MARINE A^GJB, OF BEAUFORT, N. C. 389 



range of variation. The averages, however, are probably fairly accurate, since they 

 are obtained from a large number of readings where the errors probably balance each 

 other. The general average, 1.0205, m ay, therefore, be taken as closely approximating 

 the mean density of the water at the laboratory wharf. At other places in the harbor 

 the density will, of course, be different from this. Since algae grow throughout the 

 harbor, some of them will be exposed to greater densities and some to lesser densities 

 than those recorded here. 



Several salinometer readings have been made by the author at other places in this 

 region. While these have not the value of the daily records made at the laboratory 

 wharf, they indicate the comparative density at other places. They are as follows: 



Newport River near " Green Rock, " low tide i. 016 



North River near Lenoxville, low tide i. 0188 



Pamlico Sound, Ocracoke, low tide i. 01 1 



Coral reef off Beaufort i. 0242 



TURBIDITY. 



The water from the open ocean outside of the inlet contains a considerable amount 

 of suspended matter, as is evident when this water is filtered, while the water within 

 the harbor has still more fine, suspended matter and is, at times, very turbid. All 

 rocks, shells, and posts under water are soon covered with a thick deposit, and at many 

 places in the harbor the bottom is covered with mud up to a meter or more in depth. 

 In the harbor and in Bogue Sound the amount of suspended matter seems to increase 

 as we go farther from the inlet, while in the sound back of Shackleford Banks the water 

 is decidedly clearer, owing to the strong current running in from the ocean at this place. 

 Farther back in this sound the water is as turbid as in the harbor. 



This turbidity not only reduces the light penetrating the water but itself affects 

 the vertical distribution of algae, since much of the suspended matter is deposited on 

 all objects in the water. The older portions of the broader algae (as Dictyota, Padina, 

 the leaves of Sargassum) are more or less thickly covered by this mud settling from the 

 water. 



MOVEMENTS OF WATER. 



The usual maximum range of tide (at the spring tides) is 0.97 m. (3.2 feet), the 

 usual minimum range (at the neap tides) is 0.7 m. (2.3 feet), and the usual mean range 

 is 0.82 m. (2.7 feet). The tides may, however, vary considerably from these figures, 

 the actual height and range attained depending in part on the direction and strength 

 of the wind. The greatest range observed by the author at the laboratory wharf is 

 1.31 m. (4.3 feet). The smallest range observed is 0.48 m. (1.6 feet). Under excep- 

 tional conditions the low tides are higher than the high tides recorded on other days 

 in the same month, while at other times the tides are unusually low. Although there 

 is not a very great difference in the height of water at high and low tides, there is a 

 great difference in the amount of light reaching the algae at these times. Except on 

 Shackleford jetties, where the water is clearer, no algae were found in the harbor below 

 1.4 m. below low water, and the majority were found within 75 cm. below low water. 

 Most of the algae have, therefore, about twice as much water over them at high tide 

 as at low tide. Furthermore, during summer and autumn the greater number of algae 

 grow almost up to the surface of the water at low tide. For these parts of the plants 



