J. E. Hobbie 



100 



FIGURE 1-4. A cross section of a typical pond. 



decomposition is calculated as percent per month of open water, then it 

 appears that the rate is very similar to temperate rates. 



Algal photosynthesis in the sediment surface is also an important 

 input of organic carbon. Although photosynthesis occurs only in the top 2 

 mm, the algal cells were found throughout the upper 5 cm and deeper. The 

 buildup of these benthic algae is prevented by the downward mixing of 

 sediment and algal cells by the animals; in the absence of this mixing, an 

 algal mat would develop, which would have a very high productivity. 



Algal photosynthesis in the water column is extremely low, as low as 

 any in the world. This is not caused by low numbers of cells, as millions per 

 liter are always present. These are all very small cells, however, and their 

 mass is also small. Thus, in temperate lakes the algal mass is 100 times 

 greater than the bacterial mass; in the arctic ponds the algal mass was 

 about equal to the bacterial mass (Note: The bacterial mass is the same in 

 both systems). These planktonic algae also show a paradox found in other 

 extremely oligotrophic systems, such as the Sargasso Sea. This is the rapid 

 turnover of cells (the amount of carbon produced per day equals the 

 carbon of the cells) in oligotrophic waters. Grazing by zooplankton, 

 especially Daphnia, is likely responsible for much of the rapid turnover in 

 the ponds. 



The result of the distribution of primary production, low in the 

 plankton but high in the macrophytes and benthic algae, is a shift of 



