660 Ecology 



FRESH-WATER COMMUNITIES 



Located within most of the major communities are the numerous 

 fresh-water or aquatic communities. Though these are distinct from the 

 surrounding biomes, even possessing their own coactions and reactions, 

 they are affected by their terrestrial surroundings. 



Two general types of fresh-water communities are recognizable : 

 the standing-water and the flowing-water ones. Among the standing- 

 water communities there are the lakes, ponds, bogs, and temporary pools. 

 The flowing-water communities are greatly varied as to size and swift- 

 ness of flow. 



In studying any of these communities certain habitats with their 

 peculiar fauna are recognizable. The bottom supports an assemblage of 

 organisms collectively known as the benthos; this includes many bottom 

 protozoans, crustaceans, rotifers, gastrotrichs, snails, clams, worms, and 

 many others. When rocks are present, additional habitats are available 

 for sessile invertebrates. The upper region of the water also has its 

 characteristic fauna, the plankton, which consists of numerous small 

 organisms with either no ability to swim or at best a very limited ability. 

 In the fresh-water plankton are numerous crustaceans such as the cope- 

 pods, cladocerans, and ostracods ; rotifers, protozoans, and large num- 

 bers of unicellular algae are also found in this important region. Last 

 there is that assemblage of organisms capable of strong swimming ; thus 

 they are not at the mercy of the wind and waves. These are the fish, 

 frogs, and large aquatic insects which collectively form the nekton. 



These fresh-water communities are of great importance as sources 

 of drinking water, areas for recreation, and for the production of fish 

 for food. As a result, a special science, limnology, is devoted to their 

 study. 



Standing-Water Communities. — Of all the numerous types of 

 standing-water communities, only the large lakes such as the Great 

 Lakes may be considered climax. All smaller bodies of water are ac- 

 tually only initial stages in the sere leading to terrestrial climaxes. 

 Unlike the major terrestrial communities, each fresh-water community 

 is a distinct entity, that is, it is not part of a more extensive biome. 

 Thus each lake may be regarded as a single microcosm ; in fact, the fam- 

 ous scientist, Forbes, wrote a classic paper entitled, "The Lake as a 

 Microcosm" in which he described in detail the dynamics of a typical 

 lake community. 



