Primary Producers 243 



leaves rapidly fall over. The water also gives some protection to the plants 

 and may allow a slightly longer growing season for the aquatic plants. In 

 addition, the green leaves and stems frozen into the ice are able to 

 photosynthesize the next spring. Thus, these aquatic plants start the 

 growing season with 5 or 6 cm of chlorophyllous tissue instead of the few 

 millimeters that the terrestrial plants have. Another reason for the 

 increased productivity may be an increased rate of nutrient supply. This is 

 difficult to prove experimentally and the only evidence comes from studies 

 of a series of terrestrial sites. 



The field measurements described here show that Carex aquatilis 

 populations growing in microhabitats (pond, meadow, ridge, basin, or 

 trough) only a few decimeters from one another have different nutrient 

 uptake and metabolic processes. When plants from these different 

 populations were grown in a greenhouse, the nutrient uptake and 

 metabolic differences were found to be genetically based (Shaver et al. 

 1979). These authors suggest that the genetic variability was present in a 

 population of Carex that colonized a uniform, drained lake basin. As the 

 microhabitats developed (when ice wedges and polygonal ground formed), 

 there was competitive interaction and segregation into ecotypes which 

 were maintained by vegetative reproduction. 



SUMMARY 



Phytoplankton 



Algal biomass, measured by the amount of chlorophyll present and 

 by the weight of cells, was very small in the water column of a pond. The 

 chlorophyll rose from 0. 1 /ig liter 'in early June to a peak of 0.8 to 1 .2 ng 

 liter ' in late June or early July, then remained between 0.3 and 1.0 Mg 

 liter ' until the end of August. Usually there was another peak, up to 2.0 

 ixg liter ', at the end of August. The cell weight, determined from the 

 direct count of the algae with an inverted microscope, was mostly between 

 200 and 400 ^g wet wt liter ' except for the late June — early July peak 

 when the weight reached 800 ^g- 



The algae are all nannoplankton, mostly small flagellates less than 10 

 Mm in length. There were usually several million cells per liter. The early 

 season was dominated by chrysophytes {Chromulina, Ochromonas) but 

 cryptophytes {Cryptomonas, Rhodomonas, Chroomonas) became 

 dominant in early July and remained dominant for the rest of the summer. 

 Some 105 different species of plankton algae were found in the ponds. The 

 overwhelming importance of nannoplankton, of flagellates, and of 

 chrysophytes and cryptophytes is typical of arctic ponds and lakes. 



