Abstract-The effective design of field studies requires that sample size requirements be estimated for important endpoints before conducting assessments. This a priori calculation of sample size requires initial estimates for the variability of the endpoints of interest, decisions regarding significance levels and the power desired, and identification of an effect size to be detected. Although many programs have called for use of critical effect sizes (CES) in the design of monitoring programs, few attempts have been made to define them. This paper reviews approaches that have been or could be used to set specific CES. The ideal method for setting CES would be to define the level of protection that prevents ecologically relevant impacts and to set a warning level of change that would be more sensitive than that CES level to provide a margin of safety; however, few examples of this approach being applied exist. Program-specific CES could be developed through the use of numbers based on regulatory or detection limits, a number defined through stakeholder negotiation, estimates of the ranges of reference data, or calculation from the distribution of data using frequency plots or multivariate techniques. The CES that have been defined often are consistent with a CES of approximately 25%, or two standard deviations, for many biological or ecological monitoring endpoints, and this value appears to be reasonable for use in a wide variety of monitoring programs and with a wide variety of endpoints.

Abstract-Regional assessments on large rivers often are complicated because these ecosystems receive multiple, interacting effluent discharges. Confounding factors, such as complicated mixing hydraulics and historical loading effects, can result in equivocal field data that lend weak inference to ecological risk assessments. Our approach to this problem develops a strategy that defines important mechanisms of pollutant effects through the combined use of laboratory and field measurements, riverside mesocosm experiments, and the incorporation of indicators at several trophic levels. We integrate these different types of information through weight-ofevidence postulates that provide logical guidelines for establishing causation in ecological risk assessment. Using this approach, retrospective risk assessments on the Fraser River, British Columbia, Canada, indicated that the major effect of present effluent discharges has been one of nutrient enrichment and stimulation of food web productivity. In fact, the Fraser River study suggests that small increases in effluent concentration in the river may produce negative ecological effects because of contaminant stresses. The combination of field experiments with this weight-of-evidence approach yielded the scientific justification for a conceptual model that describes community shifts across a nutrient-contaminant gradient. We conclude that the use of stream mesocosms with the weight-of-evidence approach is highly useful for establishing a mechanistic understanding of community responses to stressors at a regional scale. In addition, this approach will be useful when greater understanding of a particular class of anthropogenic stressors (e.g., specific types of effluent) is required to improve regulatory guidelines.

Theory predicts that selection should increase the ratio of the performance of a biological structure or system to the requirements placed upon it (that is, its safety factor) as conditions become increasingly unpredictable. Although conventional safety factors are rarely measurable, an alternative, truncation safety factor (the ratio of mean strength to maximum possible load), can be measured quantitatively for certain load-bearing structures. For intertidal limpet shells subject to prying forces, truncation safety factor was found to increase with increased variability in shell strength, thus providing direct support for the theory.

To determine the food‐dependent effects of biologically treated, bleached‐kraft pulp mill effluent on mayfly (Baetis tricaudatus Dodds) growth and development, mayflies were exposed to effluent for 2 weeks within artificial streams arranged in a 2 × 3 factorial design (two periphyton food levels ‐ low, high; three concentrations ‐ control river water, 1% effluent, 10% effluent). Although survival was not affected, the effluent had a significant stimulatory effect on growth (resulting in 20 to 50% greater body weights) and development of the wing pads. Furthermore, the initial growth and development trajectories suggest that effluent‐exposed mayflies may emerge sooner and at a larger size than nonexposed individuals. A key result was that the stimulatory effects occurred within both the low‐ and high‐food treatments. Thus, the effluent‐exposed mayflies grew‐faster than even the high‐food control animals, which already had access to more food than they could eat throughout the experiment. This shows that the stimulatory effect of the effluent on the mayflies involved more than just an increase in food‐availability due to enhanced algal growth in response to nutrients in the effluent. Three possible mechanisms for this growth‐enhancement effect are that the effluent (a) increased the nutritive value of the food, (b) enhanced the palatability of the periphyton, inducing increased mayfly feeding, and/or (c) directly stimulated increased mayfly growth via hormonal or other growth‐stimulation effects. Further work is required to determine whether the growth stimulation occurred at the expense of future reproductive output.

Two species of limpets appear to partition microhabitats according to substratum color within the mixed—barnacle zone of the Pacific Northwest rocky intertidal. A predominantly light—shelled species, Collisella digitalis, occurs most commonly upon the light—colored skeletons of barnacles (Pollicipes polymerus and Semibalanus cariosus), while a predominantly dark—shelled species, Collisella pelta, occurs most commonly on the dark valves of mussels (Mytilus californianus). Field experiments revealed significantly higher mortality of limpets mismtched to their background, due to predation by both fish and birds. When given a choice, in the absence of predation, both species moved actively onto the substratum where they were most cryptic. Strong selection by visual predators appears to maintain the partitioning of space by these limpet species. Competitive interactions are not required to explain this microhabitat partitioning, although they cannot be ruled out as one of the original selection pressures.

No abstract

Abstract-Northern river ecosystems are subject to a variety of stressors having multifaceted (and sometimes opposing) effects, making interpretation at a regional scale difficult. We have addressed this problem by using a weight-of-evidence approach that combines analysis of field data (to determine patterns) with experimental hypothesis testing (to determine mechanisms). Two of the more important sources of aquatic impacts in western Canada are pulp mill and municipal effluents. Their regional impacts on benthic biota were evaluated for two major river systems, the Thompson and Athabasca rivers, using an integrative approach. In the more southerly Thompson River, several lines of evidence (including field and laboratory experiments, field sampling over a 20-year period, and isotopic analysis) led to the conclusion that, although some toxic effects were apparent, these effects were usually masked by the (sometimes excessive) nutrient enhancement effects of these effluents, sometimes via novel pathways. Furthermore, analysis of the data revealed a fairly delicate balance in effluent treatment involving trade-offs between the negative effects of toxic contaminant loading versus a switch to a more eutrophic community. In the more northerly Athabasca River, effluent effects can be modified by the added impact of another stressor: widespread winter freeze-up, which prevents reaeration of oxygendepleted waters, coupled with low dissolved oxygen levels in the substratum where benthic invertebrates are found, resulting in a net shift in effluent effect from one of nutrient enhancement to a more inhibitory effect. Advantages to applying formalized causal criteria, as outlined in this weight-of-evidence approach, include helping to tie together diverse assemblages of data on the effects of multiple stressors and identifying important informational gaps, thus making ecological risk assessments more rigorous and robust.

The feeding behavior of rocky intertidal crabs in the tropical and temperate eastern Pacific was studied in relation to specific mechanical properties of the shells of their limpet prey. A series of laboratory experiments, involving direct observations, records of shell remains, and measurements of the forces generated by a feeding crab, showed that by far the most common feeding technique was to pry the margin of the limpet shell away from the substratum. The pattern of deformation in models of limpet shells subject to a similar prying force indicated (1) that the greatest stress on the shell was at the point of force application at the shell margin and (2) that the thickness of the shell margin contributed more to shell strength than did thickness in more apical regions of the shell. Measurements of the strength of real shells provided further support for this latter conclusion. In addition, the strength of foot attachment, which sets the maximum prying force that the shell can experience, closely paralleled shell strength. This linkage between foot tenacity and shell strength appeared to be maintained via the degree of allometry between foot area and the thickness of the shell margin. The potential for a particular predator feeding behavior to lead to selection for a defensive feature in shell morphology should be a function, not only of the frequency of occurrence of attacks, but also of the frequency of successful attacks. In particular, for selection to occur, some individuals must survive an attack so that they may pass on to their off spring the defensive feature that enabled survival. Compared to other crab feeding techniques, prying attacks on limpets occurred frequently and with low success. These data support the hypothesis that selection to resist prying forces has been an important feature in the evolution of limpet shell morphology.