Chemistry 161 



Michigan, the A',+5'n was 2.7 and 43.0 /xg C liter"' for glucose and 

 acetate, respectively. The average turnover times for the year ranged from 

 16 to 35 hours in the two Michigan lakes. 



Crawford et al. (1974) measured concentrations directly in an estuary 

 and found that the concentration of 15 amino acids was much lower than 

 the estimated K,-\-Sn calculated from the kinetic analysis of the uptake of 

 radioactive substrates. For proline, the average of Sn {K,+S„)^^ was 

 5.9%; for aspartate, 7.5%. If we correct our mean estimate of K,+S„ by 

 the same percentages as for proline and aspartate, then proline = 0.31 ng C 

 liter"' and aspartate = 0.08 Mg C liter"'. If the measured DFAA in the 

 estuary ranged from 10 to 30 Mg C liter"' when proline was 0.56 Mg C 

 liter"' and aspartate 1.02 Mg C liter"', then, in proportion, the DFAA in 

 Pond B would be 9 to 27 Mg C liter"'. It should be mentioned that amino 

 acids seem to differ from the general rule that the concentration of Sn is 

 close to that of A',. 



All the data show that compounds readily metabolized by bacteria 

 are maintained at very similar, very low concentrations in all natural 

 waters. At most, the pool of amino acids, simple sugars, and two or three 

 carbon acids (acetic, lactic, glycollic) found in the ponds could not be more 

 than 120 Mg C liter"'. This pool is produced rapidly, but is used just as 

 rapidly by the 10*" bacteria per ml (Hobbie and Rublee 1975). Hence, the 

 turnover of this pool is measured in tens or hundreds of hours, and its total 

 concentration is at a near steady state between production and utilization. 



The sources of this small pool of readily-used DOC include a 

 proportion of the leachate of dead vascular plants (5.6 g C m"^ yr"'), 

 leakage by the photosynthesizing plants, and excretion by grazing animals 

 (Table 4-23). Phytoplankton algal secretion or leakage ranged from 16 to 

 36% of the photosynthate (Table 4-26). In Pond B this is equivalent to 0. 14 

 g C m~^ yr"' when the planktonic production is 0.7 g C m"^ yr"' 

 (Stanley 1976a). The annual contribution from epipelic algal production, 

 8.9 g C m "^, is much more significant and an average secretion rate found 

 in 1971 was 10%. Hence, 0.89 g C m"^ yr"' of usable DOC was released 

 from this source (Table 4-26). 



Higher aquatic plants also leak a portion of their recent 

 photosynthate; this rate of loss could vary from 1 to 100% of photo- 

 synthesis rate as a function of divalent cation concentrations (Wetzel 

 1969a, Wetzel and Manny 1972). Glucose was a major compound 

 produced by Najas flexilis in the laboratory (Wetzel 1969b). On the 

 average, Wetzel et al. (1972) found an average of 4% leakage in the field. 

 At Barrow, in a single set of experiments in the laboratory at 16°C with 

 450 foot-candles of light, completely submerged Carex plants released 

 3.75% of their photosynthate as DOC during a six-hour incubation with 

 '''CO 2 and a subsequent six-hour incubation in non-labeled filtered pond 

 water. If the production of Carex ranged from 600 to 1000 mg C m"^ 

 day"' in Pond J in 1971, then the leakage would be 22.5 to 37.5 mgC m"^ 



