Macrobenthos 315 



enclosure was then heated about 5 to 6°C above the surrounding water 

 with buried heat tapes and the larval populations were sampled again after 

 3 weeks. Fourth instar Chironomus in the heated enclosure grew 2 to 3 

 mm, while the same cohort in the control enclosure did not grow. Third 

 instar Chironomus in the heated enclosure appeared to grow about 1 mm, 

 but their density was low at the end of the experiment and the data are not 

 definitive. 



Vertical Distribution 



Most chironomid larvae occupy the top 6 cm of sediments. In several 

 series of analyses of cores at 2-cm intervals in 1971 and 1972, less than 

 10% of larvae occurred deeper than 6 cm (Figure 7-9). Between 55 and 

 90%, in fact, were in the top 2 cm. This pattern is consistent with those 

 observed in many temperate lakes (Cole 1953, Milbrink 1973). 



There is also a strong stratification of chironomid larvae by size. 

 When larval size was plotted against depth in the sediments using data 

 from August 1972 samples from Pond J, the relationship had a slope of 1 .1 

 and an intercept of 5.4. Thus, the predicted size of an average larva at the 

 mud-water interface was 5.4 mm, and for every centimeter of increasing 

 distance into the sediment, average larval size increased by 1.1 mm. A few 

 larvae in any large sample were found below 10 cm and these were usually 

 in the fourth instar. This appears to represent superior ability to burrow 

 and force water through a longer tube (Brundin 1951). 



Trophic Structure 



Most of the species of chironomids are detritivores and most of these 

 are deposit feeders. Chironomus, the Tanytarsini, and probably most of 

 the Orthocladiinae feed on material either gathered from near the surface 

 of sediments or filtered from the water with nets; the gathering has been 

 observed, but the filtration is presumptive and is based on previous studies 

 of other Chironomus species. Chironomus has been found to feed on 

 Oligochaeta opportunistically (Loden 1974), but the importance of this 

 behavior is not known in arctic systems. Procladius, Derotanypus and the 

 rare Cryptochironomus are primarily predators, although particularly in 

 the first two instars both the tanypodines will ingest diatoms and detritus. 

 Trichotanypus, Corynoneura and Cricotopus feed on fine detritus and 

 algae adhering to the macrophytes. 



The most common invertebrate predator in the pond centers was 

 Procladius (900 to 1800 m"'). Its stomach often contained head capsules 

 and other sclerotized structures of chironomid prey; laboratory trials 

 indicated a short-term predation rate (at 10°C) of about one third-instar 



