350 



J. E. Hobbieet al. 



» 0.20 



u 

 « 



« 

 u 



o 



(o) Pond B 

 (.) Pond C 



0.10 



y=0.0264x + 0.0892 

 r=0.62 (n = l3) 



1.0 



2.0 



Primary Production, ^g C liter"' hr 



3.0 



-I 



4.0 



FIGURE 8-4. V^^^/or acetate uptake by planktonic bacteria at different 

 levels of primary production of phytoplankton algae. Data are from 

 Ponds B and C in 1971. June data omitted. 



a mean of 0.635 Mg C liter "^ hr '. It was further noted that velocity of 

 uptake of both glucose and acetate in these ponds correlated significantly 

 with primary production at the 99% level (Figure 8-5). For glucose r 

 equaled 0.84 (n = 25) and for acetate r equaled 0.76 (n = 25). 



The data on heterotrophic activity suggest that the interaction 

 between bacteria and an active fraction of the DOC is the same in the 

 ponds as in other aquatic systems that have been studied. That is, bacteria 

 take up small organic molecules (sugars, amino acids, fatty acids) from the 

 water even though the concentrations are only a few parts per billion. 

 Presumably each bacterium takes up a number of substrates 

 simultaneously in order to obtain enough energy. Over a certain range, the 

 systems for transporting the substrates through the cell walls are 

 undersaturated so any increase in concentration of the substrate is met by 

 an automatic increase in the transport rate. It appears that there is a 

 nearly constant level of substrate; this level is controlled by the bacteria. 

 Therefore, the changes in uptake rates and turnover times are a reflection 

 of changes in the rates of supply of the substrate. 



In the ponds, much of the substrate comes directly (excretion) or 

 indirectly (decomposition) from algae. A primary production rate of 10 to 

 15 mg C m'^ day"' for the plankton and 100 to 150 mgC m^^ day ' for 

 benthic algae could come close to supplying the 6 mg C and 30 mg C m "^ 

 day"' that is the calculated uptake for the planktonic and benthic 

 bacteria, respectively. Even larger amounts of organic carbon are 

 photosynthetically fixed by the grasses and sedges. Some will be excreted 



