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National Assessment of Pulp and Paper Environmental Effects Monitoring Data

4.1.1 Benthic Invertebrate Community Survey Data Analysis

National EEM Data Assessment

The benthic invertebrate community survey is one of the two core elements of the EEM program. Two main types of benthic invertebrate study designs were used in Cycle 2: 62 Control/Impact and 21Gradient studies. The Control/Impact design involves one or more discrete groups of sampling stations exposed to mill effluent (typically downstream from the mill) and another group of sampling stations that are not exposed (often located upstream). The Gradient design involves the use of sampling stations distributed more continuously in a gradient from more exposed to less exposed areas -- that is, progressing from areas close to the effluent discharge point to more distant areas where the effluent is much more dilute (for more details, see Environment Canada, 2002; Glozier et al., 2002; Lowell et al., 2002).

The national EEM data assessment was, in part, carried out using statistical comparisons of benthic invertebrate community endpoint values between exposure and reference areas based on the Control/Impact design. Analyses of Gradient design data were carried out using regressions of endpoint values against distance from the outfall. The results of the analyses based on the Control/Impact and the Gradient designs were then integrated using a detailed statistical analytical procedure termed meta-analysis, which facilitates the synthesis of a large number of independent studies (Gurevitch and Hedges, 1993; Rosenberg et al., 2000;). For the national EEM data assessment, meta-analyses gave a quantitative indication of the general conclusions that could be drawn concerning the nature of effluent-associated effects and provided some answers to questions that would be very difficult to address at the individual mill level, such as the influence of habitat type, effluent treatment type, and mill process type on effects observed in the field. As such, the national EEM assessment addresses a number of issues that have received little (if any) attention in the past.

The results from the meta-analyses are presented in the summary format shown in Figure 1. The x-axis represents the standardized effect size, and the vertical line represents no effect. The results for each mill grouping are presented as a 95% confidence interval (horizontal line segment), with the tick mark indicating the average effect size for that grouping of mills. Mill distributions (i.e., 95% confidence intervals) to the right of the zero effect line indicate that the average effect associated with effluent exposure was an increase in the measured endpoint. Similarly, mill distributions to the left of the zero effect line indicate an effluent-associated decrease in the measured endpoint. As long as the 95% confidence intervals do not overlap with the zero effect line at any point, the increase or decrease is considered statistically significant for the group as a whole, although the group may also contain subgroups exhibiting statistically significant effects.

Meta-analysis summary figure

Figure 1: Meta-analysis summary figure