Temperature (°C) – thermal energy of the water.

Temperature affects a number of biological processes and is a common source of stress to organisms if it does not fall within a range typical for organisms. Water temperature also affects the ability of solutions to hold dissolved gases – gases are more soluble in colder water than in warmer water. In aquatic environments, this relationship is particularly important with respect to dissolved oxygen (see below). Obviously, water temperature varies seasonally with air temperature, but it can also vary in response to shading (or lack of shading) by terrestrial vegetation. In addition, human effluent from power plants requiring cooling water is frequently warmer than receiving water bodies and can cause temperature to increase in streams, rivers, and lakes.

Specific conductance (μS/cm) – ability of water to conduct an electrical current standardized to a particular temperature (“specific” to 25°C); units are the inverse of resistance (microSiemens) over a 1 cm path length.

Conductivity is affected by dissolved substances in the water, particularly salts. Conductivity values naturally vary considerably depending on regional geology, proximity to oceans, and rainfall. High amounts of dissolved salts in freshwater can be stressful to aquatic life and indicate possible contamination from a variety of human sources. Examples include runoff from roads (particularly after winter deicing), effluent from certain industries and sewage treatment plants, mine drainage, and fertilized cropland or lawns. In general, distilled water has a conductivity of 14 μS/cm, and seawater has a conductivity of 40,000 μS/cm. Fresh surface water typically has conductivity in the range of 40-1000 μS/cm.

Depth (m) – water depth at the instrument based on pressure of water column above sensors; instruments are suspended in mid-water column.

Water depth at these sensors is actually controlled, so this measurement is a bit arbitrary. For river stage, which is related to depth (although not a direct measure), check the real-time river gage data maintained by the USGS (http://waterdata.usgs.gov/pa/nwis/rt). Depth is generally more important in non-flowing water bodies

pH – activity of dissolved hydrogen ions in exponents of 10 (a change of 1 pH unit represents a 10-fold difference in H+ ion activity), generally considered an indication of acidity.

pH controls a wide variety of physiological processes, many of which are accustomed to a “neutral” pH (near 7). pH of surface water is typically between 6 and 9, but pH could be higher or lower depending on sources of acids or bases. pH can fluctuate in response to changing concentrations of carbon dioxide in water. For example, during daytime, photosynthesis consumes carbon dioxide produced by respiration and results in higher pH, but pH decreases at night when respiration continues but photosynthesis stops.

Oxidation-reduction potential (mV) – the tendency of a solution to move electrons by accepting them from or donating them to ionic chemicals in the solution. Also called redox potential or reduction potential.

Oxidation of a substance occurs when it loses electrons, so a substance in a solution with high ORP will likely be oxidized. Reduction of a substance occurs when it gains electrons, so a substance in a solution with low ORP will likely be in reduced form.
ORP of a solution affects the state of many substances in the water. For example, iron will rapidly oxidize to rust in solutions with high ORP. In addition, many biological processes are controlled by the redox potential of the environment where the organisms live. ORP is affected by the amount of dissolved oxygen in the water, along with the concentrations of other dissolved solids (especially metals).

Chlorophyll (μg/L) – the concentration of chlorophyll in the water column at the sensor.

Chlorophyll is the most abundant pigment responsible for photosynthesis by aquatic plants. Its abundance in the water column indicates productivity by algae. Algae in the water column could be phytoplankton in the river, phytoplankton reaching the river from lakes or reservoirs, or algae that had been attached to rocks.

Turbidity (NTU) – the cloudiness of water caused by suspended particles, such as clay, silt, algae, or organic particles, in nephelometric turbidity units (NTU).

Turbidity can clog gills of fish and aquatic invertebrates, interfere with feeding mechanisms of filtering organisms, reduce prey detection by fish and other visual predators, modify substrate to affect benthic organisms, and reduce light availability to algae. Particles that cause turbidity can be generated from a number of sources, such as mining, erosion of stream banks, runoff from agriculture or developed land, and organic particles. Algal productivity in the water column also contributes to turbidity.

Dissolved oxygen ODO (mg/L) – concentration of dissolved oxygen in the water column. ODOSat (%) – Dissolved oxygen concentration as a percentage of the amount that could be dissolved in the water at a certain temperature (see above) and atmospheric pressure.

Dissolved oxygen is perhaps the single most important property in aquatic environments to organisms. Oxygen is required for survival of all aerobic organisms (pretty much every organism other than some bacteria). Dissolved oxygen is related to temperature (see above), atmospheric gas exchange (related to water velocity, depth, and turbulence), and biological “metabolism” (i.e. balance of photosynthesis and respiration by all organisms in the ecosystem). Oxygen depletion below levels required to support many organisms can occur if amounts of biological respiration exceed replenishment from the atmosphere or photosynthesis, particularly in warm water where oxygen concentrations are already lower than in cold water.

Interesting relationships among parameters:
Chlorophyll, turbidity, dissolved oxygen
Dissolved oxygen, specific conductance, oxidation-reduction potential

More Information
Links to other websites with useful descriptions of water quality parameters and their measurement