320 



MARSHALL AND MELLINGER 



TABLE 1 



INCUBATION (EXPOSURE TIME) AND LINEAR REGRESSION 



COEFFICIENTS FOR RELATIONSHIPS AMONG RATES 



OF INCREASE AND ADDED CADMIUM FOR MAJOR GROUPS 



OF CRUSTACEANS FOR IN SITU EXPERIMENTS IN 



LAKE MICHIGAN* 



Experiment 



Exposure time, 

 days 



Cladocera 



Calanoida 



Cyclopoida 



*Added cadmium ranged from to 50 Idg/Mter. 



are -0.0131 ± 0.0032, -0.0058 ± 0.0009, and -0.0010 ± 0.0003, 

 respectively (mean ± standard errors). 



The effects of cadmium on different species or other categories 

 within each major crustacean group in experiment 5 are represented 

 by the linear regression equations and corresponding lines fitted to 

 the mean values, r (Fig. 1). The data from v^hich these values w^ere 

 calculated are summarized in Table 2; summaries of the data from 

 experiments 1 to 4 are available (Marshall and Van Reken, 1977). 

 Differences in apparent sensitivity among species or other categories 

 v^^ithin each major crustacean group, as indicated by the differences 

 in the slopes (steepness) of the lines in Fig. 1, are quite small in 

 comparison v^ith the differences among the major groups. The 

 immature copepodites of both calanoid and cyclopoid copepods 

 appear to be no more sensitive to cadmium than the adults are. 

 Furthermore, the mean number of copepod nauplii in samples at 

 each added cadmium level (Table 2) w^as not significantly different 

 (P > 0.1) from that of the controls. 



At higher ranges of added cadmium (> 50 jug/liter) in experi- 

 ments 3 and 4, the relationships between r and cadmium are better 

 represented by curves (not shown) whose slopes decrease with 

 increasing cadmium. This is most pronounced in Cladocera and is 

 noticeable even for the to 20 ng Cd /liter range in the data for 

 Bosminidae plus Daphnidae in Fig. 1. Even in this range the data are 

 fitted better by an exponential function. In other words, the 

 sensitivity of Cladocera in the range of to 5 fig Cd /liter is probably 

 greater than that indicated by the slope of the line in Fig. 1. 



