Primary Producers 247 



grazing pressure which had kept the Uroglena at a low level or whether it 

 was caused by some other interaction between the Daphnia and the 

 Rhodomonas. 



In the sediments, the algae are grazed by protozoans, chironomid 

 larvae, and other larger animals such as oligochaetes and nematodes. 

 Whether or not this grazing is an important control of these algae depends 

 upon the degree of selectivity these animals have because the sediments are 

 composed of about 0.06% algae, 0.17% bacteria, and 99.7% detritus (all 

 are percent of total organic matter). The measured feeding rate of 

 protozoans and micrometazoans was a maximum of 8 mg algal Cm" 

 day ~ ' out of a total algal biomass of 500 to 1000 mg m \ Chironomid 

 feeding is less selective and totalled 325 mg C m ^ day '. If there were no 

 selectivity, this would equal 0.2 mg algal C; if there were a 5-fold selection 

 of algae over detritus, the consumption would be 1 mg m^ day \ Thus, 

 total consumption may be about 9 mg algal C m^^ day^ which is about 

 10% of daily production. 



In some shallow ponds, the benthic algae have an extremely high 

 productivity because they form thick mats. This does not happen in the 

 tundra ponds because of the continued mixing of the sediments by the 

 chironomid larvae. This mixing of the algae down below the lighted zone is 

 likely the dominant control in view of the abundant nutrients in the pond 

 sediments. 



Aside from the example of the zoo'(A2iX\k\.ox\-Rhodomonas link 

 mentioned above, there was no evidence about what controls algal species 

 composition. Autoradiography did show that Chromulina and 

 Rhodomonas, which were present at the same time in the plankton, have 

 photosynthetic peaks at different times of the day; thus, there is always 

 physiological diversity present in the plankton and different species can 

 take advantage of seasonally changing conditions. Diatoms may have been 

 kept out of the plankton by the low concentration of silica, always less 

 than 0.5 mg liter"'. 



Biology of Rooted Aquatic Plants 



In the Barrow ponds, the dominant higher plant is the sedge Carex 

 aquatilis. This plant is actually amphibious as it grows on the tundra as 

 well. It occurs as a bed around the edge of the pond out to a depth of 15 

 cm. A grass, Arctophila fulva, occurs only in ponds and is found in 15 to 

 25 cm of water. The percent of the surface area covered by plants varies 

 from pond to pond; in the "typical" pond we studied intensively, C. 

 aquatilis covered 32% and A . fulva covered 13%. 



These plants reproduce vegetatively and so have a network of 

 horizontal rhizomes and stems (the tillers) connecting mother and 

 daughter plants. Every fall there is almost complete die-back of the 



