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Water temperature is a key variable affecting aquatic organisms. Understanding their response to elevated water temperatures is important for estimating upper thermal limits, and ultimately for assisting with setting defendable, biologically-relevant water temperature guidelines for lotic systems. Sublethal effects impacting on an individual organism or species may manifest at higher levels of the hierarchy, namely, populations, communities and entire ecosystems. Sublethal effects typically include those affecting an organism's physiology and metabolism (e.g. growth rates, secondary productivity, respiration); phenology (e.g. development time, voltinism, emergence); reproductive success and fitness (e.g. fecundity, rates and success of egg development and hatching); behaviour (e.g. migration, movement, drift); and broad-scale ecological effects (e.g. species richness, composition, density, distribution patterns). Sublethal effects are discussed with examples drawn from freshwater studies, in particular those focused on aquatic insects. Commonly-used methods, which vary from simple, cost-effective, laboratory-based methods to more elaborate, expensive, laboratory- and field-based studies, are assimilated to serve as a toolbox for future thermal research. Ultimately, the method adopted depends largely on the question(s) being asked and available resources. [ABSTRACT FROM AUTHOR]

Micro-scale heterogeneity in water temperature was examined in 6 upland sites in the Western Cape, South Africa. Hourly water temperature data converted to daily data showed that greatest differences were apparent in daily maximum temperatures between shallow- and deep-water biotopes during the warmest period of the year. Pool depth affected water temperature with deeper pools creating a more stable thermal environment. Groundwater-dependency affected water temperature differences with less groundwater-dependent rivers exhibiting greater differences in daily maximum temperatures. The importance of maintaining instream and riparian habitat, including pools, undercut banks, marginal vegetation, and an intact hyporheos, is discussed, giving examples of organisms utilising such habitats as thermal refugia. Considerations when choosing an appropriate model to simulate water temperatures, including groundwater-dependency, temporal and spatial scale, and study objectives, are discussed. [ABSTRACT FROM AUTHOR]