under ideal conditions, green algae and most diatoms seldom, if ever, 

 form a bulky phytoplankton which can compare with the enormous 

 and overwhehning blue-green algal blooms frequently encountered 

 in hard water lakes, for their rate of reproduction is lower. Further- 

 more, green algae do not form sticky, floating mats of vegetation as 

 do many of the cyanophycean species. Hence, the quantity of the 

 algal vegetation is related to the kinds of plants that become estab- 

 lished in a lake. 



It is well known that some oligotrophic lakes are practically bar- 

 ren of a microflora, as well as being almost entirely devoid of higher 

 aquatic plants. See Table 1, which shows a comparison of the nitro- 

 gen content and the quality of the plankton. 



WATER BLOOMS 



Serious economic problems and drastically unbalanced biological 

 conditions frequently arise in sluggish streams and in lakes which 

 are physico-chemically constituted to support a luxuriant water 

 bloom. This is particularly true in southern Michigan and Wisconsin, 

 as previously mentioned, where a body of water may become over- 

 grown with cyanophycean species, many of which have the habit of 

 floating high in the water. Their tremendous numbers cause floating 

 crusts and scums to form, wherein plants die quickly and disinte- 

 grate in the intense sunlight. The living plants and the increased 

 bacterial flora resulting from their decay after death deplete the oxy- 

 gen below the point required for fish (2-3 ppm) and other animals. 

 In my study of Iowa lakes I observed similar abundant blue-green 

 floras. After a few hot days and nights in summer, when the oxygen 

 was low in any case, a climax situation developed. The dissolved 

 oxygen dropped to zero around midnight, with the result that within 

 a few hours not a single living animal could be found in a lake with 

 such a water bloom. Even bottom organisms, adapted to low oxygen 

 supply, were killed, and there was a mass of dead Chironomus lar- 

 vae and microcrustacea near the shore and in the shallows of bays. 

 After such a climax, dead fish appeared vdthin a few days, first float- 

 ing at the surface and then collecting in heaps along the beaches. 

 Thus enormous loss in game fish was sustained as a result of super- 

 abundant growth of blue-green algae. 



I have some evidence that fish may be killed also by poisonous 

 substances, such as hydroxylamine, produced from the decay of pro- 

 teins with which blue-green algae are abundantly supplied. Fitch 

 et al. ( 1934 ) have described the poisoning of domestic animals by 

 toxic substances produced by certain species of these plants. 



[44] 



