8 J. E. Hobbie 



systems is dramatic; the living mass of organisms is more than 150 times 

 greater per square meter of sediment than of water, and the activity rates 

 (e.g., respiration) reflect the same ratio. In spite of the relatively high 

 sediment activity, most of the detritus pool, nearly 4 kg C m~\ is not 

 being broken down. Instead, the food for the biota comes from recently 

 formed detritus (about 0.02 kg C m Mn Figure 1 -5). 



In the water column, there is a similar large quantity of carbon, the 

 dissolved organic carbon (DOC), that consists of a large pool of inactive 

 carbon and a much smaller pool of rapidly-cycling carbon. Some of this 

 rapidly-cycling pool of DOC comes from the sediment, as the mass and 

 activity of bacteria is quite high considering the low primary production of 

 the planktonic algae. 



The detritus, algae, and bacteria support a large standing crop of 

 zooplankton grazers, a crop that is much larger than algae alone could 

 support. Actually, the relationship between the zooplankton and detritus 

 may be more complicated than this. We observed that from year to year 

 the amount of planktonic algae and bacteria remained about the same (5 

 to 10 ng C liter "^) but the amount of detritus fluctuated from 300 to 1400 

 Mg C liter ~\ Zooplankton production was highest in years when the 

 average amount of detritus was lowest and vice versa. This could be cause- 

 and-effect but it is impossible to tell if the high detritus loads prevented the 

 zooplankton from harvesting very much of the nutritious algae and 

 bacteria (blocking) or if the high numbers of zooplankton removed the 

 detritus. It is also possible that the zooplankton excreted enough 

 phosphorus back to the water to increase the phytoplankton production. 



Carbon dioxide moves rapidly from the water into the air. In fact, an 

 amount of dissolved CO 2 equal to that in the water is replaced each day. 

 The flux of CO 2, appears to balance the primary production but in spite of 

 the intensive study, we could not say whether or not annual respiration 

 equaled photosynthesis. At best, our measurements were only within 20% 

 of the true value and an accumulation of only 10% of the total primary 

 production each year would easily account for the organic sediments of the 

 pond. (Ten percent each year is 5 cm of sediment in 400 years.) 



In the pond ecosystem it was obvious that grazing food chains are 

 unimportant relative to the detritus food chain (Table 1-1). In the 

 sediments, the detritus is either eaten directly by animals or is attacked 

 first by microbes. Our evidence for direct utilization comes mostly from 

 studies of the energy requirements of the chironomid larvae (the 

 "animals" in Figure 1-5). At the rate of particle ingestion that we 

 measured, the larvae had to be digesting mostly detritus. Previous workers 

 postulated that the animals were obtaining enough energy by stripping the 

 microbes from the detrital particles. We actually measured the quantity of 

 bacterial and algal biomass that is included in the detritus and found it to 

 be only 0.06% of the total carbon (Figure 1-5). This amount of carbon is 

 0.3% of the organic carbon requirement of the larvae. Although these 

 animals may select microbe-rich particles or locations for feeding, they 



