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National Assessment of Pulp and Paper Environmental Effects Monitoring Data

4.1.2 National Trends

National EEM Data Assessment

Previous studies have provided insight into the expected effects of pulp mill effluent on benthic invertebrate communities (Lowell et al., 1995, 1996, 2000; Lowell and Culp, 1999, 2002; Culp et al., 2000a,b), and the overall nature of these interacting effects for the endpoints of abundance, taxon richness, and community structure is summarized in Figure 2. In the figure, the x-axis progresses from better quality effluent and less deleterious effects on the left to poorer quality effluent and more deleterious effects on the right. Historical effects (e.g., smothering effects of fibre mats generated over many years of mill operation) may also come into play.

Nutrient enrichment (or eutrophication) generally increases from left to right, along with increasing toxicity or smothering effects. In fact, nutrient enrichment can often be measured at lower effluent concentrations than toxicity, and toxic effects are often masked by eutrophication at low to medium concentrations. Mild eutrophication is typified by increases in both abundance and taxon richness. Progressing to the right, moderate eutrophication is typically associated with lessened increases in taxon richness, although further increases in abundance may still occur. More pronounced eutrophication is commonly associated with decreases in taxon richness, even while abundance is greater than that found in reference areas. Finally, decreases in both taxon richness and abundance are typically a sign of overall inhibitory effects, such as toxicity or smothering.

In the EEM program, changes in invertebrate community composition are measured by changes in the Bray-Curtis index of dissimilarity. The value of this index increases as poorer quality effluent leads to greater changes in community composition. This point is illustrated at the bottom of Figure 2, although it should be noted that changes in community composition are not always tied to changes in total abundance and taxon richness. There are many different ways in which benthic communities may be affected in the field; due to complex direct and indirect effects, effluent exposure may lead to pronounced effects on community composition without large effects on abundance or taxon richness, and vice versa.

Predicted response patterns

Figure 2: Predicted response patterns (modified from Lowell et al., 2000; see also Culp et al., 2000a)

The average overall effects on benthic invertebrate communities associated with exposure to effluent are illustrated in Figure 3. These means represent the averages across the country for all habitat types and, as for the following figures, focus on mills using a Control/Impact study design. Essentially, total abundance was well to the right of the zero effect (or vertical) line, meaning that the most common effect associated with exposure to effluent was an increase in invertebrate numbers. This is a well-known eutrophication-type response to exposure to pulp mill effluent (Hall et al., 1991; Dubé and Culp, 1996; Chambers et al., 2000; Culp et al., 2000a,b; Lowell et al., 2000; Lowell and Culp, 2002) and is due to the nutrient enrichment effects of the phosphorus, nitrogen, and organic content of the effluent.

Grand mean of benthic invertebrate endpoints for Control/Impact study designs

Figure 3: Grand mean of benthic invertebrate endpoints for Control/Impact study designs

Figure 3 shows that in contrast to abundance, the taxon richness grand mean was close to zero. This reflects a more even distribution of mills with increases, mills with decreases, and mills with no significant change in number of taxa. The abundance and taxon richness distributions, taken together, indicate that the most common effect across the country was an increase in abundance and either an increase or no change in taxon richness, indicative of mild eutrophication (see Lowell et al., 2003). The smaller number of mills with increases in abundance and decreases in taxon richness were showing typical signs of more pronounced eutrophication. The evenness grand mean was close to zero, also reflecting a more even distribution of mills with increases, decreases, or no significant change in evenness. On the other hand, the Bray-Curtis grand mean was highly significantly positive. This shows that there were clear changes in benthic invertebrate community composition in exposure areas across the country. Because of the way in which the index is calculated, any changes in community composition in exposure areas result in positive (not negative) Bray-Curtis values, with larger effects yielding larger Bray-Curtis values. This endpoint is very sensitive and as such tends to yield more responses. Effects detected by the Bray-Curtis index therefore tend to be highly significant.

Effluent-associated effects on benthic invertebrate communities varied significantly among different habitat types. The main habitat types were riverine (rivers) -- erosional or depositional; lacustrine (lakes); and marine/estuarine. Abundance was significantly increased in both erosional and depositional riverine habitats, whereas the distribution of mill responses for taxon richness mostly ranged from increases to no significant change (Figure 4). This combination of responses is indicative of an average effect of mild eutrophication in riverine exposure areas. In contrast, both abundance and taxon richness decreased in marine/estuarine habitats, a response indicative of a toxicity or smothering effect. This inhibitory response in marine/estuarine habitats is likely due to a combination of factors, including the deterioration of habitat at several of the marine mills due to the long-term deposition of fibre mats. In erosional riverine habitats, the data suggested that evenness was reduced in exposure areas, which may reflect the sensitivity of organisms in these habitats to even mild eutrophication. In contrast, evenness was almost significantly increased in marine/estuarine exposure areas, suggesting that toxicity/smothering effects may have led to the loss of more rare taxa, leaving behind a smaller number of more evenly distributed, pollution-tolerant taxa.

The largest exposure area effects for the habitat subgroups were observed for the sensitive Bray-Curtis endpoint. By far the largest exposure-associated changes in community composition were measured in lacustrine habitats, in contrast with other endpoints, where the measured lake effects were more intermediate. This illustrates that, under different environmental conditions, effluent-related effects may be manifested in different ways, and it underscores the value of measuring changes in community structure using multiple endpoints to reduce the probability of missing important effects.

Pulp mills in Canada use a wide variety of effluent treatment and mill process types. Mills were grouped into broad categories in order to determine whether certain effluent treatment and mill process types were associated with greater or lesser effects in the environment or whether the different groupings are more or less equivalent in terms of their environmental effects. The two most common mill process types were kraft and thermomechanical, and the two most common effluent treatment types were activated sludge treatment and aerated stabilization basin. Since these represented the majority of the mills, the patterns observed in the meta-analyses apply on a national scale.

Abundance and taxon richness by habitat (Control/Impact mills + Gradient mills)

Figure 4: Abundance and taxon richness by habitat (Control/Impact mills + Gradient mills)

The abundance/taxon richness response patterns for the two prevalent effluent treatment types were similar and reflected the mild eutrophication effect that was most commonly observed for benthic invertebrate communities in Cycle 2 (Figure 5). The two major effluent treatment types were also quite similar in terms of effects on evenness and were not significantly different from zero. Overall, the magnitudes of effects were larger for the Bray-Curtis endpoint, and these measured changes in community composition were fairly similar between the two treatment types.

Abundance and taxon richness by effluent treatment (Control/Impact mills)

Figure 5: Abundance and taxon richness by effluent treatment (Control/Impact mills)

Exposure area abundance was significantly increased for the two most common mill process types (Figure 6). The two types differed for the taxon richness endpoint, with the results suggesting a milder eutrophication response for kraft mills and a more pronounced eutrophication response for thermomechanical mills on a national scale. The two mill process types had fairly similar responses for the evenness endpoint. The results indicated that individual mills that had either significant increases or decreases in evenness could not be distinguished on the basis of mill process type. The magnitude of effect was greater for the Bray-Curtis endpoint, but the two mill process types did not significantly differ.

Abundance and taxon richness by mill process (Control/Impact mills). Kraft = kraft mills; TMP = thermomechanical pulp mills.

Figure 6: Abundance and taxon richness by mill process (Control/Impact mills). Kraft = kraft mills; TMP = thermomechanical pulp mills.

Although apparently less pronounced than in previous years, effects of pulp mill effluent on benthic invertebrate communities are still widely present. For the invertebrate community survey endpoints, between 20 to 40% of the mills detected effects larger than two standard deviations. The most predominant response pattern observed in benthic invertebrate communities across the country was one of mild to moderate eutrophication, particularly in riverine habitats, although more pronounced eutrophication was observed at a few mills. In contrast, the average response pattern observed in marine/estuarine habitats was more indicative of toxic or smothering effects. The benthic invertebrate response in lakes was intermediate between those in riverine and marine/estuarine habitats. These response patterns were national averages, and exceptions occurred in each of the habitat subgroups. Further monitoring and accompanying research will help provide a better understanding of the extent and ecological significance of these effects.