Does the Proportion of Crop Fields in a Small Catchment Predict Coupled Benthic-Hyporheic Invertebrate Responses?
Junjiro N. Negishi
Abstract.—The applicability of catchment indicators for predicting aquatic invertebrate responses in both vertical and lateral dimensions of river ecosystems is poorly understood. Therefore, our objective was to determine whether landscape indicators can predict coupled benthic-hyporheic responses in river and riparian environments. To do so, we assessed the relationships between the proportion of crop fields, instream habitat conditions, and abundance of larval and adult Ephemeroptera, Plecoptera, and Trichoptera (EPT) taxa in (and from) benthic and hyporheic zones of a 15-km segment of the Satsunai River (catchment area: 725 km2), eastern Hokkaido, northern Japan. Invertebrates were collected using colonization traps (hyporheic taxa), Surber sampler (benthic taxa), and Malaise traps (adults). We first identified insect taxa belonging to each of the benthic dwellers (EPT, excluding chloroperlid Plectoptera) and hyporheic dwellers (chloroperlid Plectoptera) based on the relative unit-volume abundances in benthic zone (7-cm surficial part of riverbed) and hyporheic zone (30 and 50 cm deep). Nitrate concentration and total chlorophyll, the abundance of larval insects in both zones, and adult insects from the benthic zone increased with an increasing proportion of crop fields. In contrast, the abundance of adult invertebrates originating from the hyporheic zone maximized in the area with an intermediate proportion of crop fields. We attributed this disparity to the spatial variability in total availability of functional hyporheic habitat in a vertical dimension, which could not be explained by the abundance estimates on a unit-volume basis. Overall, the proportion of crop fields in a catchment can be used to predict numerically similar (coupled) abundance responses of EPT larvae in river benthic and hyporheic zones. Furthermore, the use of landscape indicators in predicting coupled benthic-hyporheic responses in the riparian zone can be improved with information on the vertical extent of functional hyporheic habitat.