Zooplankton 287 



concentrations. Measured uptake rates in the Barrow ponds ranged from 

 13 to 320 Mg P liter day ' (Chapter 5), indicating a large capacity despite a 

 residence time only slightly longer than has been described for more 

 eutrophic lakes (Rigler 1956, 1964, Lean 1973b). 



Excretion of phosphate by zooplankton is an established fact. At 

 least 40% of the phosphate excreted in the Barrow ponds is reactive 

 phosphate (SRP), a value that agrees with the literature (see Chapter 4). In 

 fact, when computed, zooplankton excretion could supply the growth 

 needs of the phytopiankton in the ponds. Each Daphnia, in filtering 5.0 ml 

 hr ' (Chisholm et al. 1975) can increase the ambient concentration by 4% 

 hr~' if it excretes 90% of ingested phosphorus, half of which is SRP. 

 However, if available phosphorus is only 10 to 20% of the total, actual 

 increase attributable to each individual is conservatively 20% day ' . Thus, 

 if the Daphnia can be increased from one to five individuals per liter and 

 ambient concentration of reactive phosphorus can be doubled, the 

 increased grazer density would still sweep only half the water volume free 

 of algal cells (69% of the water may be swept free of cells while 

 maintaining steady density if all cells divide daily). Clearly, a stimulating 

 effect on algal growth that can result from excretion of phosphate by the 

 Daphnia grazing on the food supply can be a simple explanation for 

 maximum fecundity in Daphnia at intermediate concentrations of 

 animals. 



Perturbations 



One planned disturbance revealed something about the stability of 

 zooplankton in the Barrow ponds. A deliberate addition of crude oil to one 

 of the ponds (Pond E) in 1970 destroyed the Crustacea and prevented 

 reproduction until at least the summer of 1973. Yet each spring a 

 substantial number of naupliar stages of copepods and fairyshrimps were 

 discovered in the pond. If the inoculation is from adjacent ponds via the 

 annual flood water from melting snows, then a substantial alteration of 

 each pond is possible. The fact that some degree of differentiation in 

 relative abundances is possible yet does not occur, suggests a functional 

 relationship of species to characteristics of the environment in each pond. 

 Control may be functional and based on the biological characteristics of 

 each species. That is not a new idea but intended merely to illuminate the 

 experimental potential of arctic ponds. 



Conclusions 



The community in arctic ponds contains a stable assemblage of 

 consumers. Reproductive and other ecologically pertinent traits of the 

 dominant species seem similar to those of so-called equilibrium species 



