The amount of oxygen released by plants varies 

 with their abundance and time of day ; photosynthesis 

 can take place in light. Phytoplankton and the 

 rooted vegetation restricted to the shore line are im- 

 portant sources of oxygen to the water. With rapid 

 photosynthesis in relatively small volumes of water, 

 the water may be supersaturated with oxygen for 

 short periods of time. 



The oxygen supply of lakes is reduced in various 

 ways; most notably through the respiration of ani- 

 mals and plants and the decomposition of organic 

 matter. As lake waters warm up during the summer, 

 their capacity to hold oxygen is reduced and oxygen 

 may be released into the atmosphere. The saturation 

 capacity of water at 0°C is 10.2 cc per liter, but at 

 25°C it is only 5.8 cc per liter. In some lakes, decom- 

 position of organic material at the bottom may deplete 

 the hypolimnion of its oxygen content for several 

 weeks during the summer ; perhaps lower than the 

 level minimal to the support of life. This is called the 

 summer stagnation period. During the winter, if the 

 lake is covered with ice and snow, there may be a zvin- 

 ter stagnation period. The oxygen supply of the deep 

 waters is renewed with the autumn and spring over- 

 turns. Before decomposition can proceed very far 

 there must be calcium in the water. Hence, decom- 

 position is slow in soft or acid waters. 



At temperatures of 15°-26°C oxygen concentra- 

 tions of less than 2.4 cc/1 (3.5 ppm) are fatal within 

 24 hours to several species of fish. From 0°-4°C, 

 oxygen concentrations can decline through 48 hours 

 to 1.4 cc/1 (2.0 ppm), or even to 0.7 cc/1 liter (1.0 

 ppm), before the same mortality results (Moore 

 1942). Some planktonic invertebrates can tolerate 

 oxygen concentrations as low as 0.2 cc/1 (0.3 ppm) 

 and, for short periods, even 0.1 cc/1 (0.1 ppm). 

 Some bottom-dwelling protozoans, annelids, mol- 

 lusks, and insect larvae may survive actual anaerobic 

 conditions for periods of days, even weeks. Organ- 

 isms that tolerate a lack of oxygen do so by creating 

 an oxygen debt ; that is, the lactic acid and other 

 breakdown products produced in consequence of 

 muscular activity simply accumulate until conditions 

 permit oxidation of them. In true anaerobes these 

 acid waste products are eliminated from the body ; 

 no oxidation debt is established. 



European workers, principally Thienemann and 

 Naumann, have devised a classification of lake habi- 

 tats into three main categories on the basis of fertil- 

 ity and the amount of oxygen in the hypolimnion 

 during the summer concentration. The oxygen con- 

 centration in the hypolimnion is, of course, a reflec- 

 tion of the fertility of the lake, since it is inversely 

 proportional to the amount of decaying organic mat- 

 ter. Dystrophic lakes contain considerable organic 

 matter but are infertile because the organic matter 



does not completely decompose and there is release 

 of organic acids. 



Oligotrophic lakes are usually deep (over 18 

 meters) with very little shallow water, and little veg- 

 etation around margins. Bottom contours are V- 

 shaped ; they are low in fertility, rich in oxygen in 

 the hypolimnion (orthograde distribution), low in 

 CO2, and the color of the water varies from blue to 

 green. The volume of the epilimnion is usually less 

 than the volume of the hypolimnion. The fish popu- 

 lation is not large. Characteristic species are lake 

 trout, whitefish, and cisco. The midge fly larva, 

 Tanytarsus. predominates. Plankton is not abundant. 

 The Finger Lakes of New York are of this type. 



Entrophic lakes are usually less than 18 meters 

 deep, the bottom contour is U-shaped, water color 

 varies from green to yellow or brownish green, and 

 there are larger areas of shallow waters and more 

 marsh vegetation. Fertility is high, and because of 

 rich bottom humus the oxygen content of the hy- 

 polimnion is greatly reduced during the summer 

 {clinograde distribution). The COo content is ac- 

 cordingly high. The volume of the epilimnion is 

 usually greater than that of the hypolimnion. Plank- 

 ton is abundant. The midge fly larva Tendipes is 

 very numerous and the culicid larvae Chaobonis is 

 usually present. The bottom fauna is rich, and there 

 is a large fish population in the epilimnion. Charac- 

 teristic fish species are the largemouth bass, perch, 

 sunfish, and pike. These lakes occur in relatively 

 mature river systems ; many lakes in Minnesota and 

 Wisconsin are of this type. 



Dystrophic lakes are bog-like, very rich in mar- 

 ginal vegetation and organic content. Oxygen is 

 likely to be scarce at all depths. The water is usually 

 conspicuously colored, yellow to brown, and may be 

 acidic because of organic acids and incompletely oxi- 

 dized decomposition products. Plankton, bottom or- 

 ganisms, and fish are usually scarce, but blue-green 

 algae are sometimes abundant. Tendipes may pre- 

 dominate among the bottom forms, but at times only 

 Chaoborus is present. Characteristic fish are stickle- 

 backs and mud minnows. Many lakes of northern 

 latitudes are dystrophic in type. 



All gradations exist between these three types of 

 lakes, and individual lakes are often difficult to 

 classify. Oligotrophy is indicated if the loss of oxy- 

 gen in the hypolimnion during the summer is not 

 over 0.025 mg/cm-/day ; eutrophy. if it is over 0.055 ; 

 mesotrophy, if it is between the two (Hutchinson 

 1957). A lake may change from one type to another 

 as succession proceeds (Lindeman 1942). Probably 

 all lakes start as oligotrophic, but as they accumulate 

 vegetation and decaying organic matter, they change 

 into eutrophic lakes ; or, if the organic matter does 

 not completely decompose, into dystrophic lakes. 



64 Habitats, communities, succession 



