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2 FRESHWATER HABITATS OF ALGAE

The study of freshwater algae is really the study of organisms from many diverse habitats, some of which are not entirely “fresh.” Although the oceans are clearly saline ( 35 g salts L) and most lakes are relatively dilute (world average < 0.1 g L; Wetzel, 1983a), there is enormous variation in the chemical composition of the nonmarine habitats that algae occupy. Conditions in lakes and rivers vary not only in salinity, but also in size, depth, transparency, nutrient conditions, pH, pollution, and many other important factors. Aquatic ecologists also use the term “inland” waters to encompass a greater range of aquatic ecosystems. Even this term may be unsatisfactory, because algae occupy many other habitats, such as snow, soils, cave walls, and symbiotic associations (Round, 1981). Organisms grouped together in this volume as freshwater algae fall into a large, but ecologically meaningful collection of environments: all habitats that are at least slightly wet, other than oceans and estuaries. One reason for such a broad scope is that inland saline lakes, snow and ice, damp soils, and wetlands are studied by phycologists and ecologists who also examine more traditional freshwater environments. Some genera with terrestrial species, such as Vaucheria, Nostoc, Chlorella, and Prasiola, also have species found principally in streams or lakes (Smith, 1950; Whitton, 1975). In North America, the variety of freshwater habitats colonized by algae is very rich, and offers an enormous and fascinating range of environments for their study. The distinction between marine and freshwater habitats is revealed in the variety of algae that occur in these environments. There are no exclusively freshwater divisions of algae, but certain groups exhibit