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We used data from 235 boreal headwater streams in Finland to examine whether macroinvertebrate assemblages constitute clearly definable types, and how well biologically defined assemblage types can be predicted using environmental variables. Twoway indicator species analysis produced 10 assemblage types, which differed significantly from each other (multiresponse permutation procedure, MRPP). However, based on MRPP and nonmetric multidimensional scaling, there was wide variation among sites within each assemblage type, and high degrees of overlap among assemblage types. Such continuous variation was also evidenced by the low number of effective indicator taxa (indicator value method) for each assemblage type. Furthermore, discriminant function analysis based on environmental variables showed a moderate yet variable prediction success (59.6% of sites correctly predicted; range 0-96% per group). Canonical correspondence analysis indicated that variation in assemblage structure was primarily related to latitude and water chemistry, especially pH and water color. The absence of discrete macroinvertebrate assemblage types in boreal headwater streams may stem from a number of reasons: (1) macroinvertebrate taxa inhabiting boreal streams tend to exhibit individualistic, taxon-specific responses to environmental gradients, (2) they have wide environmental tolerances and geographic distributions, and (3) boreal streams are characterized by frequent, unpredictable disturbances. Our results suggest that local filters in headwater streams are relatively weak, resulting in poorly distinguishable assemblage types. By contrast, the major latitudinal gradients in macroinvertebrate assemblage structure suggest that regional-scale filters, e.g., temperature, exert strong control over taxon distributions in headwater streams. We suggest that a tiered approach combining regional stratification and subsequent prediction of assemblage structure could provide a suitable framework for the bioassessment and conservation of boreal headwater streams.

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We evaluated a simple bioassessment method based on a priori river typology to predict benthic macroinvertebrate fauna in riffle sites of rivers in the absence of human influence. Our approach predicted taxon lists specific to four river types differing in catchment area with a method analogous to the site-specific RIVPACS-type models. The reference sites grouped in accordance with their type in NMS ordination, indicating that the typology efficiently accounted for natural variation in macroinvertebrate assemblages. Compared with a null model, typology greatly increased the precision of prediction and sensitivity to detect human impairment and strengthened the correlation of the ratio of observed-to-expected number of predicted taxa (O/E) with the measured stressor variables. The performance of the typology-based approach was equal to that of a RIVPACS-type predictive model that we developed. Exclusion of rarest taxa with low occurrence probabilities improved the performance of both approaches by all criteria. With an increasing inclusion threshold of occurrence probability, especially the predictive model sensitivity first increased but then decreased. Many common taxa with intermediate type-specific occurrence probabilities were consistently missing from impacted sites, a result suggesting that these taxa may be especially important in detecting human disturbances. We conclude that if a typology-based approach such as that suggested by the European Union's Water Framework Directive is required, the O/E ratio of type-specific taxa can be a useful metric for assessment of the status of riffle macroinvertebrate communities. Successful application of the approach, however, requires biologically meaningful river types with a sufficient pool of reference sites for each type.

The colonization and succession of the macrozoobenthos in a polyhumic, winter‐regulated reservoir, Lake Kyrkösjärvi in Western Finland (62° 45′N, 22°48′E, A = 6.4 km2 at summer HW, zmean = 2.5 m and zmax = 7 m) was studied from its filling in 1981 until 1989 (in 1981–83, 1986 and 1989). The zoobenthos was sampled over the whole reservoir bottom using qualitative and quantitative methods during three open‐water seasons and winter samples were taken in the regulated and submerged areas during the maximum draw down (2 m) in March‐April. Colonization during the first summer revealed two phases. The first phase featured dominance of actively swimming and adult‐dispersing taxa. The second phase was characterized by a mass occurence (high abundance and biomass) of chironomids (Chironomus, Glyptotendipes) and the isopod Asellus aquaticus. Asellus originated in a small bog pond in the northern outlet basin, and it was the dominant zoobenthic species during the study period. The species of Chironomus, most of them pool inhabitants of natural waters, succeeded each other. In the later study years, when the zoobenthos at the sites was determined more by habitat (spatially) than by the succession (temporally), the reservoir bottom could be divided into three areas: a) deep areas, ≧3 m, with an increasing occurrence of lacustrine species (eutrophic or dystrophic), b) shallower organic bottoms, ≦ 2 m, with many pool‐inhabiting and eurytopic species, and c) some eroded areas in the shallow littoral, ≦ 0.5 m, where lacustrine species became common. Water‐level regulation in winter killed only a small proportion of the fauna in the organic‐rich peatland and forest bottom areas representing the majority of the regulated zone. Hence, the regulated areas had almost everywhere high abundance and biomass values throughout the year, deviating from the results found by other authors in strongly regulated lakes, which have maximum values just below the draw down limit.