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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. Annual changes in reference condition are less directional and predictable than seasonal variability and are influenced by large-scale phenomena such as floods and drought (Bailey et al. 2004).
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. Conducting all sampling during the low and stable-flow period of the year is widely accepted based on the assumption that it increases the likelihood that samples throughout a study unit can be collected under similar flow conditions (Flotemersch and Blocksom 2005). 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.
Some Frequently Asked Questions
Question: What time of the year is best for sampling?
Answer: As depicted in the Table below, the index period will vary by region and State. Sampling within distinct periods minimizes variability due to temporal factors.
Question: What if I need to sample outside my index period?
Answer: First, an evaluation of seasonal influences can aid in developing seasonal adjustment factors for an index or biological indictor. Second, sampling a small subset of reference sites at the same time will help in the final assessment.
Question: How would I test for an appropriate index period?
Answer: For evaluating natural variability, focusing on the population of reference sites provides a basis for describing the fluctuation in the biotic assemblage over a temporal time period. Some states have restricted index periods to minimize the influence of confounding variability, and others have complemented their chosen period with “worst case” environmental conditions to gauge maximum impact.
References
Bailey, R.C., R.H. Norris, and T.B. Reynoldson. 2004. Bioassessment of Freshwater Ecosystems Using the Reference Condition Approach. Kluwer Academic Publishers, Dordrecht, The Netherlands. 170 pp.
Flotemersch, J.E. and K.A. Blocksom. 2005. Electrofishing in boatable rivers: Does sampling design affect bioassessmnt metrics? Environmental Monitoring and Assessment 102:263-283.