Assessing bed disturbance for invertebrates in lowland streams using the Pfankuch index.

Abstract

The Pfankuch stability index (PI) is known as a simple and suitable measure to assess streambed disturbance for benthic organisms. Although this method has been widely applied to unimpacted mountain streams, the applicability for human-altered lowland streams has not been clarified yet. In this study, we conducted surveys on stream invertebrates and bed-stability using the PI in ten streams flowing the Dogo Plain in Ehime Prefecture, southwestern Japan. We aimed to evaluate the applicability of the PI to lowland streams and to develop a better index (modified PI) that is specialized to assess streambed disturbance in lowland streams by considering the effect of water quality degradation. As a result, the PI varied markedly among the sites. The modified PI also showed high variation but the rank of the score did not change for each site when compared with the original PI. Our analyses demonstrated that the invertebrate abundance decreased with increasing PI. A negative relationship with abundance was also found for the modified PI but there was little improvement in the relationship compared with the original PI. Overall, this study suggests that the Pfankuch method can be applied to assess the streambed disturbance for benthic macroinvertebrates in human-altered lowland streams. Our attempt to modify the pplicability of score was not successful, implying that further trials are recommended to increase the applicability of the PI to lowland streams. We suggest that developing methodlogy for measuring flood disturbance, as in this study, is key to boosting disturbance studies in streams and managing stream ecosystems from the viewpoint of disturbance ecology.