PART VIII — AQUATIC ECOSYSTEMS 



cally speaking, this biological reduc- 

 tion is excessively wasteful, but the 

 small molecules that result from this 

 degradation can be utilized by the 

 photosynthetic plants, and thus re- 

 enter the trophic levels discussed 

 above. It must be stressed, however, 

 that a not inconsiderable amount of 

 reduction of dead algae and the 

 abundant feces of the animal plank- 

 ton occurs as these sink slowly 

 through the depth of the water. 

 There is thus a recycling of biologi- 

 cally active elements within the 

 water body itself, not dependent 

 upon the seasonal recurrence of full 

 vertical circulation and the cool- 

 season reintroduction of the accu- 

 mulation of the products of reduction 

 on the bottom back into the open- 

 water system of temperate lakes. 



Contrasting Trophic Dynamics in 

 Terrestrial Systems — Comparison of 

 some of the basic attributes of the 

 open water of a great lake, or of 

 the ocean itself, with those of a well- 

 developed terrestrial system such as a 

 forest reveals some basic dissimilar- 

 ities. The general features of trophic 

 dynamics sketched at the outset ap- 

 ply, of course, with equal validity 

 to terrestrial and aquatic systems. 

 The dissimilarities arise from the 

 differences in the structure of the 

 dominant green plants. 



Individual producers of the forest 

 attain great size, each striving to 

 spread its photosynthetic apparatus 

 so that it may be fully exposed to 

 the sun, unshaded by its neighbors. 

 The trunk and branches by which 

 each forest tree maintains its leaves 

 in the sun provide, in the aggregate, 

 a rigid three-dimensional framework 

 in relation to which the other or- 

 ganisms of the system dispose them- 

 selves. The leaves are the food source 

 for aphid and caterpillar, sloth and 

 deer, tapir and gorilla. The per- 

 manent woody plexus has made it 

 possible for this variety of sizes of 

 herbivores to evolve, each achieving 

 a different way of exploiting the 

 same food resource but each small in 



size compared with the green plant, 

 some part of which each consumes. 



How differently the photosynthetic 

 apparatus is disposed in the Great 

 Lakes! Here the individual plants 

 are tiny — microscopic solitary algal 

 cells or clumps and colonies just vis- 

 ible to the unaided eye (or, when 

 dead or moribund, evident to both 

 eye and nose as floating scum). The 

 principal herbivores in the open wa- 

 ters are small crustaceans, large com- 

 pared to the individual algal cells 

 that constitute their major food, but 

 often too small to cope with large 

 clumps of algal cells. 



To photosynthesize, the algae must 

 be in the upper, lighted water layers. 

 Under ice, algae are often concen- 

 trated at the very top of the water, 

 but in warm seasons they are swept 

 around in the Langmuir spirals in- 

 duced by the wind moving over the 

 water's surface. When wind is 

 strong and temperature low, the 

 spiral currents may carry the algae 

 too deep for adequate light to pene- 

 trate. But during the warmer half 

 of the year the myriad cells of the 

 phytoplankton are slowly spiralled 

 through the well-lighted, warmer 

 layer of lake water. Quite unlike the 

 forest situation, the green plants of 

 the open water display no semi- 

 permanent, three-dimensional pattern 

 of structure in relation to which ani- 

 mals can orient themselves and 

 evolve special behavior patterns. The 

 open waters provide no place to hide! 



One reason to stress the differences 

 between these two kinds of commu- 

 nities is that man, the observer, is 

 primatively a member of a forest or 

 grassland community, and some ecol- 

 ogists have too much betrayed their 

 experience of the forest in their in- 

 terpretation of the dynamics of open- 

 water systems. A part of this dif- 

 ficulty of interpretation has been the 

 tendency to expect, in essentially 

 structureless open-water systems, the 

 same kind of fine-grained adjust- 

 ments of organism to environment 



that have evolved in the substratum- 

 dominated terrestrial systems. 



Man-Induced Disturbances — The 

 nature of the dynamic model of rela- 

 tionships within the open-water com- 

 munity of the Great Lakes is of more 

 than academic concern. Man has 

 seriously disturbed the biotic prop- 

 erties of these lakes by his multi- 

 farious activities. If the quality of 

 these lakes is to be improved and 

 continuously maintained at an im- 

 proved level, a correct and complete 

 understanding of the ecological inter- 

 relationship is required. 



The overgrowth of the algae in 

 Lake Erie is probably the most ob- 

 vious manifestation of the disturb- 

 ances that the biological communities 

 of all the lakes have sustained to 

 varying degrees. An algal over- 

 growth, or, in ecologists' terms, an 

 increased standing crop of the phyto- 

 plankton, is a characteristic recent 

 manifestation of lakes in Europe and 

 North America on the shores of 

 which large concentrations of human 

 populations reside. 



The biological waste produced by 

 the people of cities is biologically 

 reduced, to varying degrees, into 

 small molecules of biologically active 

 elements such as nitrogen and phos- 

 phorus. When these are flushed into 

 lakes directly, or into their tribu- 

 taries, they augment the natural sup- 

 ply of plant nutrients. 



This "cultural enrichment" of lakes 

 is cumulative. Once the simple com- 

 pounds of nitrogen and phosphorus 

 enter the lake in solution, they are 

 quickly and effectively taken up by 

 the green plants — the phytoplank- 

 ton as well as the rooted water plants 

 along the shore. Henceforth, these 

 elements will reside in the complex 

 molecules of organisms. They spend 

 but little time in solution in the lake 

 water; the amount of nitrogen and 

 phosphorus that will escape through 

 a lake's outlet, dissolved in the water, 

 is remarkably small compared to that 



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