Zooplankton 



R. G. Stross, M. C. Miller, and R. J. Daley 



COMMUNITIES, LIFE CYCLES, AND PRODUCTION * 



Community Composition and Life Cycles 



Crustaceans dominate the zooplankton of polygon center ponds and 

 also of pools in nearby troughs which form between polygons. Rotifers are 

 present but rare. The zooplankton species found at Barrow (Table 6-1, 

 Figure 6-1) are commonly found throughout northern Alaska, as shown by 

 collections from the Colville River to the east (Reed 1962) and from Cape 

 Thompson to the west (Milliard and Tash 1966). 



Despite the similarity of species over the entire region, there are local 

 differences in the community structure between the zooplankton of these 

 shallow waters and the zooplankton of nearby lakes. In the shallow ponds 

 and trough pools the Crustacea are quite large and often heavily 

 pigmented. For example, the fairyshrimp frequently reach 15 mm and the 

 Heterocope is the largest freshwater copepod. The species of the genus 

 Daphnia, D. middendorffiana and D. pulex, are two of the largest known. 

 In contrast, smaller forms of zooplankton are found in lakes containing 

 planktivorous fish. Ikroavik and Sungoroak Lakes, for example, have no 

 fairyshrimp but do have a small, transparent Daphnia {D. longiremis). A 

 third large lake, Imikpuk, has no fish; it contains the same species of large 

 Crustacea as the ponds. From this evidence, it is likely that size-selective 

 predation by fish may prevent the larger species of zooplankton from 

 inhabiting deep lakes. 



In addition to the differences in zooplankton species between lakes 

 and ponds, there are also differences between the ponds and trough pools 

 at the IBP site (Figure 6-1). Calanoid copepods {Diaptomus and 

 Heterocope) and D. middendorffiana are present in the ponds but not in 

 the trough pools; fairyshrimps are present in both but are rare in pools. 

 Daphnia pulex, present only in the troughs, is somewhat smaller than D. 

 middendorffiana and has only a small amount of pigmentation 

 (Edmondson 1955). Cyclopoid copepods {Cyclops) are abundant in both 

 habitats. These tundra ponds are as species-rich as alpine ponds at mid- 

 latitudes (Dodson 1974, Sprules 1972). 



*R. Stross 



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