Effects of Chemicals Produced During Bleaching on the Aquatic Environment
Comparing the effects of changes in the impact of the bleaching process used in pulp mills on populations and communities is complicated by several factors related to changes in the local environment, seasonal differences and the continuum of process improvements that occur in most mills. In species such as fish, shifts in populations and communities caused by sublethal responses may be slow and difficult to observe because of their relatively long (5-8 year) lifespan. In these cases, bioindicators of exposure and effect may be very useful. However, to be most useful, these indicators should be indicators of effect that are linked to and predictive of responses at the population or community level.
Field studies capture a small window in time with sometimes substantial differences between sites (dilution factors, physical environment); so results are usually not directly comparable between studies. Unless field studies are conducted in a consistent and structured manner, they may offer little data for hypothesis testing. Studies in meso- and micro-cosms offer considerable experimental control and are better for hypothesis testing. However, by their nature, they may inadvertently exclude keystone species or communities that are critical to the structural integrity of the larger environment for which they act as surrogates.