76 



PROBLEMS OF LAKE BIOLOGY 



plankton just mentioned, these plants are 

 capable of ruining water supplies for do- 

 mestic and commercial use. When alive, 

 and more especially after tliej^ have de- 

 cayed, a number of species of algae impart 

 obnoxious tastes and disagreeable odors to 

 impounded waters which these plants find 

 very suitable for luxuriant growth. Also, 

 they seriously interfere with filtering sys- 

 tems and in these several respects bring 

 about economic loss. 



Problems 



In conclusion the writer wishes to outline 

 what seem to be some important problems 

 open for much desired research. 



1. We need to know more about the life 

 history of phytoplankton species; the fac- 

 tors which underly their unexplainable 

 pulses ; the causes for their periods of abun- 

 dance and decline. For many we do not 

 know in what form they pass their dormant 

 condition, if indeed they have one. We do 

 not know where they pass their dormant 

 condition : in shallow waters, on the bottom 

 in deep water, or suspended as plankters? 

 Information on some of these points will 

 throw light on the local source of phyto- 

 plankton. Also, a study of these problems 

 may help us to answer the question as to the 

 phylogenetic history of plankters. Did they 

 migrate from, or are they hold-overs from, 

 ancient seas with salinity much less than 

 now? Or, did the}" develop first in shallow 

 fresh water, warm and rich in nutrients, 

 and then migrate into open water as plank- 

 ters? On these points we can only offer 

 conjectures at present. 



2. Although we now possess much infor- 

 mation on both lake chemistry and phyto- 

 plankton composition, we need to seek out 

 the actual determining factors responsible 

 for phytoplankton distribution. We know 

 only a few of these factors. I am not en- 

 tirely in agreement with some students who 

 state that it is impossible to find phyto- 

 plankton associations in definite correlation 

 with lake types. To be sure each lake has 

 its own combination of determining factors. 

 Yet we find certain species or groups of spe- 

 cies occurring in similar or nearlv similar 



lakes. Where species seem to fail to serve 

 as indices of lake types or qualities of lakes 

 it may well be that the failure is entirely 

 due to our inability to recognize those char- 

 acteristic lake qualities of which these spe- 

 cies are representative. I believe we can 

 establish some species or groups of species 

 as indicators which will be as reliable 

 (within limits) as are terrestrial plants 

 wiien used as indicators of edaphic factors 

 or other environmental conditions. 



3. Another intriguing problem and one 

 which has far-reaching practical importance 

 has to do with the place of phytoplankton in 

 the food chain of aquatic animals. If it can 

 be established that micro-fauna feed pri- 

 marily on dissolved organic matter or col- 

 loidal substances, then our notions and our 

 textbooks will need revision. Furthermore, 

 if this should be true, the quantity and qual- 

 ity of phytoplankton cannot be used as 

 fairly direct indices and guides for the 

 stocking of lakes with fish. Programs in- 

 volving the addition of fertilizers for nurse 

 lakes will find no significance in the increase 

 of phytoplankton as a result of the added 

 nutrients insofar as food for fish is con- 

 cerned. In order to complete a useful pic- 

 ture of the value of phytoplankton in the 

 food chain, a series of coordinated experi- 

 mental studies should be made to involve 

 the following : 



a. A determination of the amount and 

 quality of plankton food required to pro- 

 duce a measured amount of micro-fauna. 



b. A determination of the quantitative 

 and qualitative values of these same species 

 of micro-fauna in the food chain of finger- 

 lings of various important species of fish, 

 expressed in weights and numbers of indi- 

 viduals of fish produced. 



e. A determination of the quantity of 

 food needed to bring these same fish to ma- 

 turity so that it will be known how much of 

 what kinds of food are necessary" to produce 

 a given number of fish pounds. 



With such data we would be able to evalu- 

 ate a lake in terms of its fish productivity 

 and fish-stocking programs could be carried 

 out all the more efficiently. 



4. The taxonomy of phytoplankton must 



