Zooplankton 293 



Another explanation of the inverse relationship between 

 zooplankton and seston is that the detritus portion of the seston is less 

 abundant in some years than in others for unknown reasons (perhaps less 

 wind). The algae and bacteria grow as usual so that their portion of the 

 seston increases and the improvement in food quality produces more 

 zooplankton. 



Experiments 



Additional experiments by Miller and Federle in 1976 and 1977 help 

 to explain the results of the pondlet experiments. In these pondlets, 

 increased numbers of Daphnia and fairyshrimp increased Daphnia 

 production up to a certain limit above which the additional numbers 

 decreased production (Tables 6-10, 6-1 1). The 1976 and 1977 experiments 

 made use of 20-liter containers of pond water incubated in Pond C. 

 Additions of Daphnia and fairyshrimp at natural concentrations did not 

 change the amount of POC, the rate of primary productivity, the bacterial 

 density, or the bacterial activity (acetate- '^C uptake) from rates and levels 

 in control chambers with no zooplankton. However, zooplankton 

 additions at 5 times natural concentrations increased the POC and the 

 bacterial activity. 



The results indicate that in these chambers the POC is not controlled 

 by zooplankton grazing at natural densities of animals. Only at the five 

 times natural level of animals was the POC affected and there the high 

 POC came from fecal material. From these and other experiments (the 

 bacteria-protozoa-P04 cycling described in Barsdate et al. 1974), it 

 appears likely that increased zooplankton production when crowded 

 comes from increased rates of nutrient cycling. 



However, conditions in the chambers are highly artificial. In the 

 natural ponds, the zooplankton do not change the POC concentrations 

 appreciably and can not affect the PO4 cycling which is so dominated by 

 sediment-water interactions. 



It still is reasonable to us that the seston-POC-food quality 

 interaction is affecting the zooplankton productivity. Control of the seston 

 concentration must come from an external agent such as wind currents. 



SUMMARY 



The ponds contain calanoid and cyclopoid copepods, two species of 

 fairyshrimp, and Daphnia middendorffiana. Rotifers are present but rare. 

 All the crustaceans except for Daphnia have a single brood per year; the 

 offspring of the overwintering generation enter diapause and overwinter in 

 this form. The embryos of calanoid copepods and of the fairyshrimp 



