316 



ECOLOGICAL FACTORS: 



life includes bacteria (Schizophyta), protozoans 

 except sporozoans (Protozoa), fresh-water sponges 

 (Porifera), free-living flatworms (Turbellaria), rib- 

 bonworms (Nemertea), roundworms (Nematoda), 

 rotifers (Rotatoria), gastrotrichs (Gastrotricha), moss 

 animals (Ectoprocta), snails (Gastropoda), clams 

 (Pelecypoda), earthworm relatives (Oligochaeta), 

 leeches (Hirudinea), water bears (Tardigrada), 

 mites (Acarina), fly and other insect larvae (In- 

 secta), water fleas (Cladocera), ostracods (Ostra- 

 coda), copepods (Copepoda), and scuds, crayfish, 

 and other malacostracans (Malacostraca). 



Periphyton might be eliminated as a category of 

 fresh-water organism types. In many cases this 

 group consists of the overflow from benthic life. 

 However, there often is some tendency for encrusting 

 forms such as sponges and colonial ectoprocts to 

 reach their greatest concentration here. 



Free-floating life, plankton, may contain the 

 greatest concentration of plants, the suspended hydro- 

 phytes, in any body of water. In fact, plankton may 

 contain the greatest mass of life in an aquatic habi- 

 tat; however, much of the life is microscopic or 

 nearly so. Among the most conspicuous fresh-water 

 organisms are blue-green algae (Cyanophyta), green 



algae (Chlorophyta), and diatoms (Chrysophyta). 

 Bacteria and true fungi often are numerous. Plank- 

 tonic protists and animals are represented by the 

 protozoans except sporozoa, reproductive structures 

 of sponges (gemmules), rare hydromedusae or hy- 

 droid jellyfish (Trachylina), rotifers, gastrotrichs, 

 reproductive structures of bryozoans (statoblasts), 

 mites, fly larvae, water fleas, ostracods, and copepods. 



Nekton are most obviously fish and shrimp. How- 

 ever, members of all the previous groups may swim 

 in a body of water. In addition, certain terrestrial 

 or aquatic margin species occasionally become part 

 of the nekton or other groups already mentioned. 

 These latter animals include salamanders, frogs, 

 snakes, turtles, loons, mergansers, pelicans, cor- 

 morants, terns, gulls, ospreys, bald eagles, swallows, 

 other so-called marsh birds, muskrat, mink, and 

 otter. 



Some neuston previously were mentioned, the float- 

 ing hydrophytes. 



LIGHT ZONES 



Light penetration is used to designate certain 

 habitats within standing waters (Figure 17.9). The 



I SUNLIGHT I 









7 Mm 





Figure 17.9 The major horizontal and vertical life zones of a lake. (From George K. Reid, Ecology 

 of Inland Wafers and Estuaries, Reinhold Publishing Corp., New York, 1961) 



