PART VIII — AQUATIC ECOSYSTEMS 



Seasonal changes as well as natural 

 and man-induced changes in the 

 fish stocks continually perturb the 

 lake ecosystem. Continuous analysis 

 of the perturbations of the plant and 

 animal plankton should make it pos- 

 sible to evaluate the concepts of 

 trophic dynamics, leading to the de- 

 velopment of techniques and concepts 



necessary for managing the Great 

 Lakes so as to maximize both water 

 quality and fish yield. 



The primary requirement is the 

 assembly of a scientific staff together 

 with the equipment and instrumenta- 

 tion (ships and collecting gear) nec- 

 essary for collecting extensive sam- 



ples. The samples should be converted 

 into data as automatically as possi- 

 ble. Taxonomic identification services 

 should be established. Methods for 

 data storage and rapid retrieval 

 should be developed. Much could 

 be done within five years toward the 

 development of effective manage- 

 ment concepts if a concerted effort 

 were made along these lines. 



Effects of Artificial Disturbances on the Marine Environment 



The capability of predicting the 

 specific consequence of a general 

 disturbance of a natural community 

 is basic to planning and evaluating 

 environmental controls. Large sums 

 of money and considerable effort 

 could be saved if we could foresee 

 the effects of a particular human 

 activity. 



History has taught us what to 

 expect from the destruction of forests 

 and prairies. But we cannot now 

 predict, with any confidence, more 

 subtle disturbances or the long-term 

 cosmopolitan consequences of drastic 

 change. This circumstance is rapidly 

 changing. Recent theoretical devel- 

 opments have directed our attention 

 to new ways of looking at the prob- 

 lem. There is reason to believe that 

 it will soon be possible to predict 

 change, at least in relatively simple 

 ecosystems such as exist in the sea. 



Ecological Generalities 



Few long-term studies have been 

 made on the changes that occur in 

 natural communities. We must there- 

 fore rely more on theory than ex- 

 perience. It is now recognized that 

 there is a fundamental relationship 

 between the number of species, the 

 number of individuals of any spe- 

 cies, and the stability of the environ- 

 ment. For example, there are fewer 

 species with relatively larger numbers 

 of individuals in severe or unstable 

 environments than in environments 

 whose fluctuations are predictable. 

 If the environment becomes more 



stable in time, the number of species 

 increases. If the environment is dis- 

 turbed in any way, the number of 

 species decreases. 



Succession and Regression — 

 Around the turn of this century 

 ecologists recognized that, wherever 

 a land surface was laid bare, it was 

 colonized by species in a regular 

 order. It was possible to predict, on 

 the basis of previous observations, 

 which species of animals and plants 

 would appear first and which would 

 later replace the earliest immigrants. 

 This process of succession of one 

 natural community by another con- 

 tinues until a stable climax commu- 

 nity is reached. However, succession 

 is a reversible process. Any disturb- 

 ance will drive the climax community 

 down to a lower level of succession. 

 The disappearance of species is also 

 in a more or less regular order. 



If we had data on the changes in 

 all natural communities, we could 

 predict the consequences of a general 

 disturbance using the principle of 

 succession. In the absence of such 

 studies, there may be another way 

 of obtaining relevant data: There is 

 evidence that natural communities 

 are continually responding to local 

 variations in the stability of the en- 

 vironment. Small-scale disturbances 

 drive down part of the system with- 

 out appreciably affecting other areas. 

 If this is the case, a community can 

 be viewed as a temporal mosaic, por- 

 tions of which are at different levels 

 of succession. In this circumstance, 

 the variations in species composition 



observed in space could be similar 

 to those observed in time. If samples 

 taken throughout a natural commu- 

 nity at one time are placed in order 

 of diversity, the array should simu- 

 late the order of species appearance 

 or disappearance in succession or 

 regression. 



The Impact of Pollutants 



At least some of the changes asso- 

 ciated with pollution resemble those 

 observed in natural sequences. For 

 example, the order in which marine 

 species disappear as a sewage outfall 

 is approached is often the reverse of 

 the order in succession. Using this 

 principle, we can take samples 

 throughout an area, arrange them 

 in order of diversity, and predict the 

 changes that would occur in the 

 vicinity of a proposed outfall. 



Some pollutants and other types of 

 disturbances are probably specific in 

 their effects upon communities, af- 

 fecting some species more than oth- 

 ers. Prediction in these cases will 

 require knowledge of the physiologi- 

 cal responses of particular species to 

 the particular compound or disturb- 

 ance. However, where the disturb- 

 ance is general, as in pollution from 

 domestic sewage or dredging, we 

 should be able to predict the effects 

 upon the community using the kinds 

 of observations and samples now 

 taken by ecologists. 



Prediction in Shallow-Water Com- 

 munities — Simple communities, low 



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