318 MARSHALL AND MELLINGER 



adding small portions in turn to each carboy. Two to four additional 

 carboys, which were not to be incubated, were filled at the same 

 time. When all the carboys were nearly filled, those to be incubated 

 were assigned prescribed cadmium additions at random. The concen- 

 trations of added cadmium in the five experiments were 0, 50, 100, 

 150, and 200 jug/liter in experiments 1 to 3; 0, 25, 50, 75, and 

 100 Mg/liter in experiment 4; and 0, 5, 10, 20, and 40 /ig/liter in 

 experiment 5. After the cadmium was added, each carboy was 

 capped, thoroughly shaken, and finally filled completely through a 

 small plughole in the cap to exclude all air bubbles. This last step is 

 very important because of the propensity of small cladocerans, such 

 as Bosmina, to become trapped at air— water interfaces. 



For in situ incubations the carboys were suspended in the 

 epilimnion at depths of 2 to 12 m in the sampling area. Inclusive 

 dates of the incubation periods for the five experiments were 1, 

 July 22—26; 2, July 27- August 1: 3, August 20-27; 4, August 27— 

 September 11; and 5, September 11—20. At the end of each incuba- 

 tion, the zooplankton in each carboy were removed by filtering 

 through a nylon net with apertures of either 153 jum (experiments 1 

 to 3) or 85 /im (experiments 4 and 5), narcotized with "club soda" 

 (Gannon and Gannon, 1975), and preserved in 4% Formalin. 



Laboratory Methods 



The zooplankton were identified vidth the aid of keys, illustra- 

 tions, and descriptions in Edmondson (1959) and Torke (1974) by 

 use of binocular compound and dissecting microscopes. The identifi- 

 cation categories in each entire sample were enumerated in an 

 open-top, chambered counting cell (Gannon, 1971) with a binocular 

 dissecting microscope. Special care was taken to count only 

 specimens that showed no signs of decomposition. 



Calculations and Definitions of Symbols 



As in the work of Edmondson (1968), rates of increase, r, were 

 calculated from the equation 



In Nt - In Ni 



where Nt is the terminal number of individjaals of a given 

 identification category in an incubated sample, Nj is the average 

 initial number in the unincubated samples, and At is the duration of 

 incubation in days. 



