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SECTION 3: ANALYSIS
Scatter plots associated with regression analyses for all chemicals, test media adjustment
groups, and data transformations for amphipods are presented in Appendix H. Additional
plots of exposure response results for selected compounds are presented in Figures 3-17
through 3-29.
Mussel (Mytilus edulis) Regression Analysis
Results of the Mytilus toxicity bioassay consisted of counts of individuals surviving and
counts of individuals displaying abnormal development. Because sample sizes were not
constant among replicates, all Mytilus effects data were expressed as the proportion of
individuals at the start of the bioassay. Proportions were arcsine-square root transformed
(Zar, 1984) prior to analyses to correct for non-normality of proportion data. The
independent variables consisted of untransformed and natural-log-transformed
concentrations of chemicals in pore water (µg/L). Simple linear regression analyses were
performed using SAS (1994) PROC REG. All models were considered significant if p#0.05.
Prior analyses indicated that Mytilus survival was poorly related to chemical concentrations
in pore water, regardless of whether data were untransformed or natural-log transformed.
Significant regression fits were obtained for only 10 of 46 compounds based on
untransformed data and for only 8 of 46 based on transformed data. Values for r 2 were low
for both approaches, ranging from 0.0058 to 0.1 for untransformed data and 0.0053 to
0.33 for transformed data. Due to the low quality of the relationship between Mytilus
survival and pore water concentrations, this analysis was not pursued further.
In contrast to Mytilus survival, the proportion of normal Mytilus was strongly related to
chemical concentrations. Significant model fits were obtained for 37 of 41 chemicals based on
untransformed data and 39 of 41 chemicals for transformed data (Table 3-18). The proportion
of normal Mytilus was negatively related to chemical concentration for all chemicals
evaluated. Among models for untransformed data, r 2 ranged from 0.029 (silver) to 0.84
(aldrin; Table 3-18). For models based on transformed data, r 2 ranged from 0.04 (4,4’-DDD) to
0.83 (4-nitrophenol, chrysene, high molecular weight PAHs, and 4-methylphenol; Table 3-18).
With the exception of eight compounds (aldrin, arsenic, barium, beryllium, chromium,
endrin aldehyde, total phenol, and vanadium), natural-log-transformed data explained more
variability in the proportion of normal Mytilus than did untransformed data (Table 3-18).
Although pore water used for the Mytilus toxicity tests was adjusted in a manner
comparable to the sediment used for the amphipod tests (e.g., derived from wet or dry
sediment, with or without salinity adjustment), because no relationship was found between
sediment concentrations and pore water concentrations (see below), regression models for
the four test media adjustment groups were not developed.
Scatterplots associated with regression analyses for all chemicals and data transformations
for Mytilus are presented in Appendix H. Additional plots of exposure-response results for
selected compounds are presented in Figures 3-30 through 3-38.
Estimated Effect Levels
In addition to determining which compounds best described effects observed in the toxicity
bioassays, regression models were used to estimate concentrations in sediment and pore
water that were associated with 20 and 50 percent lethality (i.e., LC 20 or LC 50 ) or effects (i.e.,
EC 20 or EC 50 ) concentrations. LC and EC values were only calculated for those chemicals
ERA REPORT 3-34 SAC/143368(003.DOC)
7/31/02