Macrobenthos 337 



Temperature plays an important role in larval physiology and 

 development. Respiration rates of Chironomus were similar to those of 

 temperate species and decreased as the larval size increased. When larvae 

 were placed in laboratory vessels which were 5°C above or below the in 

 situ temperatures, there was some acclimation after two days. However, in 

 the ponds the temperature changes this much each day so there was likely 

 no acclimation. At 5°C, egg development times are long and growth of 

 first instars does not occur. At a constant 15°C, growth requiring three to 

 four summers in the field will take place in 40 days. 



Between 55 and 90% of all chironomid larvae are found in the top 2 

 cm of sediment, and less than 10% occur deeper than 6 cm. The deeper 

 larvae are usually fourth instar Chironomus so there is a strong vertical 

 stratification of larvae by size. 



Species living within the sediments are primarily deposit feeding 

 detritivores or predators, while those found on the plants, including the 

 snail, the stonefly nymph, the caddisfiy larvae, and some midge larvae 

 graze on epiphytic algae. Chironomus, the Tanytarsini, and probably most 

 of the Orthocladiinae are deposit feeding detritivores. Trichotanypus and 

 the orthoclads Corynoneura and Cricotopus were observed grazing 

 epiphytic material from plant stems. The Tanypodinae Procladius and 

 Derotanypus are at least facultative predators, as many head capsules of 

 other midge larvae were observed in their guts; thus, Procladius predation 

 may account for much of the mortality observed for early instars of 

 Chironomus. In addition, chironomid larvae, pupae, and adults were eaten 

 by the red phalarope {Phalaropus fulicarius). While the insects may be an 

 important food for the birds, the impact of this feeding on the insects is 

 small. 



Estimates of assimilation and ingestion by Chironomus larvae lead to 

 a feeding efficiency of 0.10, which is normal for a deposit feeding 

 invertebrate. Yet, the source of the food is not clear; we have calculated 

 that feeding on algal, bacterial, and fungal cells in the sediments at 

 selectivities higher than those reported in the literature appears not to 

 provide enough food. Digestion of organic detritus is a possible additional 

 source of carbon. 



Estimates of total benthic production during 1972 showed greater 

 than a 4-fold variation among three ponds. One reason may be that late 

 instars of large midge species such as Chironomus contribute most to total 

 benthic production; therefore, the highly variable productivities may result 

 from differences in species composition. On the other hand, because the 

 population is made up of many cohorts, the effect of poor survival of a 

 single cohort is reduced. For example, from 1975 to 1977 Chironomus 

 pilicornis production was within 38% of the 3-year mean value of 2.0 g dry 

 weight m'^ yr~'. This production rate, which was equivalent to 1 .0 g C, 

 was well within the range of values for temperate zone midge populations. 



