218 V. Alexander etal. 



dominant species, has a high filtering rate. At 1 1°C, it filters from 2 to 6 ml 

 of pond water each hour depending on food density (Chisholm et al. 1975). 

 At average in situ filtering rates and average population densities, D. 

 middendorffiana is capable of filtering the entire pond volume within 2 

 days. 



Two different calculation procedures show that the pond zooplankton 

 probably ingest an average of 20% of the annual primary production in the 

 ponds. The first assumes that the zooplankton production (growth and 

 reproduction) amounts to about 1 mg C liter"' yr"' (Chapter 6) and that 

 zooplankton assimilation efficiency is about 10%, while 50% of the amount 

 assimilated is lost to respiration. The zooplankton would then ingest 

 approximately 20 mg POC liter '. If the maximum volume or quantity of 

 food (in mg C) that is ingested by Daphnia is unaffected by food particle 

 size (McMahon and Rigler 1965), so that the small algae are ingested at a 

 rate determined only by their volume proportion in the total pond seston 

 (Chapter 6) which is 5% on the average, then 1 mg of algal C liter ' yr ~ ' is 

 lost to zooplankton grazing. Since the average annual phytoplankton 

 production is about 5 mg liter'', 20% of pond primary productivity is lost 

 to zooplankton. In the second calculation, total volumes of water filtered 

 over several time periods were calculated from filtration rates per animal 

 and from animal densities; total ingested sestonic carbon was then 

 calculated from average particulate carbon concentrations during that 

 time period. From this procedure the same approximate annual 

 zooplankton consumption rate, 1 mg C liter ', was estimated. The 

 magnitude of the grazing losses will be determined in large part by 

 environmental conditions, which affect zooplankton survivorship, and by 

 the quantity or quality of seston available in any year, which affects the 

 assimilation efficiency of the zooplankton. 



The strong grazing pressure may be hypothesized as an explanation 

 for the apparent paradox that P to B ratios in the plankton of the ponds 

 are very high relative to eutrophic systems despite the severe limitation of 

 planktonic photosynthesis by phosphorus in the Barrow Ponds. In this 

 hypothesis, the high rates of zooplankton ingestion are a means of 

 cropping algal cells before they become "senescent", that is, before their P 

 to B ratios begin to decline. This argument has been used to explain high 

 turnover rates in bacterial microcosms grazed by a phagotrophic ciliate 

 (isolated from the Barrow ponds) in comparison with systems with 

 bacteria alone (Barsdate et al. 1974). However, as discussed earlier, it is 

 likely in this case that the short-term changes in Pio B ratios during algal 

 blooms result from the sudden onset of phosphate limitation. 



The studies of several ponds that contained added crude oil allowed 

 further evaluation of the effect of zooplankton (details are given in 

 Chapter 9). When oil was added to the entire pond (1970 and 1975), the 

 zooplankton were killed while the algae survived. At a high dose rate of oil 

 (1970), the algal production and biomass did not appreciably change from 



