Decomposers, Bacteria, and Microbenthos 355 

 Controls of Bacterial Productivity 



The primary control on bacterial productivity is the rate of supply of 

 low molecular weight organic compounds within the pond. In the water 

 column, these compounds come mainly from excretion or leakage from 

 algae during photosynthesis (Figures 8-4 and 8-5). Some of the excreted 

 material may be used directly while some must be hydrolyzed or otherwise 

 slightly broken down before it can be used. If our assumptions about the 

 constancy of the concentration of the substrate {S„) are correct, then the 

 actual flux of material into the water is about 1 to 5% of the most rapid 

 rates known for polluted waters (Allen 1969). There is also the 

 complication that significant amounts of DOC might be transferred from 

 the sediments and from the rooted aquatic plants to the water. 



Despite these other sources of DOC in the water column, there is a 

 clear and positive relationship between the potential uptake rates and the 

 rates of primary production in the various ponds. This has been observed 

 previously; for example, for a series of lakes and ponds in the temperate 

 zone (Morgan and Kalff 1972). Within the water column of a lake, Wright 

 (1970) found that the rates of uptake of glycolic acid exactly paralleled 

 photosynthesis rate. 



In the sediments, the primary source of the low molecular weight 

 compounds is the decomposition of the detrital material, mainly Carex 

 stems and leaves. As described in the DOC section (Chapter 4) and in the 

 next section (Decomposition of Macrophytes), about 20% of the organic 

 matter leaches from dead plant material within a day or two of death. The 

 remaining organic matter decomposes at about 20 to 70% per year; most 

 of this is likely due to enzymes secreted by bacteria or fungi. In contrast, 

 some plant material, such as the red marine alga Gracilaria, decomposes 

 completely within a few days (Tenore 1977). Thus, the slow rate of supply 

 of the DOC from the particulate matter is controlled in part by the 

 chemical makeup and resistance to breakdown of the Carex leaves. 



The low temperatures are also a control, but we judge temperature to 

 be relatively unimportant. Certainly microbial processes, and likely 

 decomposition too, would proceed faster at higher temperatures. For 

 example, a ^lo for bacterial growth of 1.7 was found (4° and 10°C) and 

 Figure 8-3 shows the tight coupling- between V^ax and temperature (the 

 ^10 was about 7). However, if the process did go faster, would there be 

 enough carbon to sustain the microbes at the faster rates? In the tundra 

 near Barrow an interesting experiment was conducted in which heating 

 pipes were buried in the tundra soil. Eventually, the increased rate of 

 decomposition caused the loss of so much organic matter that the volume 

 of the soil decreased, the level of the ground fell, and a small pond formed. 

 Thus, it appears that the organic matter in the soil was in a long-term 

 steady state before the disturbance. By analogy, the input and 



