the lower fox river watershed monitoring program    
University of Wisconsin Green Bay
University of Wisconsin Milwaukee

Temperature

Temperature is an important parameter in aquatic environments as it influences many aspects of stream physical, chemical, and biological health. Most aquatic organisms have limited optimal temperature ranges, which affect survival, spawning success, and metabolic rates. Annual temperature changes provide the stimulus for emergence of insects and spawning of fish. Because the high specific heat of water results in relatively slow rates of temperature change, aquatic species are buffered from the wide variations in temperature that terrestrial organisms are accustomed to. Therefore, large human-induced fluctuations in water temperature can have devastating impacts on stream biota. Increased water temperature also decreases the availability of dissolved oxygen, amplifies the stress caused to organisms by toxic compounds, and enhances algal and bacterial growth rates. Conductivity and pH are also influenced by the stream temperature, but most probes compensate for this effect.

The greatest source of heat to the stream is solar radiation. Streams with little riparian canopy cover are therefore warmer than shaded stream reaches. Deep streams are generally colder than shallow streams, as evaporation at the surface level slows heat transfer through the water column. Stream temperature is also impacted by groundwater, tributaries, rainwater, and discharge pipes. These inputs may either raise or lower the temperature, depending on season and source. Temperature also varies between different habitats within a stream reach, with backwater pools often warmer than the main stream channel.

Stream temperature can be measured with either a calibrated thermometer or with a dissolved oxygen, conductivity, or pH probe. Environmentally safe thermometers should be used to minimize the risks associated with potential mercury contamination. Data loggers which continuously record temperature measurements at set intervals may be used to show temperature fluctuations over time. For the LFRWMP, temperature measurements should be reported in °C to one decimal place.

Temperature readings for streams in the Fox River Basin typically are between 10°C and 20°C during our sampling periods. Wisconsin water quality standards state that there shall be no temperature changes that adversely affect aquatic life, and that daily and seasonal temperature fluctuations shall be maintained. By law, the temperature for warm water fisheries shall not exceed 89°F (31.7°C). A maximum temperature limit for cold water fisheries is not specified.