Chapter 62 



Release and Capillary Movement of Phosphorus 

 in Exposed Tidal Sediments* 



C.\RL H. Oppenheimer and R. A. \V.\rd 

 INTRODUCTION 



A he shallow marine bays of the central Texas Gulf Coast con- 

 tain relatively large areas of intertidal mud flats (Figs. 1 and 2A) 

 which are alternately exposed and inundated by wind driven 

 tides. The surface of the gradually sloping mud flats is covered by 

 a growth of blue-green algae accompanied by diatoms, flagellates, 

 protozoa and bacteria. Water does not stand and evaporate when 

 the tide goes down, but either runs off^ or sinks into the sediment. 

 Thus, the water table in the sediment rises and falls with the tides. 

 The sediments consist of varying size sands with some broken 

 shell and fine layers of clay which are trapped by tlie blue-green 

 algal mat. 



The winds normally blow from the south east during the 

 summer at average velocities of 5 to 35 mph. During the winter, 

 northers occur at biweekly intenals causing north west winds 

 with velocities up to 70 mph. The alternation of the wind direc- 

 tion moves the water back and forth oxer the mud flats. Tliis mo- 

 tion is accompanied by tlie normal lunar tide of approximately 

 six inches. When the flats are exposed for some days or weeks, the 

 wind draws water to the surface of tlie sediment by capillary ac- 

 tion. Salts dissolved in the capillary water are deposited and 

 encrusted on the mud surfaces as the water evaporates ( Fig. 2B ) . 

 When the flats are again covered by water or during rain fall, 

 the salts are dissolved. 



The water within the bays is highly turbid with living or- 



* This investigation was supported by Contract Nonr 375(10) from the Office 

 of Naval Research to the University of Texas. 



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