Solar radiation is vital to the metabolism, indeed to the very existence, of freshwater ecosystems. Nearly all energy that drives and controls the metabolism of lakes and streams is derived from solar energy, which is converted biochemically via photosynthesis to potential chemical energy. The photosynthetic synthesis of organic matter occurs within the lake or river (autochthonous) or within the terrestrial drainage basin (allochthonous) and is transported to the aquatic ecosystem in various forms of dissolved and particulate organic matter by “vehicles” for movement [e.g., air, water, animals; cf., Likens and Bormann (1972)]. In addition to direct biological utilization, the absorption of solar energy and its dissipation as heat markedly affect the thermal structure and stratification of water masses and circulation patterns of lakes, reservoirs, and streams. These characteristics in turn have profound effects on nutrient cycling and the distribution of dissolved gases and the biota. The optical properties of fresh waters, therefore, exert important regulatory controls on the physiology and behavior of aquatic organisms.
Light impinging on the urface of water does not penetrate completelyya significant portion is reflected and backscattered [cf., Wetzel (1983) and Exercise 4, p. 45]. Within the water, light is rapidly attenuated with increasing depth by both absorption and scattering mechanisms. Absorption is defined as diminution of light energy with increasing depth by transformation to heat [cf., Westlake (1965)]. Absorption is influenced by the molecular structure of water itself, by particles suspended in the water, and particularly by dissolved organic compounds. The result is a selective absorption and attenuation of light energy with increase in depth, influenced by an array of physical, chemical, and, under certain conditions, biotic properties of the water. These optical properties are dynamic, changing seasonally and over geological time for individual freshwater ecosystems.
Over half of the solar radiation that penetrates into water is absorbed and dissipated as heat. As we will see in subsequent analyses (Exercises 3 and 4), the distribution of this heat is influenced greatly by wind energy. In this exercise, we will evaluate methods for the measurement and description of the distribution of light and temperature in water.
KeywordsPhotosynthetically Active Radiation Secchi Disc Surface Irradiance Total Irradiance Secchi Disc Transparency
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