Implications of seasonal variation for biomonitoring with predictive models in the Fraser River catchment, British Columbia

Reference-condition models for the Fraser River catchment were developed us ing samples collected during the autumn of 1994, 1995, and 1996. The goal o f this study was to examine applicability to the reference-condition models of samples collected in other seasons and the effect of taxonomic resoluti on (genus and family) on model sensitivity to seasonal variation in the ben thic invertebrate assemblage. Samples from eight streams representing inter ior, coastal, and large-river habitats were collected in spring, summer, au tumn, and winter of 1995 and in spring of 1996. The benthic invertebrate as semblage changed seasonally such that the models could not be used for seas ons other than autumn. The models were equally sensitive to seasonal variat ion when genus-level or family-level data were used. We recommend that test samples (i.e., samples collected from disturbed sites and meant for compar ison with the reference database) be collected either during the autumn or over multiple sampling dates to reduce the possibility that seasonal shifts or stochastic events will lead to erroneous conclusions about the state of a test site.