Chemistry 1 59 



50 lOOm -78 



FIGURE 4-32. UV absorbance at 250 nm in the water of 

 some ponds near the IBP aquatic site, 18 August 1972. 



Labile Compounds 



Simple organic molecules, such as sugars, amino acids and simple 

 acids, make up a very much smaller proportion of the total DOC than the 

 humic compounds. These simple compounds are biologically very mobile 

 in contrast to the refractory humics and, as a result, bacterial uptake 

 maintains them near the lower concentration threshold for active uptake 

 by bacterial transport mechanisms. Thus, the concentration of individual 

 compounds may only be a few ^g C liter"'. These labile molecules are 

 produced by many processes, primarily by leakage from phytoplanktonic 

 algae, from epipelic algae, and from emergent plants, although the 

 egestion, excretion, and autolysis of animals can contribute significantly. 



Only a few direct analyses of these simple compounds have been 

 carried out anywhere, but these all confirm that there are only low 

 amounts present in natural waters. Using an enhanced enzymatic assay, 

 Hicks and Carey (1968) found that glucose in natural waters ranged from 

 1.3 to 3.8 Mg C liter '. Vaccaro et al. (1968) reported average values 

 ranged from 2 to 9 Mg C liter"' in Atlantic ocean waters. Crawford et al. 

 (1974) measured the concentrations of 15 amino acids in the Pamlico 

 Estuary for 1 year using an ion exchange chromatographic technique. 

 Total dissolved free amino acids (DFAA) ranged from 10 to 30 Mg C 



