390 BULLETIN OF THE BUREAU OF FISHERIES. 



the difference in the amount of water covering them at high and low tides is much 

 greater than is indicated by the figures given above. For the species growing above 

 low water the difference is, of course, still greater. Since light penetrates to such a 

 slight depth in this water, the difference in the amount of light received by the plants 

 at different stages of the tide must be very great. The difference is, however, partly 

 neutralized for the algae growing near the inlet and even as far back as the laboratory, 

 since the water of the ocean is clearer than that of the harbor. This ocean water, 

 entering the harbor at flood tide, pushes the more turbid water before it and mixes 

 with it, so that, as was shown above, the water of the harbor is clearer at high than at 

 ow tide. 



Since the harbor is a comparatively small body of water and is well sheltered by 

 land, the water is usually smooth throughout the greater part of its area. Near the 

 inlet, however, there is considerable movement, although even here there are usually 

 no waves. Even the slight movement that does occur here, however, probably affects 

 the algae growing on Fort Macon and Shackleford jetties by washing off the sediment 

 that settles on them. 



HABITATS. 



The bottom throughout the harbor consists principally of sand, with some areas 

 covered by mud or shells. (See map 2.) The mud and sand furnish no place of 

 attachment for algae. The shells furnish excellent places for attachment, but do not 

 bear algae, probably because of the turbidity of the water above them. Algae are, 

 however, found attached to single shells and other supports below low water along the 

 shore and, sparingly, on the shoals. 



The numerous wharf piles occurring here would seem to offer excellent habitats, 

 but during the summer and autumn no algae were ever found on them, while, on the 

 contrary, during the spring they bore an abundant growth of algae. The reason for 

 this difference is not apparent. 



There remain, as possible algal habitats, the jetties at Fort Macon, at Shackleford 

 and on the laboratory island, and the brick walls occurring along the town shore. 

 These jetties bear the greater number, both of species and of individuals, of the algae 

 growing in the harbor, while the. walls bear a limited number of species. Small species 

 of algae are found, too, in some abundance on the buoys that mark the channel into 

 the harbor. 



CORAL REEF. 



The physical conditions existing on the coral reef offshore have already been 

 described. Here it need be said only that the surface temperature at noon on May 15, 

 1907, was 21.11 C., the temperature at a depth of 25.5 m. was 19.44 C., and the density 

 of the surface water (measured by a salinometer) was 1.0242. At this time the tempera- 

 ture of the surface water in the harbor varied, in different places and on different days, 

 from 20.5 C. to 23 C., and the density varied from 1.0165 to 1.0212. 



