Detecting Invertebrate Species Change in Running Waters: An Approach Based on the Sufficient Sample Size Principle

The EU Water Framework Directive (WFD) is in need of a practical method and index suitable for comparing the ecological status of invertebrate fauna in rivers throughout Europe. The uncertainties that characterize lotic sampling methods can be omitted by increasing sampling effort up to a species detection probability threshold. We describe a statistical model which provides the background for applying a sampling protocol aiming to record the total number of common and abundant species. Methods, sample standards and sorting time become less significant to the result as long as the total sample is sufficiently large to detect common species. An index called the Intercalibrated Benthic Invertebrate Biodiversity Index - IBIBI - is proposed based on Observed / Expected species ratios for EPT groups. The Ecological Quality Ratio thus obtained may then be adjusted to WFD's ecological status scale within each region or river type across Europe, which then can be compared and intercalibrated. Although offering a possible solution to the problem of intercalibration, further studies are needed.