The Transition Zone: Impact of Riverbanks on Emergent Dragonfly Nymphs Implications for Riverbank Restoration and Management

Abstract

The use of riprap in the restoration and stabilization of riverine landscapes is an issue of concern for many ecologists. While current methods of bank stabilization, especially those involving the placement of rocks (riprap) along the waterline, are effective in controlling erosion their presence changes habitat components (slope, substrate composition, near-shore river velocity) at the river-land interface. The additional impacts of river current, water temperature, soil composition, slope, and water level fluctuation, may further imperil emerging nymphs. The purpose of this research is to document the effects of riprap, location (upriver or downriver of hydroelectric intake/outtake facilities), water level fluctuation, river velocity, air temperature, water temperature, substrate temperature, and soil composition on the distance traveled to eclosure site by G. vastus and S. spiniceps, and the density of S. spiniceps, G. vastus, N. yamaskanensis, D. spinosus, O. rupinsulensis, M. illinoiensis, and E. priniceps. Knowledge of the conservation status of these species is fairly limited, although S. spiniceps (threatened), G. vastus (species of special concern), and N. yamaskanensis (species of special concern) are all currently listed on the Massachusetts Endangered Species list. Species density was determined through exuviae collection, and emergence distance was recorded from the edge of the waterline to the site of attached exuviae. Results of the study indicate that nymphal response to the observed abiotic features varies both with location and species. The presence of riprap had no significant effect on densities of S. spiniceps, G. vastus, N. yamaskanensis, D. spinosus, O. rupinsulensis, M. illinoiensis, and E. priniceps, but did significantly reduce the distance traveled from the waterline by both G. vastus and S. spiniceps.