National environmental indicators

There are two national environmental indicators for freshwater.

The first provides information on water quality in rivers, lakes, and groundwater aquifers. This indicator is reported using the following measurements:

• concentrations of nutrients (nitrogen and phosphorus in rivers and lakes, and nitrate in groundwater)

• concentrations of the bacterium Escherichia coli (E. coli) in rivers and lakes, including freshwater swimming spots

• visual clarity in rivers and lakes

• water temperature in rivers

• dissolved oxygen in rivers

• richness of macroinvertebrate species (%EPT) in rivers.

The second freshwater indicator provides information on freshwater demand. This indicator is reported using the following measurement:

• volume of water allocated to human uses. This is also known as total (consumptive) water allocation.

Understanding how these indicators change over time will allow us to improve our response to existing and emerging pressures on New Zealand’s water resources.

In addition to the measurements listed above, further measurements of freshwater quality in this chapter include:

• organic pollution of rivers (as shown by measurements of biochemical oxygen demand)

• salinity and concentrations of metals (iron, manganese, and arsenic) and chemicals (sodium, sulphate, and chloride) in groundwater.

Freshwater quality

Concentrations of nutrients

Aquatic plants need many types of nutrients for growth, including nitrogen and phosphorus. (This includes the dissolved forms of nitrogen (nitrate) and phosphorus (dissolved reactive phosphorus)). However, increased levels of these nutrients in water bodies cause plant growth rates to increase excessively, especially if water flows, sunlight, and temperature conditions are favourable to them. This can lead to algal blooms, as well as an over-abundance of aquatic weeds in river channels and on lake margins. Excessive algal or weed growth can reduce the recreational and aesthetic value of water bodies, and alter water quality (for example, by changing the acidity or oxygen levels).

Fertilisers and stock effluent are major sources of the nitrogen and phosphorus in water bodies in agricultural catchments. The erosion of soil also contributes significant amounts of soil-bound phosphorus to waterways.

Concentrations of bacteria

E. coli is a bacterium that indicates the presence of faecal material in freshwater. This, in turn, indicates the presence of disease-causing (pathogenic) micro-organisms caused by discharges of treated human sewage (from wastewater plants, septic tanks, or faulty sewerage systems) and dung from birds and animals.

A high concentration of E. coli indicates an increased risk of digestive and respiratory system diseases among people who come into contact with, or drink, the contaminated water. Very young children, the elderly, or people with impaired immune systems are particularly vulnerable to this risk. The health of livestock that drink contaminated water may also be affected.

Visual clarity

Visual clarity refers to how far you can see through the water in rivers and lakes. It provides an indication of the levels of suspended sediment: high clarity indicates low levels of suspended sediment.

Rivers and lakes with high clarity appear clean and are often highly valued for fishing and other recreation. A river or lake with low clarity will have murky water, which may indicate significant erosion in the catchment (producing suspended sediment) or abundant algal growth in the water. Murky water prevents sunlight penetrating, while sediment can smother aquatic habitats, which affects the feeding and spawning habits of fish and other animals, and the growth rates of plants.

Visual clarity is reported on its own as an indicator for rivers. However, it is combined with measurements of nutrients and algae (to form a Trophic Level Index) to indicate the water quality of lakes (see box ‘What is the Trophic Level Index’ in the ‘Current state and trends’ section of this chapter).

Water temperature

If water temperatures increase beyond their usual ranges for too long, plants and animals in waterways can become stressed and die. (Low elevation streams and rivers in New Zealand typically have a water temperature that fluctuates within the range 10–20°C across seasons. Alpine or spring-fed streams and rivers can be much colder, and the water temperature in un-shaded shallow streams may rise to nearly 30°C in the peak of summer.) Temperature changes can be caused by changes in climate, or by human activities such as removing stream-bank vegetation, storing water in dams, or discharging heated or cooled water after it has been used in industrial processes (for example, in power generation). Taking too much water from a river or stream (referred to as ‘over-abstraction’) can also increase its temperature.

Dissolved oxygen

Dissolved oxygen is an indicator of the health of freshwater ecosystems. Fish and other aquatic life require dissolved oxygen to breathe. When dissolved oxygen levels are depleted, aquatic animals can become stressed and die. Oxygen depletion is commonly caused by organic pollutants breaking down in waterways, elevated water temperatures, or night-time respiration by dense algal blooms in nutrient-rich waters.

Macroinvertebrate richness (%EPT)

Freshwater macroinvertebrates are aquatic animals such as insects, worms, and snails. Sampling both the type of macroinvertebrate taxa (that is, groups of similar individuals) present in a waterway, and the population of each of these taxa provides an indication of overall river or stream health and water quality.

Some macroinvertebrates are particularly sensitive to pollution, so are good indicators of water quality degradation that has been caused by human activity. In particular, Ephemeroptera, Plecoptera, and Trichoptera taxa together form a measurement called ‘%EPT’. Low %EPT indicates a river or stream is under pollution stress.

Freshwater demand

Allocation of water to consumptive uses

Regional councils are responsible for granting resource consents in New Zealand. These consents are generally required before surface water or groundwater may be removed for irrigation, drinking water supply, industrial and manufacturing works, and other activities. Smaller volumes of water, such as for low-level home supply, can also be allocated through permitted activity rules under councils’ regional plans.

The consent process, which is generally set up through regional plans, is also known as ‘the allocation of freshwater’. It can determine both the maximum volume of water that may be taken, and the maximum rate at which water may be taken.

The allocation indicator in this report assesses consumptive water use only (that is, water that is taken from a river, lake, or aquifer, and not returned directly to that source). Allocation of water for hydro-electric power generation is not a consumptive use, although hydro-electric schemes can significantly alter the pattern of water flow in rivers and lakes.

Limitations of the indicators

While the indicators for freshwater provide an overview of the quality and level of allocation in New Zealand, they do not provide a complete picture of the health of the freshwater resource. The indicators do not include several other pollutants that can affect water quality, such as herbicide and pesticide residues, dissolved metals, pharmaceuticals, and hydrocarbons (although some information on metals and pesticides in groundwater is presented in the section ‘Current state and trends’ later in this chapter).

The indicators also do not cover many aspects of freshwater ecosystem health, such as the type and abundance of fish and aquatic plants. While many regional councils undertake such monitoring as part of their freshwater management programmes, data is not yet readily available for this to be reported at the national level. Some aspects of freshwater biodiversity, including the distribution of native and invasive (introduced) aquatic animals and plants, are discussed in chapter 12, ‘Biodiversity’.

Similarly, while many councils and other agencies monitor changes in water quantity (river and groundwater flows) in New Zealand, this data is not yet readily available for national reporting. Therefore, the indicator for water quantity focuses on demand (allocation).