Zooplanklon 271 



Coincidentally, a particulate concentration of 100 ng C liter ', which 

 is the saturation level for filtering, is near the actual amount of living 

 biomass in the ponds. However, there is also a much larger component of 

 detrital-like particles in suspension which dilutes the living material. The 

 quantity of detritus ranges from 250 to 800 ^lg C liter '. Although no 

 actual calculations have been made, the effect of dilution can be accounted 

 for in the calculation by lowering the assimilation efficiency. However, if 

 the assimilation efficiency were reduced from 70% to 14%, then 500 ^g C 

 liter ' would be needed to produce the same energy for the animal as 100 

 ng C liter ' of living biomass. We could not go further with the question 

 of how zooplankton handle low quality detritus; it is an unknown area in 

 our understanding of this or any other system. 



When Daphnia were held under conditions of constant light and 

 temperature, they had peaks of feeding activity at 1400 and 2400 

 (Chisholm et al. 1975 and Figure 6-lOa). It may be significant that these 

 are the same times that the water temperature of the pond, measured on a 

 clear day (Figure 6-lOb), passes through the temperature optimum for the 

 feeding of this species. These peaks could be caused by oscillations in 

 metabolic activity similar to those found by Ringleberg and Servass (1971) 

 for the phototactic response in Daphnia. Daily rhythms are also known for 

 arctic algae (Miiller-Haeckel 1970), invertebrates (Mendl and Miiller 

 1970) and fishes (Miiller 1970) and double activity peaks have been found 

 in copepods (Spindler 1971a, 1971b) for egg laying and molting. There is 

 also the possibility, suggested by Remmert (1969), that two discrete 

 populations are present, with one active only in the daylight and one active 

 at night. It is difficult to assess the importance of the daily rhythms at 

 Barrow because clear days are so rare (as described in Chapter 2, cloud 

 cover averages 80 to 90% during July and August). 



Synchrony 



The Crustacea in each of these ponds exhibit a certain degree of 

 seasonal synchrony in their major life cycle events such as hatching and 

 molting. This was measured in a 1971 experiment in which overwintering 

 embryos (ephippia), collected from a trough pool {D. pulex) and from 

 Pond B (Z). middendorffiana) on 15 June, were incubated in glass tubes in 

 water from their own pond under natural light and temperatures. A 

 parallel set was also incubated under constant light and a constant 

 temperature of 10°C. All tubes were inspected six times each day during 

 the hatch and the young removed. 



A seasonal synchrony for each species was certainly evident but a 

 daily synchrony of hatching was not demonstrated (Table 6-9). The best 

 example is for the D. pulex embryos incubated under natural conditions, 

 whose hatch was 75% completed within the first 24 hours of the total 



