74 



PROBLEMS OF LAKE BIOLOGY 



of aquatic animals, mention should be made 

 of the work of Welch (1916, 1924) and 

 Frohne (1938) in Michigan who have shown 

 tlie dependence of many fish-food organisms 

 on higher aquatic plants. 



When anah^zed, tlie problem seems to me 

 to be one for Avliicli insufficient data are 

 collected to establish a correct idea as to 

 the dependence of animals on phytoplank- 

 ton. Weighing the various lines of evi- 

 dence, it would seem desirable to answer 

 the question of whether micro-fauna use 

 phytoplankton and particulate matter for 

 food, or dissolved organic substances, by 

 reducing the problem to one of determining 

 which species of animals tend to feed on 

 particulate matter (perhaps nannoplank- 

 ton alone), which use dissolved matter, and 

 which use both particulate and dissolved 

 matter. In other words, generalizations as 

 to the use or disuse of phytoplankton in the 

 food chain cannot be drawn from present 

 data, and those which we now hold should 

 be set aside, at least temj)orarily. 



Unbalanced Biological Conditions 



Phytoplankton species which enter into 

 ''waterbloom" associations thrive very suc- 

 cessfully in warm water (25° to 30° C), 

 particularly if there is an abundance of 

 carbon dioxide in reserve and an adequate 

 sui^ply of nitrogen compounds. Under 

 these optimum conditions their excessive 

 growth may come to interfere in the biology 

 of a lake to a serious degree and oftentimes 

 they are, either directly or indirectly, the 

 cause of a great deal of economic loss and 

 many objectionable conditions. 



The greatest pests are Myxophycean spe- 

 cies, especially those species which multiply 

 rapidly, float high in the water (possibly 

 because of the pseudo-vacuoles which these 

 plants contain), and which i)ossess sticky 

 mucilaginous sheaths or colonial envelopes. 

 Aphanizomenon flos-aquae and Microcystis 

 aeruginosa are two such species which very 

 frequently enter into destructive water- 

 bloom associations in this country. In Lake 

 East Okoboji, in Iowa, these plants periodi- 

 cally form dense, "soupy" masses of vege- 

 tation and thick, floating scums which decay 

 rapidly in the intense light and heat of mid- 



day. Spirit Lake, nearby, also has dense 

 growths of blue-green algae during the 

 summer period, but here the flora is made 

 up of species which do not float high in the 

 water but remain suspended throughout 

 and which do not possess sticky sheaths, 

 such as Lynghya Birgei, for example. Con- 

 sequently this lake is spared the climax 

 conditions which are very disturbing in 

 Lake East Okoboji. 



One of the results of these overwhelming 

 growths of phytoplankton is the death of 

 fish, produced either directly or indirectly. 

 Occurring, as they do, in the warm seasons 

 when the ox.ygen content is low in any case, 

 the enormous numbers of plants withdraw 

 more oxygen from the water than they 

 return through photosynthesis. Myxo- 

 phyceae as a class are poor oxygenators. 

 During warm, still nights (with oxygen 

 release by photosynthesis interrupted) they 

 may entirely deplete the oxygen supply or 

 reduce it to an amount which will not sup- 

 port fish life. In these lakes the oxygen may 

 be decreased from 4.5 to 5.5 ppm to 1.5 or 

 2.0 ppm within a day or a few hours. The 

 exhaustion of the oxygen brings about the 

 death of both micro-fauna and phytoplank- 

 ton and the decay by bacteria of this mass 

 of organic matter quickly reduces further 

 the oxygen content and then fish and other 

 aquatic animals are suffocated. It is not 

 uncommon in the Middle West of North 

 America to find small lakes or bays of 

 larger lakes with scarcely a living creature 

 after an excessive growth of algae has 

 broken up and decayed. The writer has 

 seen the shore line whitened with windrows 

 of tens of thousands of fish destroyed in this 

 manner. All fish organisms are found to 

 be dead, even the bottom-living Chirono- 

 mous larvae, and the lake becomes a veri- 

 table graveyard and "desert." Hence eco- 

 nomic loss of game fish is considerable and 

 there are drastic effects on the general biol- 

 ogy of the lake. 



Fish and game agencies undertake to in- 

 crease, by various means, the oxygen con- 

 tent of such infested lakes, often at great 

 expense, in order to prevent the wholesale 

 death of fish. 



It is apparently possible for algae to 



