THE RECEIVING ECOSYSTEM

The aquatic ecosystems that receive pulp mill effluents in North America are extremely varied and the effluents themselves may contribute to the physical and chemical loading of the ecosystem in a number of ways. Almost all receiving systems have sediments that may act as sinks for organic chemicals that are in solution or are bound to particles. Where flow of water is of sufficient velocity, these sediments may be moved for some distance. Biological components of these systems are perhaps even more varied. However, these systems do have some common biological components. Primary producers in the form of algae and macrophytes are usually present. Both planktonic and benthic invertebrates will be present and fish will also likely be present. These organisms will be present in a complex interactive food web with various trophic levels and degrees of dependency.

The primary focus of past assessments of the effects of bleached kraft mill effluents has been on the community of organisms restricted to the water column and the sediments. Contaminants may be transferred from aquatic ecosystems to terrestrial animals by fish consumption. It is important to appreciate that the chemicals present in the effluent, which ultimately cause biological responses (such as, for example, enzyme induction), can undertake several pathways that may involve appreciable temporal delays (adsorption to particles, sedimentation, resuspension, bioaccumulation and food chain transfer). It may thus be some years before field studies can reliably reveal the results of process change. These observations may be further confounded by year-to-year variability and by changes in community structure resulting from changes in discharge of organic matter.

Organism-level responses of aquatic biota to environmental contaminants are often determined in controlled laboratory exposures or "toxicity tests". Acute lethality is usually measured via the median lethal concentration or LC₅₀. For changes in reproduction or growth, the NOEC (no observable effect concentration) or the lowest exposure concentration tested having a significant effect (LOEC) is used. The NOEC or LOEC is normally measured over a time period that spans the duration of a critical lifestage (e.g., 21 days for reproduction in Daphnia sp., 7 days for reproduction in Cenodaphnia sp., 60 days for larval growth in trout).

Traditionally, ecosystem-level effects have been assessed through examining the structure and function of communities. A toxic insult to an ecosystem may have a number of consequences (Ellis, 1989). These may start at the highest level of sensitivity (lowest concentration) with a metabolic response in the organism to the presence of low levels of the toxicant (Figure 5). At higher concentrations or after a longer time interval, potentially more serious effects may occur. The most important eco-toxicological premise used in risk assessment is that, before a population, species or an ecosystem is adversely affected, some biological response to the toxicant -- a physiological, biochemical or cellular adaptation (biomarker) -- will have occurred and will be observable earlier than, or at lower concentrations than the adverse organismal or population-level effects. Similarly, for a decrease in diversity to be observed in an ecosystem, extinction or significant changes in numbers in the species must have occurred.

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