Simple large wood structures promote hydromorphological heterogeneity and benthic macroinvertebrate diversity in low-gradient rivers.

The presence of large wood (LW) in river channels adds an important habitat feature for benthic macroinvertebrates. However, there has been a lack of studies focusing on the effects of simple wood structures on hydromorphology and macroinvertebrate diversity in surrounding channel areas. This study explores whether consistent patterns in LW-related benthic habitat complexity and macroinvertebrate diversity can be identified across a set of low-gradient streams dominated by fine sediments. While the presence of LW did not change the average values of standard hydromorphological variables (flow velocity, turbulence, median sediment grain size and sorting index), the coefficients of variation of such variables for wood rich sites were consistently higher than those for wood poor sites (velocity: 85 % higher, turbulence: 89 %, grain size: 126 %, sorting index: 67 % higher). In parallel, beta diversity was on average 31 % higher in the wood rich sites, and positively correlated with the amount of LW at the site. The hotspots of local (alpha) diversity were located in the river-bed areas surrounding the LW, where taxonomic richness was 83 % higher and Shannon-Wiener diversity 39 % higher compared to the sites with less wood. These results demonstrate that the presence of LW in sandy lowland rivers induces consistent patterns of increased spatial variability of benthic habitats in the surrounding channel areas and this significantly enhances alpha and beta diversity of macroinvertebrate communities. Therefore, LW should be conserved in river channels wherever possible, and its potential for introduction into degraded systems should be explored further because even simple pieces of LW introduced to lowland streams can deliver benefits. [ABSTRACT FROM AUTHOR]