cause problems. In 2004 outbreaks of Didymosphenia geminata smothered South Island riverbeds, affecting everything living there.