respire and produce carbon dioxide. The amount 

 of oxygen produced during active photosynthesis 

 is many times the amount of carbon dioxide re- 

 leased during the night or on cloudy days when 

 photosynthesis is inhibited or stopped. 



Limited concentrations of algae are not trouble- 

 some in surface waters; however, overproduction 

 of various species is considered undesirable for 

 many water uses. A relatively abundant growth of 

 planktonic algae in waters 3 feet or deeper will 

 shade the bottom muds sufficiently to prevent 

 germination of seeds and halt the growth of prac- 

 tically all rooted submersed and emersed aquatics, 

 thus removing an important source of food for 

 ducks and other water fowl. 



Some blue-green algae, many green algae, and 

 some diatoms produce odors and scums that make 

 waters less desirable for swimming. Dense growths 

 of such planktonic algae may limit photosynthetic 

 activity to a layer only a few inches beneath the 

 surface of the water. Under certain conditions, the 

 populations of algae may die and their decomposi- 

 tion will deplete dissolved oxygen in the entire 

 body of water. Certain sensitive people are allergic 

 to many species of planktonic algae blooming in 

 waters used for swimming. 



It is claimed that some species of algae cause 

 gastric disturbances in humans who consume such 

 infested waters. Several species of blue-green algae 

 produce, under certain conditions, toxic organic 

 substances that kill fish, birds, and domestic ani- 

 mals. Some of the genera that contain species 

 which may produce toxins are Anabaena, Ana- 

 cystis, Aphanizomenon, Coleolosphaerium, Gloeo- 

 trichia, Microcystis, Nodularia, and Nostoc. Some 

 species of Chlorella, a green alga, also are toxic. 



Various species of single, as well as branched 

 filamentous forms of algae, grow in both cool 

 and warm weather and when they become over- 

 abundant are generally considered to be a nui- 

 sance in whatever body of water they occur. Most 

 species of these algae are generally distributed over 

 the United States. 



Many forms of plankton and filamentous algae 

 clog sand filters in water treatment plants, produce 

 undesirable tastes and odors in drinking water, 

 and secrete oily substances that interfere with do- 

 mestic use and manufacturing processes. Some 

 algae cause water to foam during heating as well 

 as metal corrosion and the clogging of screens, 

 filters, and piping. Algae also coat cooling towers 

 and condensers causing these units to become in- 

 effective. In Lake Superior, complaints have been 

 made that diatoms such as Tabellaria, Synedra, 

 Cymbella, and Fmgilaria, and the chrysophyte, 

 Dinobryon, may be the cause of slimes on fishnets. 



Filamentous algae may interfere with the opera- 

 tion of irrigation systems by clogging ditches, 

 wires, and screens and thus seriously impede the 

 flow of water. Filamentous algae in ponds, lakes, 

 and reservoirs may cause depletion of naturally 

 occurring and added nutrients that could other- 

 wise be used to produce unicellular algae that are 

 more commonly used as food by fish. Dense 

 growths of filamentous algae may reduce the total 

 fish production and seriously interfere with har- 

 vesting the fish either by hook and line fishing, 

 seining, or draining. Such growths can also cause 

 overpopulation, resulting in stunting and the pres- 

 ence of large numbers of small fish. Under cer- 

 tain conditions, growths of filamentous algae on 

 pond or lake bottoms become so dense that they 

 eliminate spawning areas of fish and possibly inter- 

 fere with the production of invertebrate fish food. 



Submersed plants are those which produce all 

 or most of their vegetative growth beneath the 

 water surface. In many instances these plants have 

 an underwater leaf form, a totally different floating 

 or emersed leaf form, and flowers on an aerial 

 stalk. Abundant growth of these weeds is depend- 

 ent upon depth and turbidity of water, and sub- 

 stratum. For most submersed plants in clear water, 

 8 to 10 feet is the maximum depth for growth in 

 clear water as they must receive sufficient light 

 for photosynthesis when they are seedlings. Most 

 of these submersed aquatic plants appear capable 

 of absorbing nutrients as well as herbicides through 

 either their roots or vegetative parts. 



Emersed plants are rooted in bottom muds and 

 produce a majority of their leaves and flowers at 

 or above the water surface. Some species have 

 leaves that are flat and float entirely upon the 

 water surface. Other species have leaves that are 

 saucer-shaped or whose margins are irregular or 

 fluted. The latter types of leaves do not float 

 entirely upon the water surface. 



Marginal plants are probably the most widely 

 distributed of the rooted aquatic plants. Members 

 of this group are varied in size, shape, and prefer- 

 ence of habitat. Many species are adapted for 

 growth from moist soils into water up to 2 feet 

 deep or more. Other species are limited to moist 

 soil or entirely to a watery habitat. 



There are some species of floating plants that are 

 rather limited in their distribution while others 

 are widespread throughout the world. Plants in 

 this group have true roots and leaves, but instead 

 of being anchored in the soil they float about on 

 the water surface. Buoyancy of the plant is ac- 

 complished through modification of the leaf (in- 

 cluding covering of the leaf surface) and leaf 

 petiole. Most species have well-developed root 



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