€H. YIl] THE DISTRIBUTIOX OF THE PLANKTON 149 



at about the same time in adjacent parts of the sea give different 

 results and if these differences are greater than can be explained 

 by experimental errors then we can only attribute them to the 

 irregular manner in which the plankton is distributed from place to 

 place in the sea. And just as one is inclined to minimise or magnify 

 the results of quantitative plankton work, so one is inclined to lay 

 stress on, or partially to ignore these irregularities of distribution. 

 In considering them one ought, however, to remember that the 

 plankton is far more irregular in its distribution near the shore 

 than further out to sea. 



Many causes combine to render the plankton very irregular in 

 its distribution near the land (say within ten to twenty miles from 

 the shore). The tidal streams are far more rapid there than out 

 at sea. In estuaries and bays they run with great force, curving 

 in round the latter and forming " races " round headlands. 

 Temperature changes too are far more rapid and extreme near the 

 shore than further out. A wide area of sandy shore may be laid 

 bare by the tide and rapidly cooled in the winter, or as rapidly 

 heated in the summer, and when next the flood tide covers this 

 area the water becomes cooled or heated as the case may be ; and 

 thus irregularly heated areas of water are brought into existence. 

 Soon the movements of the sea smooth down .these irregularities, 

 but generally speaking as we sail out from the land in the summer 

 the sea becomes colder, while in the winter months it becomes 

 w^armer. The salinity of the sea is always lower near the land 

 because of the fresh water brought down by rivers. But land 

 water always brings down dissolved matter, such as salts of 

 nitrous and nitric acid and ammonia, and other substances which 

 are the ultimate food-stuffs for the plankton. Thus a river grossly 

 polluted with sewage matters will have a most remarkable effect 

 in increasing the planktonic and benthic life in the neigh- 

 bourhood of its outlet. Further the richest benthic fauna 

 is to be found in relatively shallow water, and it will at once 

 be seen that accumulations of such animals as molluscs, crabs, 

 cirripedes, echinoderms and the like must add considerably to the 



the time of the change of the surface movements. This is perhaps the main cause 

 of the irregularity of distribution of the plankton in an area of uniform physical 

 conditions. 



