338 M. Butler etal. 



As long as a new cohort is recruited every year, slow larval growth 

 imposed by the arctic environment does not reduce potential productivity 

 below levels expected for univoltine populations. Overall, the abundance 

 of larvae is the determinant of productivity for a given species. The size of 

 a cohort at recruitment can be influenced by weather conditions, and 

 predation by invertebrates (especially predaceous midge species) can 

 further reduce numbers and affect production. 



The biomass of emerging adult chironomids, about 0.2 to 0.4% of 

 total primary production, is similar to values reported from other aquatic 

 systems. This may reflect a general dependence of benthic secondary 

 production on the rate of carbon fixation by plants. 



Tadpole Shrimp 



The tadpole shrimp {Lepidurus arcticus) is a large (up to 26 mm) 

 crustacean found in small numbers (about 1 m ") in or on the sediments of 

 the ponds. This animal overwinters as a resistant egg and hatches in late 

 June (about 2 mm long). The early instars are planktonic but in the first 

 week in July the animals (2.6 mm in length) move into the upper layers of 

 the sediment. When they reappear at the surface in early August, they are 

 17 mm in length. When the animals become adult, in mid-August, they 

 begin to lay 1 to 2 eggs every 3 days. These eggs then overwinter. 



Temperature is important to the growth rate of Lepidurus. For 

 example, in a very shallow, warm pond the adults reached 34 mm in length 

 while in a deeper, cold lake they were 22 mm long. Their respiration rates 

 were highest at 1 5° and lower at 5, 10, and 20°C. 



Production of 11.5 mg C m~~ yr~' was calculated from the increase 

 in biomass, the mortality, and the eggs. Some 69% of the total was 

 increase in biomass while 9% was mortality and 1 5% was exuviae given off. 

 In the cold summer of 1973, 67% of the carbon assimilated (production 

 plus respiration) went to production, which is a very high percentage 

 indeed. In part, this could reflect an error of the respiration measurement 

 which had to be carried out on restrained animals. 



The nauplii, pre-adult, and adult forms are likely all predators. In 

 experimental vessels, adult Lepidurus preferred flatworms but also 

 captured large Daphnia quite effectively. Given the low density of the 

 tadpole shrimp, the predation rate in the pond should only be about 1 

 Daphnia m " day ' out of a population of 800. 



Perhaps the most important effect of Lepidurus was in disturbing and 

 mixing the top 0.5 cm of sediment. The adults moved about 10 cm min ~ ^ 

 in the pond, which is equivalent to mixing 0.7 to 1.2 m" day"^ for each 

 animal during the month of August. 



The tadpole shrimp maximizes its reproductive output by having all 

 the individuals reproducing and self-fertilizing. The actual number of eggs 



