ELUCIDATING ENERGY SOURCES FOR HYDROPSYCHID CADDISFLIES ALONG A STREAM CONTINUUM USING STABLE-ISOTOPE ANALYSIS

R.R. Doucett¹, D.R. Barton¹, D.J. Giberson², and G. Power¹. ¹Department of Biology, University of Waterloo, Waterloo, ON N2L 3G1, Canada, ²Department of Biology, University of Prince Edward Island, Charlottetown, PEI C1A 4P3, Canada

Hydropsychid caddisflies were collected every 500 m from headwaters to mouth along Catamaran Brook, New Brunswick, for stable-isotope analysis of carbon and nitrogen. More than 600 late-instar larvae, comprising five species of Hydropsyche, one species of Arctopsyche, and one species of Cheumatopsyche, were obtained in June and October 1998. Stable-nitrogen-isotope ratios were more enriched upstream (6-8 per mil) than downstream (0-2 per mil), while stable-carbon-isotope ratios were similar over the entire stream (-30 to -28 per mil), except near the outfall of the major tributary (-35 to -31 per mil) and just below the headwater lake (-27 to -25 per mil). Larvae collected in June were consistently 4 per mil more enriched in carbon and 2 per mil more enriched in nitrogen than those sampled in October. Species composition did not differ along the stream, but some species (Arctopsyche sp. and Hydropsyche morosa) were most abundant in October. Arctopsyche sp. were always more enriched in N¹⁵ than most other hydropsychids, suggesting a higher trophic position for these larvae. In general, stable-carbon-isotope ratios implied a shift in energy sources for conspecific Hydropsyche, varying between purely allochthonous (-26 per mil) to purely autochthonous (-35 per mil), depending on season and location along the stream continuum