The use of chironomid pupal exuviae in river biomonitoring : the importance of sampling strategy

Due to the seasonal variation in the emergence patterns of chironomid taxa, a combination of three monthly samples from spring to autumn has been recommended for biomonitoring of rivers using the Chironomid Pupal Exuvial Technique (CPET). The main purposes of this study were to establish an effective sampling strategy (i. e. strategy that would allow effective discrimination between study sites) for biomonitoring of boreal rivers using the CPET methodology and to estimate the taxonomic coverage of any three monthly sample combinations. First, we collected pupal exuvial samples from three oligo- to mesotrophic study sites in a large boreal River Kymi, southern Finland, in 4-week intervals from the beginning of May to September 2003. The four most effective combinations of three monthly samples resulted in a significant difference between the sites, while the six least powerful combinations indicated no differences in chironomid communities between the sites. A corresponding data set was collected from four oligo- to eutrophic rivers, one sampling site representing each river. In this data set, eight combinations out of ten found significant differences between the rivers. Thus, the sampling strategy seemed more decisive at the within vs. among-river scale of comparison. No single combination of three monthly samples was consistently the most effective one in discriminating among the studied sites and rivers. Nevertheless, a combination of CPET samples from different seasons (i. e. spring, mid-summer and late summer/autumn) clearly constitutes the best sampling strategy for the detection of environmental impacts in boreal rivers. In the studied rivers, and probably in other boreal rivers as well, any combination of three monthly samples from spring to autumn will yield a reasonably high coverage of the available genera (at least 60%). We suggest that the efficiency of different sample combinations and the distribution of the emergence periods of locally abundant intolerant taxa are crucial information when the CPET methodology is used for river biomonitoring.