Macrobenthos 335 



fifth molt Lepidurus nauplii are probably predaceous and are larger (2.6 

 mm) than any other benthic organism except the chironomids. 



Intraspecific competition may be one mechanism operating to 

 determine density of Lepidurus adults. We noted that the ponds with the 

 largest individuals had the lowest density and vice versa (Table 7-4). This 

 could be a density-dependent effect such as competition for food or 

 cannibalism. On one occasion in the laboratory cannibalism was observed 

 (Kallendorf 1974). This interpretation of competition is confounded by the 

 positive relationship between cumulative degree hours above 10°C and the 

 maximum size of adults. Still we have observed that where the highest 

 total biomass of Lepidurus was found the individuals were the smallest (on 

 20 August) and where the total biomass was smallest the individuals were 

 the largest. 



The length of time before the freeze probably exerts the dominant 

 control over egg production and the next year's initial density. Yet many 

 animals died before the freeze and in 1972 many Lepidurus were found 

 dead in the pond in August before the ponds had frozen or reached a 

 temperature of close to 0°C. In 1971, animals died during a mid-August 

 snowstorm (19 August). It would appear that the probability of dying 

 increases with normal stresses in the environment in the late summer. 

 Animals taken into the laboratory in late summer frequently died within 

 hours, whereas they would live for days under the same conditions in mid- 

 summer. Hence, variations in density of Lepidurus from year to year are 

 caused, at least in part, by random weather events. 



SUMMARY 



Macrobenthos 



The larger benthic organisms, mostly insect larvae, dominate animal 

 biomass and productivity in these ponds. They affect the pond system by 

 mixing the surface sediments and promoting exchanges of dissolved 

 materials between sediments and water. They also dominate major trophic 

 pathways as detritivores, grazers, predators, and prey. 



Larvae of the dipteran family Chironomidae constitute most of the 

 macrobenthic fauna, although oligochaete worms, snails, mites, and 

 turbellarians are also important. Non-dipteran insects and some larger 

 crustaceans are poorly represented in the Barrow ponds relative to some 

 other arctic freshwater systems. Many other invertebrate classes and 

 insect orders common in temperate ponds, such as mayflies, odonates, 

 leeches, and clams, are totally missing. 



The soft sediments of the pond centers and the borders of emergent 

 Carex and Arctophila represent two distinct habitats for the 



