PART VIII — AQUATIC ECOSYSTEMS 



medium. When planktivore stocks 

 are sufficiently high, the populations 

 of large Daphnia are reduced to in- 

 significant numbers. Since the smaller 

 crustacean competitors that replace 

 them (see Figure VIII-2) are less 

 effective in collecting small algae, the 

 algal populations will tend to in- 

 crease, making the lake water less 

 transparent. 



This theory, in essence, states that 

 the composition of the open-water 

 community is determined by the 

 trophic actions of the highest (third 

 and fourth) trophic levels. The for- 

 mulation suggests a management 

 concept for controlling the effects of 

 the continued enrichment pollution 

 of the Great Lakes. In essence, the 

 plan would be to reduce planktivore 

 pressure in such a way as to maxi- 

 mize the populations of Daphnia 

 which are most effective in removing 

 algae from suspension. The plank- 

 ton-eating fish could be removed by 

 man through fishing. Removing the 

 fish would remove some "packets" 

 of nitrogen and phosphorus in the 

 lake ecosystem at the same time as it 

 permitted the proliferation of Daph- 

 nia. The fish themselves, depending 

 on their species, could be variously 

 used as human food, animal food, or 

 as a source of oils and other material 

 for chemical manipulation. 



The stocks of planktivores could 

 also be kept in check by introducing 

 and manipulating stocks of piscivo- 

 rous fish. For example, the introduc- 

 tion of coho salmon into Lake Mich- 

 igan is an attempt at controlling the 

 burgeoning population of the alewife 

 (Alosa pseudoharengus — originally 

 a marine planktivore that, despite its 

 abundance in many freshwater lakes, 

 is still imperfectly adapted to the 

 peculiarities of a freshwater exist- 

 ence). While this method of con- 

 trolling planktivores has the ad- 

 vantage of permitting the nitrogen 

 and phosphorus to be removed in 

 large packets that tend to find greater 

 acceptance as human food, the total 

 amount of these elements that could 

 be extracted from the fourth trophic 



level of the lake is at most one- 

 seventh of that which could be re- 

 moved via the third. It is thus less 

 satisfactory as a means of decreasing 

 the total amount of nitrogen and 

 phosphorus from a lake than is re- 

 moval of fish from the third (plank- 

 tivore) level. 



The entire matter of the use of 

 the fish removed from the Great 

 Lakes as food for man or beast has 

 been complicated by the fact that 

 various stable and toxic chlorine- 

 containing compounds such as DDT, 

 DDD, DDE, and PCB's are concen- 

 trated in the oil and body fat of the 

 fish of both trophic levels. 



Theoretical Formulations: 

 Control from Below 



In contrast to the concept of con- 

 trol of the composition of the open- 

 water community indicated above, 

 the alternate concept — widely held 

 a decade ago — still has adherents. 



The control-from-below theory en- 

 visions the composition of the com- 

 munity as being primarily determined 

 by competition within each trophic 

 level. In this view, the composition 

 of the first level — phytoplankton — 

 is determined by the particular con- 

 figuration of physico-chemical con- 

 ditions at the season in question. 

 The species composition of the sec- 

 ond level — zooplankton — is deter- 

 mined primarily by competition 

 among populations of the various 

 species of crustaceans and rotifers 

 that could occur within the lake for 

 the kinds of phytoplankton thriving 

 at that moment. Each species is most 

 effective in collecting only a portion 

 of the total range of sizes and kinds 

 of algae available. The planktivores 

 feed on whichever species of zoo- 

 plankter is available at the time. 



It can be appreciated, therefore, 

 that changing the intensity of plank- 

 tivore predation upon the zooplank- 

 ton would be expected, by the 

 control-from-below hypothesis, to al- 

 ter the total quantity of zooplank- 



ton — but not necessarily its specific 

 composition. Since this concept does 

 not consider that planktivore preda- 

 tion has any pronounced effect on 

 the species composition of the zoo- 

 plankton, there is no theoretical basis 

 for attempting to modify the com- 

 position and standing crop of the 

 algae by manipulating the stock of 

 planktivorous fish. 



Requirements for 

 Scientific Activity 



Examination of the simultaneous 

 changes in the abundance of all the 

 various species that comprise each 

 trophic level is necessary to evaluate 

 the alternative concepts of trophic- 

 dynamics outlined above. This is an 

 enormous task, even in the Great 

 Lakes where the variety of species 

 on all levels is very much less than 

 it would be in an equal volume of 

 the ocean. 



The greatest difficulties of enumer- 

 ation and categorization are pre- 

 sented by the extremely numerous 

 small organisms of the plankton. 

 Automatic methods of counting the 

 plankton and categorizing them ac- 

 cording to size must be developed. 

 The Coulter method of counting and 

 sizing particles by the drop in elec- 

 trical potential that each generates 

 while passing through a small aper- 

 ture through which an electric cur- 

 rent passes is not entirely satisfac- 

 tory. This data must be stored 

 electronically so as to be immediately 

 available for use with data on phys- 

 ico-chemical conditions, on the one 

 hand, and data on the characteristics 

 of the fish populations, on the other. 



In addition to methods of auto- 

 matic data collecting, it will be nec- 

 essary to make provision for the 

 proper taxonomic assignment of spe- 

 cies of the plant and animal plankton. 

 This information, gathered from ali- 

 quots, must be applied to the auto- 

 matically acquired data on size 

 categories. At present this is an 

 operation that is tedious at best and 

 nearly impossible at worst. 



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