mid-May. The May peak was tlie higliest for the 

 year (13.6 p.p.m., 137 percent saturation) in the 

 laboratory supply water, but the harbor surface 

 did not reach its annual maximum until June 8 

 (12.3 p.p.m., 133 percent saturation). Thereafter 

 the dissolved oxygen in both water sources de- 

 creased irregularly througliout the summer, al- 

 though the highest average for August was in 

 196G. Throughout the fall and winter, saturation 

 was usualh' between 90 and 100 percent. 



OBSERVATIONS IN 1967 



Tlie dissolved oxygen remained near or below 

 saturation through February, but the concentration 

 reached a record high for the period of study : 14.8 

 p.p.m. on March 27. 1 )espite the relatively low tem- 

 perature, saturation reached 130 percent at this 

 time. The next major peak appeared during the 

 third week in May, when the concentration reached 

 13.5 p.p.m. and saturation 135 percent. Values for 

 pumped water and harbor surface water diiTered 

 only slightly. As in the 2 preceding years, dissolved 

 nitrogen rose to about 120 percent saturation for a 

 few days in May. 



By the end of June the supersaturation of oxy- 

 gen declined markedly, and the July average was 

 the lowest for that month in the 4-year period. In 

 general, oxygen supersaturation was lower during 

 the whole sunnner of 1967 than in the 3 preceding 

 years. This drop may have indicated a long-term 

 downward trend, or could liave been due to an 

 unusual amount of cloudy or foggy weather. 



ANNUAL TRENDS AND COMPARISON 

 WITH OFFSHORE WATERS 



The seasonal and annual trends in temperature 

 and in oxygen concentration, percentage satura- 

 tion of oxygen, and temperature are shown in 

 figure 1. The values shown are averages of all 

 afternoon measurements made each month. They 

 can be considered to be the monthly means of daily 

 maxima. 



A study of the figure reveals several points of 

 interest. Both concentration and saturation show a 

 cyclic pattern that bears some relation to tempera- 

 ture. Oxygen concentration tends to be highest 

 when the water temperature is low, but the peaks 

 lag the temperature minima by about a month. 

 Percentage saturation is highest during periods of 

 rising temperatures. The probable reasons for these 



phase relations are that oxygen is more soluble 

 at low temperatures but the peak of oxygen pro- 

 duction by photosynthesis occurs later than the 

 time of minimum tempei'atures. Maxi)num concen- 

 trations, therefore, occur when the interaction of 

 liigh solubility and maximum production is op- 

 timal. Maximum saturation is similarly the result 

 of optimal interaction between high production 

 and low solubility. Saturation is further increased 

 when the rate of Manning outpaces the rate at 

 which the dissolved gas can come into equilibrium 

 with the atmosphere. 



Tlie supersaturation of oxygen appeared to be 

 somewhat higher at Boothbay Harbor that at other 

 localities along the coast, although the number of 

 samples taken elsewhere may not have been suffi- 

 cient to provide a fair comparison. Values for 

 coastal waters were generally higher in 1965 and 

 1966 than those observed for comparable periods 

 in offshore waters during the Albatross IV cruises 

 (Colton et al., 1968). In 1965 no ofi'shorc samples 

 exceeded 120 percent saturation in May and June, 

 and in 1966 only 3 percent of all samples from the 

 top 20 m. in the open Gulf of Maine were greater 

 than 120 percent. At Boothbay Harbor during the 

 comparable period of both years, 78 percent of the 

 samples in 1965 and 82 percent in 1966 exceeded 

 120 percent saturation. Elsewhere along the coast 

 during a comparable period, 14 percent of the 

 samples in 1965 and 43 percent of those in 1966 

 showed over 120 percent saturation. 



CAUSATIVE FACTORS 



Oxygen supersaturation in the sea can result 

 from several processes, of which two have major 

 significance. The most important is probably the 

 actual increase in oxygen concentration resulting 

 from photosynthesis ; the other is a change in the 

 physical properties of the water which affects the 

 solubility of gases. Air that diffuses into water 

 from the atmosphere at the sea surface can produce 

 100 percent saturation of oxygen and nitrogen at 

 the ambient temperature, salinity and barometric 

 pressure; if these physical properties change 

 rapidly, the water can become at least temporarily 

 supersaturated. Rapid warming of saturated 

 water, or the mixing of warm and cold saturated 

 water results in supersaturation until the gases are 

 able to once again acliieve equilibrium with the 

 atmosphere. 



SUPERSATURATION OF ATMOSPHERIC GASES IN GULF OF MAINE COASTAL, WATERS 



119 



