Hyporheic annelid distribution along a flow permanence gradient in an alluvial river
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In this study, we examined hyporheic annelid assemblages along a gradient of flow permanence (FP) and compared assemblages in gaining (groundwater-fed) and losing (runoff-fed) sections of the alluvial Selwyn River, New Zealand. To reduce the effects of poor taxonomic resolution, we used a dataset with most taxa identified to the genus or species level. We predicted that annelid assemblages would vary in structure and composition along FP gradients due to differences in desiccation resistance between taxa. We also predicted that groundwater-fed (gaining) and runoff-fed (losing) river sections would be inhabitated by dissimilar annelid assemblages due to differences in river-aquifer connections and recolonization sources. We found a negative relationship between taxon richness and FP, indicating that, on average, two annelid taxa are lost from hyporheic assemblages in the Selwyn River with every 10% decrease in FP. Low FP appears to favour annelid taxa that tolerate moist or dry conditions in sediments, as shown by a negative relationship between FP and the proportion of desiccation-tolerant taxa. A high proportion of hypogean taxa distinguished the groundwater-fed and perennial-gaining reach from the other reaches. In spite of the large differences in physical structure between the intermittent-gaining and the ephemeral-losing reach, we found few between-reach differences in annelid assemblages and, in particular, no differences in % hypogean taxa. These varied results illustrate the need to employ both categorical and continuous variables in ecological analyses: the combined categorical and gradient approach used in the present study is likely to explain more variability than either univariate approach alone.