290 R. G. Stress et al. 



From his studies, Saunders (1969, 1972) concluded that not only was 

 detritus a food for zooplankton but it also acted as a biological buffer that 

 evened out the fluctuations in food supply. He states (1969) that ". . . low 

 calorie detritus in large amounts . . . acting as a dilutant, . . . may also 

 dampen the assimilation of high quality algae, particularly when the seston 

 is present in quantities approaching the incipient limiting concentration 

 for feeding. This dampening would tend to suppress the growth rate of an 

 exploding zooplankton population and perhaps permit the phytoplankton 

 community to adjust to a high rate of grazing." 



Zooplankton Feeding and Seston 



As Saunders states above, it is important to establish that the seston 

 concentration in the tundra ponds is above the limiting concentration for 

 zooplankton feeding. We were not able to determine what this limiting 

 concentration was, using the natural seston. However, we did determine 

 that dilution of the water sample, which reduced the concentration of 

 seston, increased the filtration rate of Daphnia. The seston in these ponds 

 is made up of only 5 to 13% living matter (algae and bacteria) and is 

 always greater than 200 ^g C liter"'. In Pond B in 1971, the detritus was 

 300 to 2000 Mg liter ' while the algal biomass was 0.6 to 1 5.0 Mg C liter ' . 



The ingestion rate of Daphnia middendorffiana rises with increasing 

 food concentration then reaches a constant, saturated rate at about 10^ 

 Chlamydomonas cells liter ' (Figure 6-9, Chisholm et al. 1975). The point 

 at which the rate became constant is a little less than 450 Mg C liter"'. 

 (The experiment was carried out with natural pond water containing 200 

 Mg detrital C liter " '. The Chlamydomonas cells were 5% carbon and had a 

 volume of 500 ^m^ cell ' to give 250 ^g C liter " '.) This is also the point 

 above which the volume cleared per hour drops sharply from 8 ml or so 

 down tq 7 ml (Chisholm et al. 1975). It appears that above this point the 

 filtering apparatus is delivering all the food to the food groove that the 

 Daphnia can handle. Thus, the energy intake of the animal is limited by 

 the quality of the food. If the filtered seston is of poor quality, that is, 

 made up mostly of unusable detritus, then the animal may not be able to 

 obtain enough food for growth despite the abundance of seston and the 

 presence of algae. It should be stressed that we imply in this section that 

 carbon or energy is limiting the zooplankton growth. In reality it may just 

 as well be digestibility or even nitrogen content. Sick (1970) proved that 

 growth and reproduction in the brine shrimp, Artemia, correlated best 

 with the nitrogen content of the different algae they were fed. 



The concentration at which saturation of the feeding apparatus 

 begins, around 450 ^g C liter ', is about the same for D. middendorffiana 

 as for other large zooplankters. For example. Bell and Ward (1968) found 

 a constant ingestion rate in D. pulex when the food was above 200 jug C 



