72 



PROBLEMS OF LAKE BIOLOGY 



Macro-flora 



Notwithstanding some conclusions to the 

 contrary, there is overwhelming' evidence 

 that open water lakes which support a rich 

 macro-flora also maintain high phytoplank- 

 ton productivity (Fig. 1). We have excel- 

 lent examples of this in lakes of Midwest 

 North America and New England. The 

 work of Rickett (1922) and Wilson (1935, 

 1937) have given us a mathematical picture 

 of macro-flora production. In the lakes 

 studied, Sweeney Lake for example, I have 

 found a very direct correlation between the 

 large amount of vegetation per square 

 meter, as recorded by Wilson, and the 

 amount of phytoplankton together with the 

 number of component species. This lake 

 showed a greater variety of phytoplankton 

 than any other lake I have ever surveyed, 

 and many of the species were represented 

 by enormous numbers per liter. That there 

 should be such a correlation is only to be 

 expected since the nutrients needed for 

 growth of large aquatics are practically the 

 same as those required by the micro-flora. 



Disregarding wave action and other dis- 

 turbing influences, the nature and chem- 

 istry of the bottom sediment determine the 

 type of vegetation. Also, the type of bot- 

 tom and the surrounding geological features 

 determine the chemistry of the water, and 

 we think we can see many ways in which 

 the chemistry of the water, in turn, de- 

 termines the nature of the phytoplankton. 

 It follows that a lake with an abundance of 

 organic material on the bottom ("Gyttja" 

 of Naumann, 1917), that is the eutrophic 

 lake with shallows, has a rich macro-flora 

 and, as has been pointed out, the eutrophic 

 lake is the type which usually possesses a 

 correspondingly rich phytoplankton. Aside 

 from the fact that the eutrophic lake with 

 its beds of vegetations and high percentage 

 of shallow water is physically suitable for 

 a large population of fish, it is also the type 

 of lake which has the greatest quality and 

 quantity of food. It has been suggested 

 that large aquatics, rather than phyto- 

 plankton, be used as indices as to the ade- 

 quacy of a lake in fish-stocking programs. 



I do not know how much actual evidence 



there is, but the statement has been made, 

 and it seems possible, that an abundance 

 of rooted vegetation has, in itself, a very 

 direct determining relationship to phyto- 

 plankton production. This relationship, 

 supposedly, is brought about through the 

 nutrients of tlie bottom being carried into 

 lake circulation by the absorbing action of 

 roots. When plant parts are thrown off, 

 or when the plant as a whole dies and de- 

 cays in the upper levels, nutrients are re- 

 leased for the use of plankters. 



Fish Food 



In their relation to the food chain of 

 aquatic animals, phytoplankters have im- 

 portant roles in the biology of a lake. First, 

 they are used directly by bottom fauna and 

 by some fish, especially in the fingerling 

 stages. It is known that the gizzard shad, 

 the blunt-nosed minnow, and some other 

 species feed on phytoplankton almost to 

 the exclusion of other forms of food. 

 Kraatz (1928) reports that young suckers, 

 shiners, yellow perch, and wall-eyed pike 

 feed on algae and other plankton. In my 

 own experimental observations to de- 

 termine the character of food of lake fish, 

 I have found the gizzard shad gorged with 

 Aphanizomenon flos-aquae and other Myxo- 

 phyceae, golden shiner less so, while black 

 bass fingerlings and sun fish showed a mix- 

 ture of blue-green algae and micro-fauna 

 in the intestinal tract. 



Great importance is usually attributed 

 to phytoplankton as the basic food supply 

 of micro-fauna and other fish food organ- 

 isms. The convincing studies of Bond 

 (1933) on the feeding of the zooplankter, 

 Artemia sp. and other similar studies, sup- 

 port the long held and rather general belief 

 that phytoplankton, as such, constitutes the 

 so-called "pasturage" for aquatic animals, 

 both marine and fresh water. In this con- 

 nection we should cite the work of Martin 

 (1922, 1923, and 1928) on the food of the 

 oyster. One of Bond's summarizing re- 

 marks is : "It is perfectly obvious that zoo- 

 plankts generally do ingest a considerable 

 quantity of particidate matter, and that a 

 large proportion of the ingested material is 



