Breakdown and macroinvertebrate colonization patterns of leaf packs in impaired streams

The contribution of shed leaves by riparian vegetation is the most important source of organic matter in streams and forming leaf packs are often colonized by macroinvertebrates for food and refuge. Aim of the work has been to determine (1) the effects of wastewater on composition and functional organization of colonizing macroinvertebrate assemblages and (2) if leaf packs formed by native leaves can improve diversity of macroinvertebrate community structure in control and impaired sites. The study was performed in Seveso, Lura and Bozzente streams (Lombardy). Leaves and twigs of various species (native mix, locust tree, laurel), in litter bags, were placed in the riverbed as artificial substrates. Breakdown rates of leaf packs did not differ significantly (ANOVA split‐plot, α=0.05) among sites, indicating that water quality do not influence the breakdown rate. Within each site group, different leaf packs did not show dissimilar rate of decomposition, but different degradation ways (fragmentation in native mix and locust tree, delamination in laurel). Taxonomic composition differed between the two site groups. Tukey HSD test revealed a significantly higher value of Shannon index (p<0.001) and Taxa Richness (p<0.05) in the control sites. Macroinvertebrate colonization of leaf packs seems to be influenced principally by water quality and not by leaf types, as shown by all biological metrics and functional traits. Use of native leaves improves microhabitat quality and, thus, macroinvertebrate biodiversity: within control group, taxa richness showed significant differences among leaf types (p<0.01), with native mix being the best substratum.