Effects of instream restoration measures on the physical habitats and benthic macroinvertebrates in an agricultural headwater stream

The effectiveness of instream restoration measures in improving habitats has been extensively examined; however, the evidence is inadequate to infer that these measures have positive effects on benthic macroinvertebrates. In this study, we compared the effects of the instream wetland (IW), groin (GR), artificial drop (AD) and boulder placement (BP) measures on the physical habitat and benthic macroinvertebrates. On a five-year scale (two to six years after the implementation of the restoration project), the sample sites treated with the four restoration measures and the unrestored upstream control sample site were compared. The results show that the instream restoration measures had significant positive effects on the physical habitat and benthic macroinvertebrates in the agricultural headwater stream, that the physical habitat quality was a key factor affecting the restoration of the benthic macroinvertebrates, and that the restorative effects were affected by the interaction between the restoration measure and time. The five-year observation period showed continuous improvement in the habitat quality as well as a continuous increase in the taxon richness and diversity of the benthic macroinvertebrates at the restored sample sites. The AD and BP measures had the most significant positive effects on the richness and diversity of the benthic macroinvertebrates. The density of the benthic macroinvertebrates in the habitat at the sample site treated with the IW continuously remained at a high level. The benthic macroinvertebrates in the habitat at the sample site treated with the GR exhibited outstanding durability against and resilience to a flood. Schemes involving densely placed instream restoration measures had continuous positive effects on the physical habitat and benthic macroinvertebrates in agricultural headwater streams on a medium time scale (six years).