THE SUBSTRATUM 



161 



Stand high in comparison with any other 

 given factor of the inshore httoral environ- 

 ment. 



The kind of bottom is also important in 

 lakes where rocky, eroding shores support 

 animal communities that resemble those on 

 rocky bottoms of streams of the same gen- 

 eral area. Sandy depositing shores of lakes 

 usually maintain a sparse population, and 

 if the sand is much battered by waves, as at 

 the south end of Lake Michigan, the loose 

 sand tends to be as bare of life as is a 

 sandy desert during the heat of the day. 



cated in Figure 33. When mud is mixed 

 with the sand, bottom-dwelling animals, 

 burrowing or otherwise, show a great in- 

 crease in numbers of species and in popula- 

 tion density. On favorable tide flats of New 

 England, the substrate may be completely 

 stippled by the siphon openings of the bur- 

 rowing clams, Mya and Venus. On other 

 coasts, thickset colonies of oysters grow on 

 suitable substrata between the tide lines 

 (Fig. 34). Numbers of species and indi- 

 viduals decrease as the bottom becomes 

 almost sandless mud, especially in regions 



Fig. 33. The usual positions of a number of characteristic animals on the sand beach along the 

 coast of Carolina. (Redrawn from Pearse.) 



The relation between population density 

 and the type of substratum within the six- 

 foot contour line in western Lake Erie is 

 summarized in Figures 31 and 32. Sandy 

 beaches and shoals are most sparsely inhab- 

 ited, with only about 100 macroscopic 

 invertebrates per square yard. Flat shelving 

 bedrock supports the densest population, 

 with an average of some 7700 individuals 

 per square yard. The animals on bedrock 

 were mainly dipterous midge larvae. Sand 

 substrate also supports fewer species, six 

 per square yard; pebbles, shelving rock, 

 clay, block rubble, and flat rubble follow 

 in an ascending series. There were approxi- 

 mately seventeen species of these inverte- 

 brates per square yard of flat rubble. 



Along sea coasts, loose sand similarly 

 supports relatively little animal life. The 

 Pismo clam of California, mole crabs (Eme- 

 rita talpoida), and "terraqueous" bur- 

 rowing copepods of the sandy littoral of 

 eastern Nordi America are some of the 

 characteristic inhabitants. Others are indi- 



of stagnant water, where, if the mud has a 

 high organic content, hydrogen sulphide 

 develops. 



Near land and in the Arctic Ocean, the 

 sea bottom is characterized by rain wash 

 along the coast, and by stream erosion often 

 from far back upcountry. Such eroded de- 

 bris falls to the bottom mainly on or near 

 the continental shelf. Glaciers carry a heavy 

 load of miscellaneous soil and rock, some of 

 which may be deposited hundreds of miles 

 out at sea, as may also the air-borne dust 

 from wind erosion of arid land or from vol- 

 canoes. The "mud hne," which marks the 

 seaward hmit of terrigenous deposits, usu- 

 ally hes somewhere outside the 200 meter 

 contovu*. 



In deeper water, in 2000 meters or more, 

 gravels, sands, and silts from the land are 

 mainly replaced by pelagic oozes or red 

 clay. In exceptional cases, as in the Arctic 

 Ocean and in a broad strip between Ant- 

 arctica and South America, terrestrial de- 

 posits apparently predominate much beyond 



