SECT. 1] THE INFLUENCE OF ORGANISMS ON THE COMPOSITION OF SEA- WATER 55 



this period with the result that there was little change in the content of the 

 water column. During the summer the oxygen content of the water decreased 

 at a rate which corresponded closely to the rate of consumption by biological 

 activity. Consequently the exchange with the atmosphere was small during 

 this period. 



These estimates show that the changes in the oxygen content of the water 

 are small compared to the processes on which they depend. Thus, during the 

 period between 4 September and 22 March, about 84 x 10 4 ml/m 2 of oxygen 

 entered the water. The change in the oxygen content of the water column 

 during this time was only one-fourth of this amount, three-quarters being 

 consumed in the decomposition of organic matter. This demonstrates the 

 degree to which the oxygen content of the water is stabilized by exchanges 

 with the atmosphere. The quantity of oxygen crossing the surface of the Gulf 

 of Maine annually is equivalent to that in a layer of air about 4 m in thickness. 



The rate of diffusion of oxygen across the sea surface may be expected to 

 vary with the difference in oxygen pressure in the atmosphere and in the 

 surface water, according to the expression AQ s/At = E(P — p). P is the partial 

 pressure of oxygen in the atmosphere, and p its tension in the water, p was 

 determined from the degree of saturation of samples drawn from a depth of 

 one meter. E is the exchange coefficient of oxygen. 



It w T as found that the pressure head of oxygen (P — p) was positive during 

 the winter and negative during the remainder of the year, as required by the 

 sign of AQsjAt. The estimated values of the exchange coefficient E are about 

 20 x 10 6 ml per m 2 per month per atmosphere during the winter period and 

 agree in magnitude with estimates of this constant based on laboratory experi- 

 ments. The values for other periods are not satisfactory and suggest that water 

 samples drawn from a depth of one meter do not give a correct indication of 

 the tension of oxygen in the water at the surface at times when the surface 

 water is stable. 



b. Vertical exchange as a function of depth and season 



The factors on which the concentration of nonconservative elements in a 

 vertical water column depend vary greatly with depth. In the sub -surface 

 water of the euphotic zone the growth of phytoplankton tends to reduce the 

 concentration of nutrient elements while at greater depths regeneration tends 

 to increase their concentration in proportion to the quantities of organic 

 matter sinking to decompose at each level. The motion of the water on which 

 the vertical diffusion coefficients depend also varies with depth. The pro- 

 cedures which may be employed to determine the rate of biological activity at 

 different depths and of the vertical diffusion required to maintain the observed 

 concentrations as they varied with the season are illustrated by an investigation 

 of these factors in Long Island Sound made by Riley (1956, 1956a). 



For the purpose of the analysis the water column underlying a unit area was 

 divided into a series of segments of which the length, Az, was equal, except 



