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Index Period
The capacity of aquatic assemblages to respond to environmental stressors over time provides a foundation for developing bioassessment strategies. An index period is a consistent seasonal time frame for sampling the assemblage that is a cost-effective alternative to sampling on a year-round basis to account for seasonal variations. Ideally, the optimal index period corresponds to recruitment cycles of the organisms (based on reproduction, emergence, growth, and migration patterns). Sampling during an index period minimizes between-year variability while optimizing accessibility of the target assemblages and maximizing efficiency of sampling crews and gear. Understanding the dynamics of how an ecosystem functions seasonally allows for more consistent interpretation of data from prescribed index periods.
The use of an index period also allows a practical concentration of sampling effort during a period when reference conditions have been characterized. USEPA recommends establishing index periods for a particular season, time of day, or other window of opportunity when signals to stressors are determined to be strong and reliable. There is an added value of sampling “worst case” environmental conditions; the inherent assumption is that other times of the year will be at least as good. Further, USEPA recommends that only the results from similar index periods be compared for decision-making regarding assemblage status. The linking of administrative needs and decisions with calibrated index periods provides for a most robust program.
Figure 1:
Examples of Bioassessment Index Periods for Various State Water Quality Agencies.