primarily on the stage of the tide. Strong gradients occur during 

 flood and somewhat weaker gradients during ebb. Salinities ranged 

 from 19%o to 27%o (parts per thousand) throughout the year. Negative 

 temperature gradients or isothermal conditions occur during summer. 

 These conditions depend almost completely upon the meteorological 

 conditions. During periods of slight winds or calms the surface is 

 strongly heated, and a sharp temperature gradient develops between 

 the surface and depths of 5 to 10 feet. Following periods of moderate 

 or strong winds, the temperature structure is either isothermal from 

 surface to bottom or shows an isothermal layer overlying a moderate 

 negative gradient extending to near the bottom. During winter the 

 temperature gradient is weakly positive. 



The transparency of the water is generally low. Secchi disc measure- 

 ments, taken with a white (Secchi) disc approximately 12 inches 

 (30 cm.) in diameter, range from 1/2 to 3 feet. Water transparency 

 measurements with a white light hydrophotometer indicate that through- 

 out the year the light transmission at the surface ranges from 20 

 to 40 percent of light transmission in air, whereas at 5 feet above 

 the bottom the transmission ranges from about 5 to 15 percent of 

 that in air. 



The question immediately arises as to whether the 1956-57 period 

 was typical wi-h regard to environmental factors. Biological activity 

 is dependent to a large degree on water temperature. A preliminary 

 means of determining how closely the test period approximated an 

 average year is to compare observed sea surface temperatures at the 

 test site with average temperature data for the surrounding area. 

 The Coast and Geodetic Survey has compiled surface temperature 

 observations for the tide gage station at Old Point Comfort for the years 

 1943 to 1945 and 1948 to 1954. Monthly mean maximum, mean, and 

 mean minimum temperatures for the tide gage station are shown 

 in Figure 5 along with the observed sea surface temperatures at 

 the fouling rack location. The figure indicates that temperatures 

 at the test location during the test period were within the average 

 range for the area, although during the first part of the period they 

 were consistently lower than the mean, and during the latter part 

 of the period they were consistently higher than the mean. On the 

 basis of water temperature, therefore, biological activity in 1956-57 

 was probably close to average. The collection of an additional year 

 of data will serve to indicate the validity of this conclusion. The 

 plot of mean and observed salinities, shown in Figure 6, also indicates 

 that this element varied within the average range for the test period. 



