COMPONI. ONSHIPS 



leaving the lake in the tissues of 

 emerging insects or organisms other- 

 wise removed from the lake. 



Approaches to Quality 

 Management 



Management of lakes to maintain 

 quality seeks two goals, both of 

 which involve maximizing the rate 

 at which the energy-rich compounds 

 of nitrogen and phosphorus fixed in 

 algae are passed to higher trophic 

 levels. One goal is to reduce the 

 standing crop of phytoplankton, 

 thereby making the water more trans- 

 parent; the second is to find an eco- 

 nomical way to remove nitrogen and 

 phosphorus from the lakes. 



The third trophic level in the open- 

 water community is the lowest at 

 which nitrogen and phosphorus are 

 concentrated into packets of a size 

 that man can manipulate and use. 

 These "packets" are the bodies of 

 the fish that eat the animal plank- 

 ton; they can be fished from the 

 lake and used directly as human 

 food (as lake whitefish once were in 

 large amounts) or they can be used 

 as a protein source for animal nutri- 

 tion (as alewives can be). 



We began this discussion of man- 

 generated changes in lakes by sug- 

 gesting that our conception of trophic 

 dynamics within the open water is 

 crucial to attempts to redress some 

 of these biological imbalances. There 

 are two alternate concepts of these 

 relationships (to be sketched below). 

 They differ in their relevance to 

 achieving the two management goals 

 set out above. The more recent for- 

 mulations stress the role of predation 

 by plankton-eating fish in control- 

 ling the species composition of the 

 plant and animal plankton. This con- 

 cept offers hope that the two goals 

 are not only compatible but might 

 be achieved by the same manipula- 

 tions of the system. On the other 

 hand, the older concept — which 

 stresses competition within a trophic 

 level as the prime determinant of 



plankton composition — presents no 

 simple dynamic model of relation- 

 ships among the first three trophic 

 levels. Attempts at management of 

 disturbed lakes will, therefore, not 

 only hope to achieve practical goals 

 but also to test and extend the con- 

 ceptual models. 



The Scientific Data Base 



In general, the data base for evalu- 

 ating and extending knowledge of 

 the trophic dynamic systems of the 

 Great Lakes is inadequate. This dy- 

 namic approach demands knowledge 

 of the interrelationships of the ele- 

 ments of the lake ecosystem, while 

 all that is now available are unre- 

 lated segments of data concerning 

 various aspects of the ecosystem. 

 Data on the seasonal changes in the 

 physical and chemical parameters for 

 more than a few stations at a time 

 in any one lake have become avail- 

 able only within the past decades. 

 Attempts to relate these physico- 

 chemical to biological changes have 

 only been sporadic. Of the biologi- 

 cal data, that on changes in the com- 

 position of the fish stock is probably 

 most nearly adequate. That on the 

 plant and animal plankton, which 

 comprise the bulk of the biomass, is 

 spotty and inadequate. A recently 

 published bibliography of the Great 

 Lakes plankton studies lists over 400 

 papers, but, as the bibliographer 

 added, 



The biology and ecology of the 

 plankton remains poorly known. 

 Most papers are descriptive and 

 concentrate heavily on taxonomy 

 and distribution of certain orga- 

 nisms. Experimental work on the 

 dynamics of Great Lakes plankton 

 is urgently needed in light of rap- 

 idly changing environmental con- 

 ditions and fluctuating fish stocks. 



The last sentence makes the essential 

 point: Significant studies of the 

 trophic dynamics involve simultane- 

 ous studies of physico-chemical pa- 

 rameters, the phytoplankton, the zoo- 



plankton, the planktivorous fi 

 the piscivores. 



Various bits of work done recently 

 in Lake Michigan can be put together 

 to provide some insight into the 

 trophic dynamics of that lake. This 

 has provided the reassuring informa- 

 tion that changes in the composition 

 of the animal plankton following 

 changes in stocks of planktivorous 

 fish (establishment of alewives, to be 

 specific) have been precisely what 

 would be predicted from knowledge 

 of the dynamics of much smaller 

 lakes. Furthermore, the time required 

 for the changes to be manifest in the 

 animal plankton of Lake Michigan 

 is not inordinately greater than the 

 time required in smaller lakes. This 

 is not surprising, because the total 

 size of the system should be less 

 significant than the mean ratio of 

 planktivore/zooplankter. 



Theoretical Formulations: 

 Control from Above 



A recent theoretical formulation 

 states that the composition of the 

 first trophic levels in the open-water 

 communities of large lakes is deter- 

 mined in large measure by the selec- 

 tive feeding habits of the planktivo- 

 rous fish. The prey selections by the 

 schools of zooplankton-eating fish 

 directly determine the species com- 

 position of the animal plankton. This 

 indirectly affects the quantitative and 

 qualitative composition of the phyto- 

 plankton (algae, bacteria) because 

 species of animal plankton differ in 

 the effectiveness with which their 

 populations can collect algae and 

 other small particles from the lake 

 water. 



Large crustacean zooplankters of 

 the genus Daphnia play a crucial role 

 in the indirect control of the first 

 trophic level resulting from the selec- 

 tive feeding of the third level. The 

 large Daphnia are both the favorite 

 food of freshwater planktivores and 

 the most effective collectors of small 

 particles (1-50 microns) from the 



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