Chemistry 95 



58% of the variability in the CO > gradient. The direct relationship between 

 sediment temperature and the CO2 gradient is likely caused by the 

 increased respiration with increased temperature. The negative correlation 

 of water temperature with gradient might be a result of increased 

 photosynthesis in the water when the algae are more important than the 

 bacteria. As expected, the gradient was proportional to the wind speed. 

 The theory of gaseous exchange between atmosphere and water has been 

 described by Kanwisher (1963); one important control of exchange was the 

 thickness of a thin, stagnant layer of water at the surface. Increased wind 

 decreases the thickness of this layer and increases the rate of exchange. 

 This inverse relationship is obvious in Figure 4-9 for the pond and also 

 present, but less obvious, in the data for the lake (Figure 4-10). 



There are only a few bits of information on PCO2 in freshwaters. For 

 example, Park et al. (1969) found that two rivers were supersaturated in 

 CO2 with gradient values up to 340 ppm. There are more data for oceans 

 but oceans are strongly buffered against CO2 fluctuations by high 

 alkalinity of about 2.3 meq liter"' (vs. 0.4 in the ponds). Thus, the same 

 amount of CO2 added to seawater produces only about 1/13 the change in 

 PCO2 that occurs in distilled water. Teal and Kanwisher (1966) found that 

 the small changes in PCO2 that result from the biological activity in the 

 oceans persisted for long periods. Coyne and Kelley (1974) concluded that 

 the changes in pC02 in freshwater are so rapid that the PCO2 must be 

 intensively monitored if biological activity is to be followed in this way. 



The lake began to freeze on 1 1 September 1972 and a permanent ice 

 cover formed on 27 September. Ice thickness was 30 cm by 26 October and 

 the lake was completely frozen by 4 January 1973. The values of PCO2 

 were 800 ppm on 28 September, 2,000 on 12 October and over 3,000 ppm 

 on 22 October. The limit of the instrument was 3,000 ppm, so a PCO2 

 value was calculated on 16 October from precise pH measurements and 

 accurate CO2 reference gases of 12,000 ppm (Kanamori personal 

 communication). Thus, decomposition in this lake (and presumably in the 

 ponds) continues until the water is frozen to the bottom. 



CO 2 Evasion Rate 



During 1972, more than 25 attempts were made in North Meadow 

 Lake to assess the rate of transfer of CO2 from the water to the air. Details 

 of technique and calculation are given in Coyne and Kelley (1974). The 

 technique was similar to that for measurement of PCO2 except that the air 

 returning to the floating cuvette (at 0.5 liter min ') entered above the 

 water level rather than by bubbling through the surface layers. In an 

 experiment, the instrument was set up so that air circulated through the 

 cuvette. Next, the IRGA was set to zero, the cuvette placed in the water, 

 and the time required for the CO2 to come to equilibrium was measured. 



