318 M. Butler etal. 



square meter contains several thousand larvae of appropriate size, the 

 impact of bird predation must be small in comparison to the invertebrate 

 predation. 



Feeding Rates and Assimilation 



Assimilation can be estimated and divided by ingestion as a measure 

 of feeding efficiency. Assimilation is approximated by the sum of 

 production and respiration, which was 3.01 g C m^ or 32.7 mg C m"^ 

 day ~\ This estimate combines production calculations from 1975 to 1977 

 and respiration estimates from 1971 and 1972. Production as dry weight 

 was twice carbon (Waters 1977). The growing season was taken as 92 

 days. Ingestion was estimated by feeding individual Chironomus larvae 

 with ^^P-labeled sediments. Natural sediments from the ponds were 

 incubated in a solution of radioactive phosphate for 1 week with frequent 

 shaking. Larvae were permitted to feed on the sediments at 12 to 15°C. 

 Every 3 hours, the larvae were individually rinsed and transferred to 

 scintillation vials. Cherenkov radiation in the water was measured, then 

 larvae were returned to the labeled sediments for another 3 hours. When 

 the rate of increase in ^^P uptake slowed, larvae were presumed to have 

 filled their guts with labeled sediment. From the concentration of ^^P and 

 organic carbon in the sediments, the amount of carbon in sediments 

 ingested by the larvae was estimated as 120 to 150 mg C (g C larvae) ' 

 day"' or 12 to 15% of their carbon content per day. This is probably an 

 overestimate because of relatively high experimental temperatures. Again 

 using data on larval biomass from Pond J and the lower ingestion 

 estimate, the community feeding rate becomes 325 mg m^ day"' in 1972 

 and the assimilation to ingestion ratio is 0.10. This is not unusual for 

 deposit feeding invertebrates (Cammen 1978, Ladle 1974). 



In Barrow ponds neither algae nor diatoms in whole sediments are 

 abundant enough to supply the chironomid demands. Data from Chapter 4 

 for the carbon content and specific gravity of the upper 2 cm support a 

 calculation of 7.5 cm^ m "^ as the volume of whole sediment ingested by 

 the chironomid larvae each day. According to studies of epipelic algae in 

 the ponds there are only about 0.21 mg of algal organic carbon in the 7.5 

 cm ^ . This is less than 1 % of the organic carbon requirements of the larvae, 

 so if epipelic algae are an important component of the diet, the larvae must 

 feed on them selectively. Kajak and Warda (1968) indicate that 

 Chironomus can achieve 5 times selectivity for diatoms {Melosira sp.) 

 relative to detritus on a volume basis in an alga-rich sediment (2.2% by 

 volume). If thaw pond larvae are selecting algae over detrital carbon with 

 a factor twice that observed in Polish lakes, or 10 times, they would obtain 

 just 2 mg C m~^ day~\ less than 10% of their requirements, from this 

 source. 



