18 



PROBLEMS OF LAKE BIOLOGY 



ments of lake organisms have been stndied 

 botli in laboratory and in the field. As 

 Welch (1935) points ont, most aquatic or- 

 ganisms appear to have a high tolerance for 

 excess oxygen. At the other extreme there 

 is a considerable group of bottom animals 

 which survive for long periods in water 

 containing no oxygen. Other organisms, for 

 example, fish, are affected adversely by 

 rather low concentrations. In this connec- 

 tion we should recall Ruttner's (Brehm and 

 Ruttner 1926) effort to recognize the effect 

 of temperature on the oxygen requirements 

 of aquatic organisms by calculating "re- 

 spiratory values." Ricker (1934) men- 

 tions both the value and limitations of this 

 procedure. 



CARBON DIOXIDE 



Carbon dioxide occupies an important 

 place in the transfer of materials that we 

 describe as the metabolism of a lake. It is 

 an essential substance for photosynthesis 

 and thus stands at the threshold of all pro- 

 duction. The forms and quantities in 

 which it occurs and its combinations with 

 calcium have been the subject of much 

 study. Pia (1933) provides a recent and 

 rather complete discussion and various 

 authors including Juday et ul. (1935) have 

 published the results of extensive field 

 studies. 



The free carbon dioxide in the surface 

 water of a lake varies rather widely accord- 

 ing to photosynthetic activity of its phyto- 

 plankton. If the lake is thermally strati- 

 fied there is usually a considerable increase 

 in the hypolimnion. Bound carbon dioxide 

 is usually more constant with only a slight 

 increase near the bottom. The phytoplank- 

 ton is able to use in photosynthesis both the 

 free carbon dioxide, the "half -bound" part 

 of the bicarbonates, and even some of the 

 monocarbonates if necessary. "When the 

 half-bound carbon dioxide is used the mono- 

 carbonate may be precipitated and sinks to 

 the bottom or it may be redissolved in the 

 hypolimnion if there is free carbon dioxide 

 present in that region. Since carbon di- 

 oxide is used in the assimilation of bacteria 

 as well as in green plants and since it is pro- 

 duced by animal respiration, we should re- 



gard the amount present as a "balance on 

 hand" depending on the various rates of 

 transfer. 



Apparently the total carbon dioxide 

 available for photosynthesis is seldom low 

 enough to make it a limiting factor for pro- 

 duction of phytoplankton. That it may 

 have some limiting effect is suggested by 

 evidence from the work of Birge and Juday 

 (1911) that phytoplankton is unusually 

 poor in soft water lakes of Wisconsin. In 

 lakes of northeastern Indiana, Scott (1931) 

 reported a correlation between the quanti- 

 ties of carbonates and numbers of diatoms 

 but not with numbers of blue-green algae. 

 Hile (1936) suggests that the amount of 

 bound carbon dioxide in lakes of northeast- 

 ern Wisconsin was directly correlated with 

 the density of ciseo populations and in- 

 versely with their growth in weight. He 

 suggests that the amount of bound carbon 

 dioxide is roughly indicative of the produc- 

 tive capacity of a lake but his data provide 

 little support for this assumption. It is 

 noteworthy that he finds the amount of sur- 

 face plankton not related to production of 

 ciscoes and cisco growth poorest in the most 

 eutrophic lake. 



The presence of large amounts of free 

 carbon dioxide may be a direct factor in 

 limiting the distribution of aquatic animals. 

 As in the ease of oxygen, increased carbon 

 dioxide is accompanied by changes in other 

 factors so that the specific effects are diffi- 

 cult to establish. It seems probable that 

 some organisms show a negative reaction to 

 increased carbon dioxide even though the 

 amounts are by no means toxic, a condition 

 which rarely occurs in nature. Powers 

 (1934), as cited above, has investigated the 

 toxic effects of carbon dioxide on fishes and 

 suggests that sudden increases have a pro- 

 nounced effect on the respiration of fishes. 

 More recently Powers (1938) and also Fry 

 and Black (1938) have shown how carbon 

 dioxide affects the level to which oxygen 

 can be removed from the water by fish. 



HYDROGEN ION CONCENTRATION 



The trend of opinion in recent years sug- 

 gests that the pH of the aquatic environ- 

 ment is of less importance as a limiting 



