Assessment of functional integrity of eutrophic streams using litter breakdown and benthic macroinvertebrates

Currently, interest is growing in evaluating stream functional integrity while river assessment schemes are still exclusively based on structural indicators determined from various aquatic communities. Although some approaches relying on macroinvertebrate functional groups and combinations of traits have been advocated as means to assess ecosystem function, there has been no attempt to test the reliability of these methods with any direct functional indicator even though litter breakdown has been recently proposed as a functional indicator of stream impairment. The purpose of this study was to compare nine benthic macroinvertebrate-based structural metrics with functional metrics based on leaf litter breakdown in coarse and fine mesh bags in nine streams distributed along a eutrophication gradient. In coarse mesh bags, a 10-fold drop in breakdown rate of alder litter indicated a high sensitivity of this functional indicator to the deleterious effects of ammonium and its associated products, ammonia and nitrite. In contrast, microbial breakdown measured in fine mesh bags did not vary substantially along the gradient. Taxonomic and functional structures of macroinvertebrate assemblages were altered in the most eutrophic streams, as shown by drops in IBGN (French biotic index), BMWP (Britain biotic index) and EPTC (Ephemeroptera, Plecoptera, Trichoptera and Coleoptera) richness. Shredder richness and an ecological diversity index calculated from functional groups defined from multiple trait combinations exhibited the highest correlation with litter breakdown rate. Our results indicate that indirect assessment of stream functional integrity by structural indicators is realistic but requires specific metrics not necessarily based on traits or functional groups. Leaf litter breakdown is confirmed as a reliable indicator for direct assessment of streams impacted by eutrophication.