Stressor - vs Effects-Based Monitoring

It is helpful to differentiate between two approaches in environmental monitoring: monitoring of stressors, and monitoring of effects.

In the context of an effluent discharge, stressors are typically specific chemicals that may cause undesireable effects on specific receptors (i.e., organisms that are exposed to these stressors). Stressor-based monitoring typically predict the risk of effects, based on comparing the observed concentration of the stressor with the concentration known or considered to produce those effects.

Advantages: because of their predictive and stressor-specific nature, stressor-based monitoring can assess potential effects that have not already occurred, and typically incorporate safety margins (in calculations and comparisons against effects thresholds) favouring precaution.

Disadvantages: stressor-based assessments may be unrealistic, as safety margins sometimes compound unrealistically – assessments can reflect the worst case of the worst case of the worst case. Also effects thresholds are typically generic rather than site-specific, and are prone to selection bias – they consider only stressors known in advance to be relevant, and may therefore miss unknown stressors.

In contrast, effects-based monitoring is observational – it accurately reflects what is actually occurring in the environment.

Advantages: effects-based monitoring examines the effects of the entire effluent, not individual constituents, and integrates other factors, unrelated to effluent, that may contribute to or diminish effects.

Disadvantages: these other factors may confound identification of the actual cause of any observed effects, and, crucially, effects may only be assessed once they already have occurred – it is a responsive, rather than a predictive, approach.

Most monitoring programs incorporate both stressor-based and effects-based endpoints.