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Environmental Water Quality Indicators


The quality of water in our rivers, estuaries and coastal waters provides valuable insights into overall ecosystem health. Physical, chemical and biological water quality indicators are vital measures of human impact on ecosystem health and the effectiveness of remedial efforts. Long-term water quality decline leads to not only ecosystem degradation, but also threatens other uses of our waterways that are valued by the community, such as the provision of water for drinking, industry and agriculture, social and recreational needs.

The Yarra River and Port Phillip Bay Report Card was developed to report on the environmental health of Port Phillip Bay and its associated catchments using water quality indicators. For information on recreational health of waterways see Yarra Watch and Beach Report programs.

2 Men freshwater sampling Freshwater sampling. Image source: EPA Victoria


What are water quality objectives?

The level of water quality needed to protect the aquatic environment and maintain waterway health for the uses we value is established through setting water quality objectives in government policies. These objectives are developed for specific geographical regions, taking into account the uses of the water and the background water quality. They are typically stated in terms of water quality thresholds for single measurements or statistical measures from routine a number of measurements.

Water quality objectives also provide a means for setting goals for improvement of degraded waterways. For example, in many urban waterways a return to pristine or natural conditions is simply not achievable because of the irreversible ecosystem changes that have occurred over decades of urban and rural growth. Water quality objectives are typically designed to account for the history of human impacts and change that accompanies waterways in populated areas.

Scientists and decision makers then assess the extent of deviation from water quality objective levels as a measure of waterway health.


How to read the graphs for each monitored site

Environmental water quality objectives (the red line) provide "benchmarks" for water quality - they represent the "natural" level that protects the health of ecosystems and ensures that the water is suitable for beneficial uses (such as agriculture, recreation, human consumption, and others). We get these benchmarks from the State Environment Protection Policy (Waters of Victoria).

Measurements that are higher than the red line (or lower in the case of dissolved oxygen) generally indicate pollution or adverse human impacts, but they could also be naturally high or low in that particular environment for a number of reasons. The red line is a general benchmark we can use to trigger more questions to be asked about what may be happening at that spot.

We also need to recognise that it may not be possible to meet these benchmarks in some locations because the streams, particularly in urban areas, are so significantly altered from their natural state, and will never return to a "natural" state for that area. However, it is still important to keep monitoring against benchmarks to provide "goal posts" for us to aim towards.

Read more about the water quality objectives and beneficial uses for different environments:


Report Card Water Quality Indicators

Water quality indicators measured during routine monitoring of the catchment and bay have been selected and processed (see Scoring Method). The selected indicators correspond to water quality objectives in the State Environment Protection Policy (Waters of Victoria), and are used to benchmark water quality.

Indicator Catchments Marine
Nutrients ? ?
Water Clarity ? ?
Metals * ? ?
Salinity ? ?
Dissolved Oxygen ? ?
pH ? ?

*Measured levels of metals in the bay have generally always been very low since sampling began in the early 1980’s, and monthly sampling was suspended in 2012.


Nitrogen and phosphorus are essential nutrients for plant and animal growth. These nutrients reach coastal waterways via rivers and streams, and through exchange with the atmosphere and ocean. Rural and urban land use activities, like farming and housing development, have led to broad scale and significant increases in nutrient loads to waterways. Excessive levels of nutrients can greatly modify aquatic plants, and subsequently general water quality, by promoting the growth of organisms like blue-green algae at the expense of other species. This in turn can affect the entire food chain.

Dissolved Oxygen

Dissolved oxygen refers to the amount of oxygen contained in water, and is a critical measure of the living conditions for oxygen-requiring (aerobic) aquatic organisms. Most aquatic organisms need oxygen to be above a certain concentration for respiration and efficient metabolism, and dissolved oxygen concentration changes above or below this level can have negative impacts on their physical wellbeing. Discharges of wastes rich in organic matter (e.g. from sewage treatment plants, paper manufacturing, food processing and other industries) can occur in urban population centres, and can substantially reduce dissolved oxygen concentrations. Even short-lived conditions of low dissolved oxygen (anoxic or hypoxic events) can lead to deaths of aquatic organisms, such as fish, and are observed from time to time.

Water Clarity
(Turbidity and Total Suspended Solids)

Turbidity is a measure of water clarity. High turbidity (low clarity) is caused mainly by large concentrations of sediments that are washed off catchments into streams and rivers and ultimately into estuarine and marine environments. High concentrations of sediments are detrimental to aquatic ecosystems because the light that is available for plant photosynthesis is reduced, and sediment also carries with it other materials such as toxicants, pathogens and organic matter that consume oxygen in the water column.

Salinity (Conductivity)

Salinity refers to how much salt is in water. The water in rivers and streams is usually fresh, oceans are salty and estuaries are highly variable depending on tides and freshwater flows. Salinity levels may fluctuate quickly (periods of hours and days) with the tidal cycles, in response to mixing of fresh and marine waters by wind and currents, and over seasonal cycles with large freshwater flows from the catchments during wetter months. Most aquatic organisms have evolved to function within an optimal salinity range and tolerate natural cycles within this range. Long term changes to salinity distributions and their natural cycles can severely affect the health of aquatic organisms.


Metals occur naturally in the Earth's crust, and are released into the environment from the physical and chemical weathering of rocks. The background concentration of metals is mostly controlled by the geological characteristics of the catchment. However, anthropogenic (human) sources of metals include industrial and municipal waste products, urban and agricultural runoff, atmospheric deposition and antifouling paints applied to marine vessels. Most metals are toxic to organisms above a certain concentration and some accumulate in animals and plants (including fish, mangrove vegetation and seagrasses). They enter the food chain through body and respiratory surfaces, and by ingestion.


pH is a measure of the acidity or alkalinity of water, ranging from acidic (pH less than 7) through to neutral (pH 7) and alkaline (pH greater than 7). Most aquatic organisms require pH to be within a particular range. If water is too alkaline or too acidic the physiological processes of an organism may be disrupted. While there are natural sources of alkaline and acid inputs due to local geology, changes are often due to chemical spills and mine waste disposal. Acidic waters also make metal toxicants more available for plants and animals to take up and accumulate, which can lead to severe ecosystem impacts (as discussed above).


Chlorophyll-a is a green pigment found in plants. It absorbs sunlight and converts it to sugar during photosynthesis. Chlorophyll-a is a commonly used measure of water quality and concentrations are an indicator of algae abundance and productivity in aquatic environments. Higher concentrations typically indicate poor water quality, usually when high algal production is maintained due to high nutrient concentrations. It is natural for chlorophyll-a concentrations to fluctuate over time and they are often highest after rain, particularly if the rain has flushed nutrients into the water during the warmer, sunnier months of the year.

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