96 R. T. Prentki et al. 



Because periods of up to 6 hours were required to attain CO2 

 equilibrium, some of the series of measurements could not be used as the 

 PCO2 in the water changed so drastically over a short period. From 17 

 experiments between 19 July and 6 September a mean evasion coefficient 

 of 0.34 mg cm "atm ' 'min ' was measured (standard deviation of 0.17). 

 These coefficients were measured under a variety of temperature and wind 

 conditions so we can make the assumption that the mean coefficient can be 

 applied to the average gradient of 1 15 ppm (Table 4-6) to give an average 

 rate of transfer from the lake of 0.56 g CO 2 m ''^ day ^ (this is 0.15 g C). 

 We also assume that the coefficient is similar for the pond. Based on an 

 average gradient of 397 ppm, the rate of transfer from the pond was 1 .95 g 

 COam 'day~'or0.53gC. 



There are no other values for CO 2 evasion rates in freshwater. Rates 

 for seawater are widely variable (Riley and Skirrow 1975) probably 

 because of the strong influence of turbulence which cannot be controlled 

 from experiment to experiment. In our study, the water was generally 

 turbulent during the rate experiments. The cuvette did prevent direct wind 

 effect on the surface but did not attenuate the wave train to any degree. 

 Accordingly, the measured evasion rates probably lie somewhere between 

 those for diffusion and turbulent exchange rates. We imagine that the rates 

 are conservative estimates, based on this information. However, as 

 discussed in the sediment respiration section (Chapter 8), this value of 0.5 

 g C m'^ day^ is about twice as high as the respiration of the sediment 

 organisms measured in incubations of sediment cores. 



Total Dissolved Inorganic Carbon 



Bacterial respiration in the sediments as well as the solution of 

 carbonate in the peat layer contribute to the total dissolved inorganic 

 carbon (DIC) in the interstitial water. Actually, four processes are 

 occurring. The first is the production of DIC by respiration of aerobic and 

 anaerobic bacteria. Most of the production (77%) occurs in the top 4 cm 

 (Chapter 8). The second process is the diffusion of the DIC through the 

 sediment towards the water. The third is the rapid diffusion of DIC across 

 the sediment/water interface. This is a rapid process, as the gradient is 

 large due to the continual water circulation that renews the water film and 

 dees not allow a buildup of DIC. The fourth process is the solution of 

 carbonates. While this occurs, we have no evidence that it is at all 

 important. The concentrations resulting from these processes increase 

 exponentially with depth (Table 4-2). 



In the areas of the sediment where there are rooted aquatic plants the 

 roots also will add CO2 to the sediments. The rate of root respiration is 

 poorly known but it may produce much of the CO2 measured in the whole- 

 pond transfer rates. 



