Chemistry 177 



The amount of DOC in the pond thus changed from 0.8 to 3.1 g C 

 m"^ (for a 20-cm-deep pond). However, DOC has two fractions. One 

 fraction (90%) is refractory DOC made up of compounds, such as humic 

 acids and tannin, that are resistant to microbial breakdown. The other 

 fraction (10%), easily broken down and removed by microbes, is called the 

 labile fraction and is composed of peptides, sugars, amino acids, and short- 

 chain organic acids. The DOC that is measured in the pond is mostly 

 refractory material, as the microbes of the sediment remove the labile 

 compounds nearly as quickly as they are produced. 



Re-solution from the sediments was determined from changes in 

 subponds open to the sediment. Tests were run with leaves and stems of 

 Carex in litterbags (mesh) and plastic bags. From these and from the 

 production values it was calculated that 3.8 g refractory DOC m"^ enter 

 the pond from macrophytes (two-thirds) and sediments (one-third) and 

 5.6 g labile DOC m~^ come from macrophytes. A survey of ponds 

 revealed that those with the most color in the water, which correlates with 

 humic material, are trough ponds. Ponds on either side of the old lake 

 shelf also had high amounts of color while the intensively studied ponds 

 had low amounts of color. 



Labile DOC is made up of a large number of compounds, each 

 present at concentrations of a few ^g liter"'. Studies of the kinetics of 

 uptake of these compounds indicated that the sum of the concentration of 

 substrate plus a half-saturation constant for uptake was: 16 Mg glucose-C 

 liter ', 28 Mg acetate-C liter"', 5.3 Mg proline-C liter"', and 15 ng 

 aspartate-C liter"'. This means that the concentration of glucose was 

 below 16 Mg glucose-C liter'. The same experiments showed that these 

 compounds were completely removed from the water every 56 hr 

 (glucose), 229 hr (acetate), 70 hr (proline), and 1 5 hr (aspartate). Thus, the 

 microbes keep the pool of labile DOC at a low level even though large 

 amounts of DOC may move through this pool. 



Labile DOC leaches from Carex and Arctophila (5.6 g C m"^ y"^), 

 leaks from the photosynthesizing plants (1.9 g C m ~ yr"'), and is 

 excreted from grazing animals (1.0 g C m '' yr '). Another source, but 

 one that could not be measured, was from the decomposition of detritus 

 (non-plant) in the sediments. 



The total input of DOC to the ponds was 12.4 g m"^ yr "' or 4 times 

 the maximum amount of DOC. Most of the loss was due to microbial 

 decomposition in the sediment but there was also some photo- 

 decomposition. The limited data indicate complete photo-decomposition 

 of the humics every 28 days. This loss could be as much as 3-4 g 

 DOCm 'yr'. 



The changes in the DOC can be accounted for by the leaching and 

 decomposition within the pond. Thus, the DOC is autochthonous. 



Most of the material in suspension in the water is particulate organic 

 carbon (POC). It is defined as the amount retained on a glass fiber filter 

 (0.8 Mm effective pore size). Amounts were high, 1 100 to 1200 Mg POC 



