RELATIONSHIP OF PIIYTOPLANKTON TO LIMNOLOGY 



69 



Carbon Dioxide and Electrolytes 



We can, however, see causal relationships 

 between some limnological features and the 

 nature of tlie micro-flora. For instance, it 

 is well known and an unfailing correlation 

 that great productivity of phytoplankton is 

 provided by a high carbonate content and 

 a carbon dioxide reserve, such as is found 

 in the eutrophic type of lake (Table 1). 

 This is easily understood, for we know the 

 direct dependence that chlorophyll-bearing 

 organisms have on carbon dioxide. Just 

 what the connection is, we do not know, but 

 it is of some significance that bodies of 

 water in which there is an abundance of 

 carbon dioxide, particularly in tropic 

 waters where there is low oxygen content, 

 have an algal flora made up predominantly 

 of filamentous and branched plants, while 

 unicellular and simple colony formers are 

 typical of well-aerated waters with a low 

 carbon dioxide content. 



A rich algal flora, particularly if it is the 

 Myxophycean type, is a visible index of an 

 abundance of electrolytes. In Wisconsin, 

 and elsewhere in the middle west, Gloeo- 

 trichia echinulafa does not occur in every 

 hard water lake in quantities easily visible 

 to the unaided eye. Yet, when it does occur, 

 the relative abundance is evidence that the 

 lake is correspondingly rich in carbonates, 

 and that the conductivity is high. 



Also, it is known that there is a definite 

 relationship between calcium and magne- 

 sium content of lakes and conductance. 

 Seasonal variation in conductivity accom- 

 panies fluctuations in pH and increases in 

 phytoplankton quantity. When the pH is 

 low, conductivity is found to be high, and 

 phytoplankton Ioav (Fig. 1). When phyto- 

 plankton increases during the seasonal 

 growth, the conductivity becomes less be- 

 cause of a consumption of electrolytic salts, 

 and the pH rises with the precipitation of 

 carbonates. In certain loAva lakes the pH 

 may rise from 7.4 to 9.8 during the summer 

 season. 



The ratio of sodium and potassium to cal- 

 cium and magnesium has an important 

 bearing on plankton composition, perhaps 

 more important than is now realized. At 



least we find that lakes with a low ratio of 

 alkali to calcium have a diatom and Myxo- 

 phycean flora, while a high ratio supports 

 a (lesmid and a more nearly Chlorophycean 

 population. For example, the silted lakes 

 of England were found to have a ratio of 

 1.1. Rocky lakes with a higher Na-K/Ca- 

 Mg ratio (3.2) have a Chlorophycean flora, 

 particularly rich in desmids as is character- 

 istic of the oligotrophic type of lake. 



As Pearsall (1922) and others have re- 

 marked, the plankton mass is not so much 

 proportional to the total quantity of dis- 

 solved substances as it is to the smallest 

 quantity of limiting ion present in relation 

 to the minimuyn amount required. This 

 calls to mind the fact that certain elements 

 do not need to be present in large quantities 

 in order to provide for the growth of phyto- 

 plankton species, but that if they are not 

 present in required minimum amounts they 

 may be important determiners in the phyto- 

 plankton flora. Herein lies a great oppor- 

 tunity for laboratory research. 



Phosphorus 



In respect to the importance of })hos- 

 phorus as a determining factor in phy- 

 toplankton composition there are some con- 

 flicting opinions and evidences. Some lim- 

 nologists contend that phosphorus has no 

 important part to play in determining phy- 

 toplankton productivity and quality. On 

 the other hand, analyses of phosphorus 

 in both marine and fresh water (particu- 

 larly those made by British limnologists) 

 offer convincing evidence that phosphorus 

 is absolutely essential and is, therefore, a 

 determining element. 



Brandt, Fisher, and Harvey have shown 

 phosphorus to be a limiting factor. Harvey 

 (1926) found that phosphorus and nitrogen 

 wei-e L'om]iIetely exhausted in the English 

 Channel with the increase of plankton dur- 

 ing the summer season. Wiebe (1930), 

 reporting on the results of tlie addition of 

 fertilizers to certain bodies of water, states 

 that there is some evidence that phosphorus 

 is a limiting factor in plankton production. 

 Here, although the amount of plankton in- 

 creased after the addition of fertilizer, no 

 complete differentiation was made between 



