194 



ANALYSIS OF THE ENVIRONMENT 



a relatively high oxygen concentration re- 

 mains in tlie epiUmnion. In the autumnal 

 overturn, hypolimnial oxygen is replaced, 

 but this vital gas may be again depleted 

 during winter stagnation under ice. Sum- 

 mer depletion of dissolved oxygen in the hy- 

 polimnion is of greatest significance in lake 

 metabohsm (Rawson, 1939), and the hy- 

 polimnial deficit may well be expressed in 

 terms of area (Strom, 1931). Hypolimnial 

 deficits may then be calculated, combining 

 area and time factors (Hutchinson, 1938). 



The lack of oxygen keeps many animals 

 away from the hypolimnion, and yet this 

 region is not devoid of life even in summer; 

 in fact, a large number of organisms may 

 be present. Fishes from the epiUmnion 

 "dive" through the thermocline to feed and 

 ascend to oxygenated water as a diver rises 

 to air (Pearse, 1920). Chaoborus {=Core- 

 thra) larvae, phantom-like in their transpar- 

 ency, migrate to the epiUmnion at night and 

 back into the deeper hypolimnion during 

 the day, a type of depth migration wide- 

 spread in waters that are almost uniform in 

 oxygen and temperature relations. These 

 larvae have air sacs that probably help 

 them to live in oxygenless water below the 

 thermocUne. Other forms live constantly in 

 the profundal zone. The source of oxygen 

 for such animals in the stagnant waters of 

 the hypolimnion is unknown. Suggestions 

 include: (1) oxygen storage, but this is in- 

 adequate for animals that do not invade 

 oxygenated waters during the entire sum- 

 mer; (2) anaerobic respiration by a spUt- 

 ting of oxygen-rich carbohydrates; and (3) 

 the use of atomic oxygen from decaying 

 tissues (Welch, 1935). Facultative anaer- 

 obes are known among animals; some pro- 

 tozoans, nematodes, mollusks, and even 

 fishes have this abiUty to a more or less 

 Umited extent. Some animals can survive 

 the abijence of oxygen for several days, and 

 the more resistant may Uve for much 

 longer periods, even though they carry on 

 many activities. 



In extreme stagnation, with low oxygen 

 and high carbon dioxide concentration in 

 the hypoUmnion, many of the usual deep- 

 water residents move out (trout) or at 

 least show a decrease in population size 

 (Oligochaeta, chironomid larvae, and fin- 

 gernail clams). This was demonstrated for 

 Lake Pinantan (British Colximbia) by Raw- 

 son (1934). He suggests that hydrogen 

 sulfide present in a gradient from rela- 



tively high concentration in the terraqueous 

 bottom upward through the hypoUmnion is 

 an additional Umiting factor in this biologi- 

 cal desert. 



Three types of relationship are known 

 between the rate of respiiation of aquatic 

 animals and the oxygen content of the sur- 

 rounding water. Some are nearly inde- 

 pendent of the oxygen tension over a wide 

 range; paramecium is an example, and 

 many fishes show this relationship. Others 

 use oxygen at a constant rate over an inter- 

 mediate range, with the rate of use increas- 

 ing above and decreasing below that range. 

 Many, perhaps most, aquatic animals that 

 use dissolved oxygen belong in this cate- 

 gory; Dugesia dorotocephala, a flatworm, 

 is a well-studied example (Hyman 1929). 

 The fishes that we have tested maintain a 

 constant rate of oxygen use in ordinary in- 

 termediate tensions and show a decided 

 reduction in rate below 1.5 to 2.0 cc./L. 

 The third type has a rate of oxygen con- 

 sumption that is highly dependent on the 

 oxygen content of the surrounding medium; 

 the echinoderms, Patira and Strongylocen- 

 trotus, and the common lobster, Homarus, 

 are examples (Hyman, 1929). 



The clam worm. Nereis virens, provides 

 an interesting example, not alone of the 

 point under discussion, but also of the ne- 

 cessity for approximating normal conditions 

 in laboratory tests, if natural reactions are 

 to be observed. When Nereis is placed in a 

 clean bare flask under excellent conditions 

 for a standard laboratory determination of 

 oxygen consumption, the rate is closely af- 

 fected by the amount of dissolved oxygen 

 in the water throughout the normal range 

 of its concentration. When allowed to crawl 

 into a glass tube, a rough approximation of 

 their normal tube-dwelling existence in 

 nature, these worms respond as does the 

 second type of animals and are able to keep 

 constant their rate of oxygen use down to 3 

 or 4 cc./L, the lowest tension tested (Hy- 

 man, 1932). 



RESPIRATION FROM GAS BUBBLES 



A number of aquatic insects, including 

 corixid hemipterans and dytiscid and hydro- 

 phyUd beetles, are air-breathing, although 

 they spend much of their life surrounded by 

 water. They obtain bubbles of air at the 

 surface and have special adaptations for 

 carrying them down as they dive. The bub- 

 bles are used as a direct source of oxygen 



