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Temperature

Stream temperatures reflect the season and a site's latitude, altitude, aspect and proximity to the ocean. Waters tend to be cooler in streams that are well shaded or rise in alpine areas. During low summer flows, water in rivers may reach warmer temperatures, especially where there is a lack of riparian cover.

Water temperature can fluctuate diurnally and seasonally. Water temperature is also influenced by altitude and degree of riparian coverage. For example top of catchment locations which may have a greater degree of riparian shading, and are typically higher in altitude are generally cooler than lower down in the catchment. Basically, on average water temperatures increase toward the bottom of the catchment.

pH

The Australian guidelines for fresh waters are 6.5 - 9.0. The pH or acidity of Tasmanian streams is typically in the range 5.5 - 7.5. For humic rich Tasmanian lakes and rivers, the pH range is 4.0 to 6.5 (ANZECC 2000). Tasmania's inland waters are also weakly buffered¯i.e. they are unable to absorb large pH changes.

The majority of Tasmanian inland waters are low in alkalinity and hence poorly buffered. This is with the exception of some West Coast streams which are slightly more acidic.

In stream pH values are strongly influenced by a variety of factors relating to catchment geology, soil chemistry, vegetation and land use practices (Bobbi, 1999). The degree to which these factors affect pH and its buffering capacity against changing environmental conditions are inter-related with levels of acidity and alkalinity (UNESCO 1992). Waters of low alkalinity ( < 24 ml/L as CaCO3) have a lower buffering capacity and are therefore more susceptible to fluctuations in pH. These fluctuations can be influenced by biological and atmospheric processes (UNESCO 1992). Thus pH levels can vary both seasonally and diurnally depending upon environmental conditions (Wilson et al, 2003).

The pH is a measure of the acidity of a solution and ranges in scale from 0 to 14 (from very acid to very alkaline). A pH value of 7 is considered 'neutral'. In natural waters, pH is generally between 6.0 and 8.5. In waters with little or no buffering capacity, pH is related to alkalinity which is controlled by concentrations of carbonates, bicarbonates and hydroxides in the water. Waters of low alkalinity (< 24 ml/L as CaCO3) have a low buffering capacity and are susceptible to changes in pH from outside sources." (Bobbi et al 1999 - State of Rivers Report for Rivers in the Great Forester catchment).

Salinity (Electrical Conductivity)

Conductivity is a measure of the capacity of an aqueous solution to carry an electrical current, and depends on the presence of ions; on their total concentration, mobility and valence. Conductivity is commonly used to determine salinity and is mostly reported in microSiemens per centimetre (mS/cm) or milliSiemens per metre (mS/m) at a standard reference temperature of 25 degrees Celsius.

The recommended default trigger values for lowland rivers are 125-2200 mScm-1, upland rivers are 30 - 350 mScm-1, and 20 - 30 mScm-1 for lakes and reservoirs (ANZECC 2000). Tasmanian streams are generally at a mid-range of 90 mScm-1. Refer to the Australian Water Quality Guidelines for proposed conductivity trigger levels for Tasmanian rivers.

Changes in conductivity can also be explained through landuse, catchment geology and flow. For example during periods of low flow, ground water has a greater influence on conductivity. Hence in some regions of the state (eg Jordan River catchment, Coal River catchment and some east coast catchments) where ground water is naturally saline, low summer flows are often typified by high levels of conductivity due to ground water dominance. During higher winter flows the lower saline surface waters dominate, resulting in a decrease in in-stream conductivity. In some catchments it is not unusual for an initial spike in conductivity to occur at the beginning of a high flow event. A flushing of salts from the soil profile in the immediate area usually causes this.

Turbidity

Turbidity in water is caused by suspended material such as clay, silt, finely divided organic and inorganic matter, soluable coloured compounds and plankton and microscopic organisms. Turbidity is an expression of the optical properties that cause light to be scattered and absorbed rather than transmitted in a straight line through the water. Standard units for turbidity are "nephelometric turbidity units" (NTU's) standardised against Formazin solution.

Refer to the Australian Water Quality Guidelines for proposed turbidity trigger levels for Tasmanian rivers.

Suspended solids

Suspended solids are typically comprised of clay, silt, fine particulate organic and inorganic matter and microscopic organisms. Suspended solids are that fraction which will not pass through a 0.45 mm filter and as such corresponds to non-filterable residues. It is this fraction which tends to contribute most to the turbidity of water.

Dissolved oxygen

Dissolved oxygen (DO) concentrations vary with temperature, salinity, biological activity and rate of transfer from the atmosphere (ANZECC, 2000) and provides a basic indicator of ecosystem health. Cooler waters are more capable of holding DO than warmer waters and hence there are generally higher concentrations of DO during the cooler months as opposed to the warmer summer months. Salinity also affects DO concentrations with the solubility of DO increasing with decreasing salinity. Diurnal fluctuations in DO can be quite marked with night-time concentrations lower than those experienced during daylight hours. This is in part a function of in-stream photosynthetic and respiratory activity with decreasing DO at night influenced by the respiratory activity of aquatic plants.

The decomposition of organic matter through microbial activity also consumes oxygen and can be evident at nutrient enriched sites. The impact of microbial activity on water quality can be measured by testing for biochemical oxygen demand (BOD) of water.

Refer to the Australian Water Quality Guidelines for proposed DO trigger levels for Tasmanian rivers.

Heavy metals

Heavy metal pollution is typically associated with mining activities or discharges from some industries.

Persistent toxicants (heavy metals) in water and sediments affected by heavy metal pollution can have serious affects on the aquatic ecosystem and can make water unsuitable for human consumption. Some animals can also 'bioaccumulate' metals, making them unsafe to eat.

Nutrients

Some sources of nutrients are surface water runoff from land, resuspended material from the river bed, and bank erosion. Discharges from dairy sheds and sewage treatment plants can also contribute to elevated nutrient levels if not properly managed.

The process of nutrient enrichment is called eutrophication. Excess nutrient enrichment can lead to algal blooms. Factors contributing to the establishment of a bloom can be the transportation of large quantities of nutrients and a prolonged period of calm dry weather. Once a bloom is established it can persist for long periods even during winter.

Turbidity can sometimes be correlated with nutrients, however, to get a true relationship between turbidity and phosphate there needs to be a sizeable amount of data. Increases in total phosphate and total nitrogen can sometimes be correlated with an increase in turbidity and total suspended solids measured, particularly during flood events. This is because nutrients such as phosphates often adhere to clay particles, and it is during flood events when much of the material suspended is made up of sediment, soil and detritus. This can be from surface run off from the land, from the river bed, or eroded river banks.

The Australian Water Quality Guidelines specify trigger levels for total nitrogen and total phosphorus in inland waters.

Further Information

EPA - Water