Light and Temperature

Abstract

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.