16 ■ MASS. EXPERIMENT STATION BULLETIN 402 



It has been pointed out that changes in the oxygen content of the water under 

 ice on a winter- flooded bog are due to a difference in the rate of photosynthesis 

 under different conditions (p. 11). Since the intensity of the light received by 

 the vines is the only factor among those affecting this rate which changes signifi- 

 cantly, the great difference in the oxygen content at different times must have 

 been due to differences in the intensity of the light received. 



Among the factors upon which the intensity of the light received by the vines 

 depends (pp. 9-11), the intensity of the incident light is of primary importance. 

 The intensity of the incident radiation during the day, in terms of percentage of 

 the June 21 mean maximum, from January 16 to February 15 is shown in Figure 

 3. This includes the periods during which the greatest changes in the oxygen 

 content of the water occurred. 



Ice and water, however, also reduce the intensity of the light received by the 

 vines. The percentage of incident light that penetrated ice 4 to 7.5 inches thick, 

 with snow included in it, and reached the vines at a depth of 12 to 15 inches in 

 clear or only slightly colored water, at several intensities of incident radiation 

 from 5 to 55 per cent of the June 21 mean maximum is show^n in Table 1. The 

 depth of the water, on January 15, was 12, 16, and 15 mches at stations 1, 2, 

 and 3, respectively and did not change significantly between January 15 and 24. 

 The intensity of the light received by the vines, particularly on Januray 16, 17, 

 and 18, therefore, must have been very low — it may have been at most not more 

 than 6 to 8 per cent of the June 21 mean maximum and then only for an hour or 

 less; during the greater part of the daylight period it was probably less than 5 

 per cent of that maximum, decreasing to zero early in the forenoon and late in 

 the afternoon. The increase in the dissolved oxygen content of the water, or 

 most of it, between January 15 and 24, therefore, probably occurred from Jan- 

 uary 20 to 24 when the intensity and duration of the incident radiation was 

 greatest. The oxygen content of the water at station 1 remained lower because 

 the ice was thicker there and had more snow in it so that the vines received less 

 light than those at the other stations. 



The importance of photosynthesis in maintaining the dissolved oxygen con- 

 tent of the water is shown by the great decrease in the oxygen content at all three 

 stations from January 24 to February 4 as the result of the exclusion of light by 

 snow on the ice. Eight inches of snow fell on January 24. About half of it after- 

 ward melted, but an additional 2 inches on February 3 made about 6 inches 

 of snow on the ice on February 4. The thickness of the ice also had increased to 

 9.5, 8, and 7 inches at stations 1, 2, and 3, respectively. The Intensity and dura- 

 tion of the daily incident radiation during this period were as great as on the 

 days between January 15 and 24 when the oxygen content of the water increased. 

 There were only 2 days when the intensity of the incident radiation was as low 

 throughout the day as on January 16, 17, and 18; on all other days it was greater, 

 and on 6 days its intensity and duration w'ere as great as on the days from Jan- 

 uary 20 to 24 when the increase in the dissolved oxygen content of the water 

 probably occurred. However, measurements showed that only about 5 per cent 

 of the incident light penetrated 4 inches of snow. Therefore, with 6 inches of 

 snow, and with 7 to 9.5 inches of ice with snow included in it, it is probable that 

 very little or no light reached the cranberry vines. Consequently, little or no 

 oxygen was given off in photosynthesis, and the dissolved oxygen content of the 

 water decreased. The low mtensity of the incident radiation on February 2 and 

 3 further increased the probability that all or most of the light was excluded on 

 those days. The dissolved oxygen content of the water at stations 2 and 3 may 

 have been even lower from February 5 to 7 than on February 4 since most of the 

 snow then on the ice remained for another three days. 



