Temperature and light.

~ Riparian vegetation shades streams, decreasing the amount of direct and diffuse sunlight reaching the water surface and reducing daily and seasonal extremes of water temperature. ~ Shading controls primary productivity within the stream to a greater extent than nutrient levels, as the growth of most aquatic plants is regulated by light availability. At sites with elevated nutrient levels, shading can therefore control the effect of nutrient enrichment. ~ In cleared streams, water temperature can exceed the lethal limits for aquatic fauna, directly influencing local biodiversity and, at lower temperature levels, the growth and development of aquatic plants and animals. ~ The temperature tolerance of Australian aquatic macroinvertebrate fauna is similar to that measured elsewhere in the world. In temperate systems, a target of 21°C is recommended, and in northern systems, 29°C for stream water temperatures. ~ The degree of shade created by riparian vegetation is influenced by several factors, including canopy height, foliage density, channel width and orientation, valley topography, latitude and season. The effect of shading on the structure and function of stream ecosystems is greatest in small streams. ~ Typically, riparian replanting is best conducted in the upland streams of a catchment, particularly those orientated east-west, as this will have a flow-on effect for temperature in the lower reaches. However, for cooler-water refugia in large rivers, replanting tributaries close to the confluence can have considerable benefits for native fish. ~ Stream shade has three components — macrotopographic shade (provided by nearby hills), bank shade and vegetation shade. Any restoration activities need to recognise the differential effects of these components.

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