Artificial Streams Pinpoint Effects of Aquatic Stresses

Aquatic plants and animals are often exposed to more than one environmental stressor at a time—for example, municipal sewage combined with the nutrients and contaminants from pulp-mill effluent. Assessing the cumulative effects of these substances on aquatic organisms has been a difficult task until recently, when scientists at Environment Canada's National Water Research Institute (NWRI) developed a system for evaluating individual and combined impacts.

Cumulative effects assessments are required for all proposed developments under the Canadian Environmental Assessment Act as a means of predicting adverse impacts and designing ways to prevent or mitigate them. The problem is that it is not always easy to determine which substance is causing which effect, or to what extent the presence of one substance lessens or increases the effect of another. Many of the standard techniques used for bioassessment cannot establish a clear cause-and-effect relationship. In the natural environment, for example, researchers cannot always be certain about the concentration or duration of exposure, nor can they easily replicate experiments.

Over the past six years, an NWRI research team has developed a mesocosm, or artificial stream system, that can "tease out" the effects of multiple stressors, such as nutrient-contaminant or metal-contaminant interactions, and multiple metal contaminants. This allows them to examine the effects of individual stressors and also to evaluate their combined effects.

The mesocosm system bridges the gap between laboratory studies, where variables are strictly controlled, and the natural world, where researchers have very little control over factors affecting their experiments. It consists of a series of circular streams—the originals had a volume of approximately half a cubic metre each—that are transported to the riverside on a flatbed truck. Under ambient light and temperature conditions, river water is pumped through the streams to simulate its current. Substrates, or stream beds, are created using rocks and other river materials, and a "biofilm"—made of sediment and tiny organisms that have settled out of the water—is given time to develop before benthic, or bottom-dwelling, invertebrates and small fish are introduced.

Researchers have used the system to study the effects of pulp-mill and metal-mining effluent in several large rivers across Canada: the Athabasca River in northern Alberta, the Thompson and Fraser rivers in British Columbia and, most recently, the Saint John and Little rivers in New Brunswick. They focussed their attention on impacts on benthic communities, looking at community structure and at the growth and reproductive health of various community members.

In the Athabasca experiment, researchers were able to separate the effects of contaminants and nutrients in bleached-kraft pulp-mill effluent on benthic algae and invertebrates using three treatments: the first containing no effluent; the second containing the one-per-cent effluent concentration typically found in the river; and the third containing a nitrogen-phosphorus solution with nutrient levels similar to those of the effluent treatment.

They found that, relative to the control treatment, algal growth and insect numbers increased with exposure to both the effluent and nitrogen-phosphorus treatments, and there was a shift in algal community structure. Algal and insect biomass did not differ significantly between the two treatments, suggesting that at concentrations found in the Athabasca River, the response to nutrient enrichment overwhelms any negative effects caused by the contaminants.

Since the Athabasca study, NWRI has refined its system and now uses much smaller mesocosms that can be transported more easily and cost-effectively to the research site. With support from the federal Toxic Substances Research Initiative, researchers are currently evaluating the system's effectiveness in assessing impacts not only on algae and benthic invertebrates, but also on fish and the aquatic ecosystem as a whole—research that contributes directly to the efforts of government and industry to reduce risks to the Canadian environment.

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