Growth of a stream-dwelling caddisfly (Olinga feredayi : Conoesucidae) on surface and hyporheic food resources

Many epigean stream invertebrates periodically occupy hyporheic sediments and may exploit epilithic biofilms and particulate organic matter (POM) as food in surface and subsurface habitats. We compared the taxonomic composition and density of biofilms coating shaded and unshaded surface sediments in a New Zealand stream with those coating hyporheic (15-30 cm depth) sediments in the same reach, and investigated whether larvae of the cased caddisfly Olinga feredayi (Conoesucidae) grew when provided with biofilms and POM from the different sources. Biofilms grown in light were densely packed with algae and fungal hyphae, whereas dark-grown biofilms from the surface and the hyporheic zone supported lower microbial biomass and were dominated numerically by bacteria. Olinga feredayi larvae ingested hyporheic foods, but increased mass only when light-grown biofilm and POM were provided as well. In contrast, larvae lost mass and shortened their cases when supplied with light-grown biofilm only, or dark- or hyporheic-grown biofilm with POM. Overall, our findings suggest that hyporheic food resources were of lower quality and/or quantity than those found at the stream surface and that the growth of aquatic insects such as O. feredayi in the hyporheic zone depends on their ability to find and move between resource patches within the hyporheic zone and at the sediment surface.