OYSTER INVESTIGATIONS IN GEORGIA 79 



the rise of the river stage rather than to local precipitation is clear 

 from the examination of Figure 11, the upper and the third curves 

 of which indicate precipitation (in inches) in Greensboro and Bruns- 

 wick. Greensboro is located at the head of Altamaha River, over 

 200 miles northwest from Doboy Island, while Brunswick is located 

 in the coastal region about 20 miles south of Doboy Island. An 

 examination of curves shows that local precipitation (Brunswick) 

 was not followed by any noticeable decrease in the salinity of the 

 water at Doboy; while the rains in Greensboro, which began on 

 August 11 -and continued for a week, were followed 10 days later 

 by a sharp decline in the salinity. Daily discharge data of Al- 

 tamaha Eiver made by the United States Weather Bureau show 

 that since August 20 the river at Everett City was at flood stage 

 (10 feet and above). Flood conditions prevailed until October 2, 

 -when the water dropped to 9.7 feet. 



TIDAL CURRENTS 



The organisms of the coastal region are exposed to regular changes 

 in their environment caused by the rhythmical tidal motion of the 

 water. The length of time they remain out of water, the tempera- 

 ture, velocity of the currents, osmotic pressure, and chemical com- 

 position of the water vary with every change of tide and are de- 

 pendent on the tidal range. Each of these factors may have a pro- 

 nounced effect on the organisms and may control their growth and 

 propagation. Hence, an understanding of the tidal conditions is 

 essential in determining the extent of changes in the environment 

 which the bottom and shore organisms of a given locality must 

 •endure. 



One of the striking features of the tidal motion is the change in 

 the velocities of the current that accompany the rise and fall of the 

 tide. We know that the direction and duration of tidal currents 

 and their velocity depend on the range of tide and on the configura- 

 tion of the tidal basin. According to the United States Coast and 

 Geodetic Survey, the mean range of tide around Doboy Island is 

 6.8 feet, with the spring range reaching 8 feet. A study of the 

 changes in the velocities of the current at Doboy Island was made 

 for both spring and neap tides. Observations during spring tides 

 were carried out on July 3, 31, and August 16, 30, 1928, and April 24, 

 1929 ; observations of July 10 and August 23, 1928, and May 2, 1929, 

 fall on neap tides. The results of the observations are presented in 

 Figures 12 to 18 and Tables 3 and 4. The range of tide was recorded 

 on the tidal gage set under the dock at Doboy Island. The heights 

 of tide in feet were recorded from an arbitrary zero level which was 

 set about 4 feet below low-water mark. The direction and the velocity 

 of current were determined by means of an Eckman current meter 

 (old model). It can be noticed from an examination of Figures 12 

 to 18 that no significant differences in current velocities were ob- 

 served between the spring and neap tides, and that the highest 

 current velocities of 97.5 and 116 centimeters per second were ob- 

 served on August 16 (spring tide) and August 23 (neap tide). 

 <Figs. 14, 17.) 



