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 alge 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 algee 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 alge. ‘The shells furnish excellent places for attachment, but do not 
bear alge, probably because of the turbidity of the water above them. Algz 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 alge were ever found on them, while, on the 
contrary, during the spring they bore an abundant growth of alge. 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 alge 
growing in the harbor, while the walls bear a limited number of species. Small species 
of alge 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. 
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