Spatial and temporal variability of diatom and macroinvertebrate communities: How representative are ecological classifications within a river system?

Evaluation of river condition is complicated by dynamic relationships between river assemblages and their environment. We assessed variation of ecological status classifications of stream diatom and macroinvertebrate communities within a boreal river system. We specifically examined whether results of ecological classifications are concordant across taxonomic groups and among main channel and headwater side tributaries of this river system. We further assessed whether ecological classifications are stable from year to year and whether classifications show predictable relationships to major stressor gradients. The estimated ecological condition varied considerably among and between reaches of main channel and tributaries for both examined taxonomic groups. Ecological classifications also differed between the two taxonomic groups and were generally lower and temporally more variable for diatoms. Ecological Quality Ratios (EQR) of the metrics were mainly related to pH representing a major disturbance gradient in our study, but in general, relationships to stressor gradients were weak. Our results suggest that ecological classifications could be weakly extrapolated among sites and, depending on the taxonomic group, also among different years in river systems disturbed by anthropogenic activities. Spatially and temporally extensive monitoring is therefore needed to reliably classify ecological condition of a water body. With this regard, it would be important to assess and monitor whole river networks instead of linear segments of large rivers.