As previously highlighted, it is important when selecting biomarkers for ERA that the naturally occurring variation in biomarker response is clearly understood. This is 1 of the most difficult aspects when making ecological predictions based on biomarker analyses (Depledge 1994). Relatively few studies have taken into account abiotic- and biotic-driven differences in biomarker response within the population of
indicator organisms. Table 2 shows that environmental factors such as temperature, salinity, turbidity, diet, and season may contribute to biomarker variation either by altering the pollutant bioavailability or resulting from natural biological
changes such as growth and reproductive stage. In a study